,,fossils ,a ,story of the ,rocks and ,their ,record of ,prehistoric ,life ,by ,harvey ,c4 ,markman ,curator of ,geology and ,paleontology ,cover ,design and ,murals by ,mary ,chilton ,gray ,,denver ,,museum ,,of ,,natural ,,history ,popular ,series ,no4 #c ,alfred ,m4 ,bailey1 ,editor ,third ,edition1 ,reprinted ,october #a1 #aied ,illustration3 ,at ,work on a ,dinosaur ,skeleton ,,contents ,page ,introduction #e ,the ,prehistoric ,record #e ,varieties of ,fossils #h ,fossilization #i ,floras and ,faunas #ac ,formations #af ,geological ,time #ah ,explanation of the ,time ,chart #bc ,the ,geological ,section #be ,before the ,age of ,reptiles #ca ,the ,pre-,cambrian ,complex #ca ,cambrian ,life #cc ,the ,ordovician ,record #ce ,silurian ,events #cf ,devonian ,progress #cg ,carboniferous ,forests #dj ,permian ,hardships #dc ,the ,age of ,reptiles #dg ,dinosaurs #dh ,plant ,life and ,climate #ef ,coal and ,fossil ,footprints #eg ,mesozoic ,invertebrates #eh ,extinct ,birds #fj ,ancestors of the ,mammals #fa ,the ,age of ,mammals #fd ,prehistoric ,horses #gj ,mastodons and ,mammoths #hj ,the ,rancho ,la ,brea ,fossil ,pits #hh ,the ,age of ,man #ic ,supplementary ,reading #ie ,,list ,,of ,,illustrations ,page ,at ,work on a ,dinosaur ,skeleton #b ,fossil ,bones in ,rock ,formation #g ,insect ,fossils #aj ,restoration of ,rhinoceros #ab ,dinosaur ,tracks #ag ,time ,chart #bb ,geological ,section ,showing ,positions of ,formations #bf ,marine ,beds of the ,benton ,formation #bh ,plesiosaur ,bones in ,place #bh ,invertebrate ,fossils #cd ,modernized ,fishes #ch ,prehistoric ,plants #da ,marine ,reptiles #df ,diplodocus #di ,trachodon #ea ,stegosaur #eb ,sea ,turtle #ed ,murals1 ,hall of ,mammals #fc ,uintatheres and ,contemporary ,life #fe ,moropus #fg ,titanotheres #fi ,oligocene ,mammals #ga ,pleistocene ,horse #gc ,structure of ,molar ,teeth #ge ,grazing ,type of ,molar ,teeth #gg ,american ,mastodon #gi ,long-,jawed ,mastodont #ha ,molar ,tooth of ,mammoth #hc ,nebraska ,mammoth #he ,rancho ,la ,brea ,fossils #hg ,folsom ,bison #ij ,man and ,mammoth #ib ,,fossils ,,introduction ,in the recent growth of knowledge there has been rapid progress in two directions4 ,the commercial exploitation of natural resources1 being fundamental to modern civilization1 attracts a liberal share of the talents and energies of workers trained for the industrial professions4 ,a second trend has specialized in the further development of the sciences which are characteristic of our time4 ,such activities1 in the natural history field1 deal largely with the refinements of exact definition1 nomenclature and classification1 all of which means little or nothing to the layman who is otherwise engaged4 ,for the latter1 however1 there is a quality of interest which may be described as a wholesome curiosity about what has happened1 how it happened1 how we know it happened1 and what it may signify to one who is neither industrialist nor scientist4 ,this booklet is intended for the many who feel that there is more to be obtained from a natural history museum than an occasional glimpse of a bewildering 8marvel40 ,in addition to being a guide to fossil exhibits it supplies parts of a great story which specimens alone can not relate4 ,,the ,,prehistoric ,,record ,all that is known of the extinct plants and animals which inhabited the earth before man began the practice of recording his observations has been obtained from a study of the rocks4 ,the few possible exceptions to this rule1 in which animal and plant remains have been preserved by freezing or drying1 are so unusual as to be hardly worth mentioning4 ,explanation of this is that plant and animal tissues quickly decay under ordinary conditions when life ceases4 ,unless protected from destructive agencies which are especially active at the surface of the ground1 even the heavier bones of animals and the large trunks of fallen trees will soon crumble into shapeless masses4 ,the usual method employed by nature to prepare a fossil specimen is so closely related to the natural process of rock making that a little knowledge of that subject will be necessary in order to know what fossils are and how they are preserved for so long a time4 ,it should be understood first that a fossil is some record1 commonly preserved in rock1 of a kind of plant or animal which no longer exists as a living type4 ,this1 at least1 is the ordinary sense of the word and more elaborate definitions are of small service to anyone4 ,it may be necessary to add1 however1 that all things which have lived at any time are regarded as either plants or animals4 ,nature's way of producing rocks and fossils remains a mystery to many of us because we are so wrapped up with the importance of finding names for things and materials that we frequently neglect the consideration of sources and histories4 ,everyone knows a rock when he sees it in a large mass1 but when he looks at sand1 mud1 dust1 or soil1 he seldom thinks of it as related in any way to rocks4 ,although the difference is almost entirely a matter of size1 our use of words makes it seem unreasonable to speak of the finer particles as rock4 ,there can be no reality or meaning in the natural record for an individual who has failed to observe a few simple facts which involve changes going on in all parts of the world at the present time4 ,with regard to rocks1 it is supposed that what happens in our day also occurred under like circumstances ages ago4 ,anyone wishing to do so may see for himself that rock masses break down wherever they are exposed to the elements1 that the larger pieces are reduced to smaller fragments1 and that the final product is sand or dust4 ,he may also note that this finely ground material is being moved and sorted1 by rain1 wind1 and streams1 transported to lower levels and accumulated in great quantities wherever it finds a resting place4 ,along with it go sticks and leaves1 bugs1 shells1 bones and carcasses of animals1 some of which in time may become fossils4 ,in large lakes and seas there is a steady distribution of such materials over broad areas1 yesterday's accumulation of sediments being buried by the contributions of today1 the most recent of the settlings always resting upon older ones until something happens to disturb that arrangement4 ,not so readily observed are other parts of the process1 such as the consolidation of sands and muds into the firm sandstones and shales which we again recognize as rocks4 ,much of this requires more than the few score years of a human lifetime for its accomplishment1 but many of us have seen muds become so solidified1 by merely drying1 that they could hardly be distinguished from prehistoric shales4 ,it is to be noted also that some ancient fossils come from sandstones which are scarcely more rock-like than the loose sands of an ocean beach4 ,thus we learn that firm consolidation of rock-making materials is not always a sign of great antiquity1 and that hardness of rock is not always essential to the preservation of imbedded plants and animals4 ,illustration3 ,rhinocerous ,bones as ,found in the ,rock ,this exhibit was taken from the famous fossil quarry at ,agate1 ,nebraska4 ,the fossilized remains are still partially imbedded in the sandstone which preserved them for millions of years4 ,the rocks themselves must explain the many things which have happened during the course of millions of years1 and this they do remarkably well when carefully studied1 for many of the factors involved in their histories leave characteristic marks4 ,changing climates1 the draining of seas1 the uplifting of mountain ranges1 all have ways of registering their occurrence which are as convincing and reliable as anything ever written by man4 ,piece by piece the story has been patched together through the efforts of thousands of investigators4 ,parts of the narrative remain buried at inaccessible depths1 and whole chapters1 no doubt1 have been destroyed by the same forces that composed this tremendous record of prehistoric times4 ,,varieties ,,of ,,fossils ,it would be a serious mistake to regard nature as divided into a number of distinct and independent schools of fossil making1 each refusing to use the methods and devices of another4 ,there are1 however1 certain features which stand out so prominently that a little classification becomes helpful4 ,while this procedure brings out differences it should be understood that processes actually work together1 several of them usually being involved in the production of any individual specimen4 7#a7 ,impressions of animals and plants1 or parts of these1 are frequently left in soft sand or mud which later becomes converted into more durable rock4 ,this type of fossils is represented by animal foot-prints and the imprints of leaves1 flowers1 insects1 and like objects which may be mingled with the finely ground materials of the common sedimentary rocks4 7#b7 ,parts of plants and animals may be gradually replaced by mineral matter with little or no change from original form and texture4 ,fossils of this class are said to be petrified or turned to stone4 ,they are also known as replacements4 ,the fleshy parts of animals do not petrify4 7#c7 ,many animals among the invertebrates use mineral substances for protective or supporting structures4 ,small plants of various kinds follow a similar practice4 ,these structures1 being produced in stony materials1 are readily converted into fossils4 ,the shells of mollusks are the best known illustrations in this field1 and all that is required for a shell to become a fossil is the extinction of the species of animal that produced it4 ,fossils of this type are extremely abundant4 7#d7 ,preservative substances other than those which produce common rocks may be mentioned among fossil-making possibilities4 ,bones are known to have been preserved in asphalt1 and insects in resins1 but such cases are few in comparison with the products of other methods4 7#e7 ,in rare instances there has been preservation of extinct creatures by the process of drying or by refrigeration4 ,occasional mummies are found with shriveled flesh and skin still in place1 but better preservation of all tissues occurs when the temperature is quickly reduced below freezing point and held there without interruption4 ,this can happen only in the colder parts of the earth and is always subject to climatic change4 ,the effect of drying also may be undone at any time by a slight increase in the amount of moisture4 7#f7 ,coal beds often produce fossils of an unusual sort4 ,in the formation of coal1 plant material gradually loses some of its more perishable substances but retains carbon which has better lasting qualities and slowly accumulates to produce the seams and beds that are mined4 ,in the early stages of the process the original vegetation undergoes little change in appearance but eventually practically all of its character is lost4 ,many fossil leaves are found as thin layers of carbon1 bedded in the clays which are commonly associated with coal deposits4 7#g7 ,concretions1 which are hardened lumps of mineral substances occurring commonly in sandstones and shales1 are often mistaken for fossils because of their peculiar shapes4 ,however1 there are localities in which the mineral solutions have been concentrated and deposited around shells1 leaves1 seeds1 or similar objects1 thus producing an abundance of fossils which may be obtained by opening the concretions4 ,fossils of this type are well known from ,mazon ,creek and other districts in ,illinois1 ,kansas1 ,colorado1 and elsewhere4 ,,fossilization ,footprints need little explanation other than a consideration of the factors which make it possible for them to be preserved4 ,the sand or mud must be neither too soft nor too hard to take the form of the foot and retain its shape when the foot is withdrawn4 ,then in some manner the impression must be protected while the rock-making process goes on4 ,when such protection is obtained it is usually in the form of more mud and sand1 deposited over the surface which received the impression4 ,at a later time the covering may be separated from the lower part of the deposit1 which serves as a mold1 and if the separation be accomplished successfully a natural cast of the foot will be obtained as well as the mold in which it was produced4 ,since conditions for perfect work are not always present in a laboratory of this kind1 it is not surprising that fossil footprints are very rare considering the number and variety of tracks left by wandering animals4 ,impressions of leaves are explained in much the same way except that the leaf remains under its protective covering until it decays4 ,similar impressions may be obtained from the bodies of delicate invertebrate animals but they are seldom preserved because of the softness of the tissues4 ,the smaller fishes provide much better material for the production of fossils according to this method4 ,while the fish is being flattened by the weight of surrounding sediments1 scales1 fins1 and soft bones retain their positions and provide the necessary resistance to leave an impression of the body form when the flesh is gone4 ,illustration3 ,insect ,fossils 7enlarged7 ,fine specimens of this type are obtained from an old lake bed at ,florissant1 ,colorado4 ,the larger and more spectacular fossils1 such as skeletons1 skulls1 and detached bones are nearly always of the replacement type4 ,replacement of plant and animal substances by mineral matter is a slow process and in younger fossils the change is rarely completed1 some of the original material being present in a partially altered condition or not modified at all4 ,since air does not often carry the necessary materials and provide other essential conditions1 replacement may be regarded as something which happens underground or in water4 ,it is perhaps best explained in connection with limestones1 because calcite or 8lime0 is frequently the replacing substance although other minerals1 especially quartz1 may serve the purpose4 ,besides converting bony or woody objects into rock substance1 mineral replacements may assist in the production and preservation of fossils in another manner4 ,it often results in the filing of cavities with some rock-making substance which retards destruction through crushing or other injury4 ,in many cases1 so-called fossil shells are not shells at all2 instead1 they are merely a stony filling which was once surrounded by shell substance4 ,in other instances the original shell remains as it was during the life of its former occupant1 preservation of the shell being due largely to the substitution of a mineral filler for the soft animal tissues once present4 ,limestone comes into existence through a more elaborate process than that which produces sandstone and shales4 ,it is one of the three types of common rocks1 known collectively as the sedimentaries1 in which fossils are found4 ,it differs from sandstones and shales1 however1 in that much of its substance has been dissolved in water instead of being transported in the form of finely ground rock particles4 ,lime occurs in many varieties of rock which are exposed to the wear and tear of the elements throughout the world4 ,slowly but more or less continuously it is taken from this source by ground and surface waters coming in contact with it4 ,particularly active is carbonated water1 moving underground through pores and crevices4 ,this underground circulation of mineral matter in a dissolved condition explains the occurrence of fossils in land areas which have not necessarily been submerged during any great length of time1 for it is well known that plant and animal remains are not invariably washed into lakes or seas1 and that all sedimentary deposits have not been built up in large bodies of water4 ,here we are dealing with what is known as the continental type of sedimentation and such fossils as dinosaurs1 mastodons1 three-toed horses1 and other former inhabitants of land areas4 ,in order to become properly fossilized1 certain conditions are absolutely necessary1 and only a small percentage of the once-living multitude secures the required treatment4 ,there must be present1 soon after death1 some protection from the activities of the carnivorous birds and beasts that would separate and scatter the parts of a carcass1 also from the smaller gnawing animals that would continue the destruction1 and finally from wind1 sun1 rain1 frost1 and bacterial and chemical activities which in the course of only a few years would remove everything but possibly a few scraps of tooth enamel1 which is the hardest of animal tissues4 ,a slight covering of earth substance in any form serves to check the disintegration1 and this may be acquired in several ways4 ,animals that perish in bogs or quicksands are soon covered over2 in many localities wind-blown dust and sand do the work2 and flooded river valleys provide an abundance of mud for the necessary burial of others4 ,even underground1 the decay of soft tissues is too rapid to permit of replacement by mineral substance in a manner that would reproduce form and texture4 ,skin and flesh are almost invariably lost1 although in a few instances the thick scaly hides of dinosaurs are known to have produced natural molds and casts by the method explained in connection with footprints and other impressions4 ,illustration3 ,the skeleton of this prehistoric ,american rhinoceros is mounted in a 8half shell0 which was modeled over the bones to show the form of the living animal4 ,the artist's reconstruction appears in the painting above the fossil exhibit4 ,with regard to the more durable tissues found in the teeth1 bones1 and shells of animals1 or the woody parts of plants1 the case is different4 ,these parts become firmly imbedded in the ground1 but moisture still has access1 and it begins to work immediately2 for all water moving underground finds soluble substances which it picks up and carries with it wherever it goes1 and much of the load consists of mineral matter which may be unloaded again when the necessary conditions are found4 ,mineral-laden waters will drop one kind of substance to take up another which dissolves more readily1 and this happens sooner or later when a buried bone or log is encountered4 ,complications of various sorts enter into the process1 but the final outcome frequently is a complete change from one chemical composition to another which is more enduring1 the transformation being brought about so gradually and thoroughly that in many fossils the inner structure of the original tissue is as accurately reproduced as the fine detail of surface features4 ,converted into stone1 however1 the result is still far from permanent4 ,while yet underground the fossil is subjected to distortion and breakage due to earth movements which bend and dislocate the rock deposits4 ,what causes these upheavals and depressions of the earth's surface remains the subject of much discussion1 but that they have occurred on a large scale and continue to occur is clearly evident4 ,at higher altitudes the surface rocks and fossils are exposed to a larger variety of destructive activities than at lower levels where protective coverings are more likely to be provided and retained4 ,once stripped of that protection there is little chance for a fossil to survive4 ,beyond a doubt there are many thousands of tons of prehistoric remains damaged or destroyed each year1 by weather and stream erosion4 ,,floras ,,and ,,faunas ,as the various types of sediments continue to accumulate on land and in water they produce deposits of sandstones1 claystones1 and limestones which in time may acquire great thickness and cover wide areas of sea floor1 or continental surface4 ,usually there is more or less mixing of sediments resulting in sandy limestones1 limy clays1 and other combinations4 ,quite commonly1 however1 the types remain fairly pure but become arranged in layers which alternate from one kind of material to another4 ,at all times the character of the deposit will depend upon the nature of the rocks which supply the materials1 and any fossils that may be produced will consist of such plants and animals as live and die during the time the rock is in the making4 ,some of the rock layers will be rich in plant and animal remains1 others quite barren1 the difference being due partly to conditions influencing the life of the region4 ,in addition1 the character and amount of rock-making materials at the time may be favorable or unfavorable to the preservation of fossils4 ,seas1 lakes1 and valleys may at any time be drained1 or enlarged and deepened1 by changes in the elevation of underlying rocks4 ,the amount and variety of mineral substances dissolved in the waters of a region not only affect the character of rock deposits but also the plants and animals living in the water4 ,some of these chemical solutions provide cementing materials which bind together the grains of sands and mud2 others have a detrimental effect upon cementing material previously deposited1 and so construction and destruction go on continuously1 more or less hand in hand1 to produce complicated and often puzzling results4 ,a little more salt1 or a little less of it1 may change completely the variety of life inhabiting a body of water4 ,a slight change in the depth of the water often accomplishes the same thing1 for plants and animals are so delicately adjusted to their environments that conditions fatal to one race of creatures may provide the exact life requirement of another4 ,this is a matter of practical knowledge which is being used today in the cultivation of plants and animals for market purposes4 ,it is being demonstrated continuously1 also1 upon living subjects in experimental laboratories throughout the world2 and1 in a bigger way1 the facts are observable wherever life is considered in relation to habitat4 ,that anything so obvious should be regarded as guesswork or theorizing1 or opposed to truth1 when applied to former inhabitants of the earth1 is somewhat surprising4 ,and1 it may be added1 the cultural worth of fossil study comes to a focus on this very point1 for men and women are now meddling1 consciously or unconsciously1 wisely or unwisely1 with an all-important environment about which they have learned very little--one called1 among other things1 8civilization40 ,for any portion of the world a complete-list of the different kinds of plant inhabitants comprises the flora of that region1 and a like summary for the animal life is known as the fauna of the district4 ,it is generally understood that different species of both plants and animals inhabit different regions of the earth1 but outside of professional circles it is only beginning to be recognized that changes in floras and faunas occur from time to time1 that slight differences may be noted in the course of observations extending over a period of only a few years1 and that everything in a fauna or flora eventually may be displaced by new forms4 ,it is1 however1 a convenient practice to use these terms in connection with time periods1 rock beds1 and types of environment1 as well as geographical areas4 ,thus we have such phrases as a 8,cretaceous fauna0 7attaching the name of a geologic period71 a 8,benton fauna0 7with reference to the fossils of a rock formation71 a 8marine flora0 7using the name of an environment71 an 8,arctic flora0 7which applies to a definite portion of the earth surface and its plant inhabitants74 ,faunas include animals which many persons do not recognize as such4 ,sponges1 corals1 insects1 worms1 crabs1 oysters1 and a host of other boneless creatures are grouped together as invertebrate animals1 while another group includes the fishes1 amphibians 7toads1 frogs1 and salamanders of today71 reptiles 7crocodiles1 lizards1 snakes1 and turtles being well known varieties71 birds1 and mammals4 ,this second lot1 provided with backbones and skeletons1 comprise the great division of vertebrate animals4 ,floras also include types which are commonly seen but not popularly identified as plants4 ,the algae are perhaps best known as seaweeds1 water-silk1 and pond scums2 fungi as toadstools and moulds4 ,both groups are large and of important rank in the vegetable kingdom2 only the algae1 however1 are recognized as important fossil producers4 ,better known types of plants are the mosses1 ferns1 evergreens1 grasses1 and the more conspicuous flower-bearing forms1 from weed size to tree size4 ,many rocks owe their character to the work of large colonies of plants or animals1 for the living organisms are frequently the active agency which takes dissolved mineral substance from the solvent liquid and gets it back into solid form4 ,the liquid is1 of course1 the water in which the creatures live1 while the mineral substance often becomes a commodity required by a plant or animal in its mode of living4 ,mollusks have a way of using lime in the production of shells1 and many a bed of limestone consists almost entirely of this by-product of molluscan life4 ,tiny coral polyps build complicated and beautiful structures from the same mineral substance4 ,either intact or in broken condition1 these structures contribute in a large way to the making of limestones4 ,algae1 among the lowliest of plants1 have done extensive work along similar lines1 and numerous invertebrate animals could be named as important factors in the production of rocks4 ,many of the shells and other fabrications retain their peculiar patterns long after the extermination of their makers1 and a highly informative part of the fossil record is provided in this manner4 ,it is also by far the larger portion of the record1 for the earlier ages of prehistoric time failed to produce a vertebrate animal of any kind1 while the invertebrate record dates back to pre-,cambrian time4 ,,formations ,if in some part of ,north ,america there had been steady accumulation of sedimentary materials under constantly favorable conditions since the beginning of ,cambrian time1 the result would have been a deposit of sandstones1 claystones1 and limestones measuring nearly fifty miles from bottom to top4 ,these figures are based on actual production in ,north ,america where extensive measurements have been made in many localities4 ,when other parts of the world are as thoroughly investigated and older deposits included in the calculations1 the total thickness of such beds will probably be more than one hundred miles4 ,no single pile of rocks offering a complete cross section of the geological record has ever been produced1 but portions of the section are exposed to view on all the continents4 ,in order to carry on desirable investigations and make comparisons1 it has been necessary to divide this great composite section into small units which may be named in some way and placed definitely with relation to lower and higher1 or older and younger1 layers4 ,to serve this purpose there has been developed the idea of rock formations1 and here we have a word which is not defined readily1 even for the use of those who are familiar with it4 ,nevertheless it is used so commonly that some understanding of its meaning becomes desirable4 ,a formation may be regarded as an extensive rock mass1 variable1 in thickness and other proportions1 as well as in composition1 but representing a period of time during which there was no great change in the character of plant and animal life1 and no serious interruption in the depositing of the rock-making materials4 ,occasionally the lower and upper limits of a formation are well defined and readily located4 ,frequently1 however1 the transition is gradual1 one formation merging into another with no apparent mark of separation4 ,in such event the original description serves to establish more or less definitely the boundaries of a formation4 ,descriptions are published whenever a worker believes he has discovered a significant part of the great section which has not previously been named4 ,the usual practice is to apply a name taken from the locality in which the beds were investigated1 and in this manner the names of formations become associated with towns1 rivers1 counties1 mountains1 states and other geographical features4 ,the locality which supplies the name is then regarded as the 8type locality0 for the formation1 but wherever these same beds may be traced or otherwise identified the one formation name applies4 ,illustration3 ,dinosaur ,tracks ,an ancient trail in sandstone of the ,dakota formation4 ,east slope of the hogback1 west of ,denver4 ,the 8,dakota formation10 to use a convenient illustration1 is mentioned in scores of reports bearing on the geology of ,colorado1 ,iowa1 ,kansas1 ,nebraska1 ,new ,mexico1 ,texas1 ,utah1 and ,wyoming1 as well as the ,dakotas4 ,on the geological map of ,colorado it appears on both sides of the ,rockies1 scattered in strips and patches from north to south boundary lines4 ,the beds are easily located in the foothills district west of ,denver because of their tendency to produce the prominent ridges known as hogbacks4 ,many formations are exposed over much less territory1 some have even greater extent4 ,thickness may vary from a few inches to thousands of feet1 and no two exposures will be exactly alike though some similarity necessarily prevails throughout4 8,exposures0 are simply portions of the beds which are not concealed by loose rock1 soil and vegetation1 or overlying formations4 ,canyon walls1 steep cliffs and mountain slopes1 gullies1 and badlands provide a large variety of natural exposures4 ,in such places rocks and fossils may be studied to best advantage4 ,since a formation may contain a variety of beds1 including sandstones1 shales1 limestones1 and all sorts of mixtures1 there is sometimes need of subdividing it2 but formations are the smallest units commonly shown on geological maps4 ,they are actual rocks which fit into a historical scheme of things and may be regarded aptly as the pages of a book which nature has done in stone4 ,,geological ,,time 8,how old are they80 8,how can you learn their names from the rocks80 ,these are typical examples of questions most frequently asked concerning fossils4 ,the second question follows the usual reply to the first1 for prehistoric plants and animals are as old as the rocks in which they are found4 ,the answer1 as to age1 must come from the rocks and what we have learned about them through many years of hard work1 thoughtful observation1 and careful study4 ,names1 however1 come from a different source4 ,nature1 apparently1 managed for a long time to carry on without the use of words4 ,since man began talking he has had no trouble inventing names for things which interest him4 ,early students of rocks and fossils likewise accomplished a great deal without being able to date events in terms of years although many of their efforts and interests centered on the problem of discovering a continuous sequence of events in the fragments of evidence that had been uncovered4 ,this relatively simple problem has not been fully worked out1 and some of the breaks in the record are recognized as 8time gaps0 which may never be converted into history4 ,the question of time1 expressed in years1 has been a puzzle which attracted some attention even in the earliest days of investigation4 ,its solution was attempted by several methods long before there was sufficient information to make them work satisfactorily1 which accounts in part for the extreme variation in results of the calculations4 ,even now it is to be expected that changes will have to be made as long as pertinent studies are continued4 ,two of the most promising methods of investigation in late years have been producing figures which are surprisingly large4 ,more accuracy than ever before is probably present in modern estimates but1 except for comparatively recent time1 there is yet no way of knowing within a range of millions of years when a creature lived4 ,astronomy and physics were used in early calculations but1 although taken seriously by some geologists1 it was soon recognized by others that certain events revealed by earth history could not be explained with so short a time allowance as these methods indicated4 ,one of the first estimates provided a total of only twenty-five millions of years and included a great stretch of time during which the earth1 according to prevailing theory1 was more sun-like than rock-like1 a time when planets were being born and the earth could not have been in its present physical condition1 which is the chief concern of the geologist4 ,since those earlier conditions could not have supported life as we know it1 our knowledge of cosmic history renders small service in the study of fossils4 ,among the methods suggested by astronomy and the laws of physics is one which is based on the probable rate at which the earth cooled from its molten condition to present temperature4 ,it is believed now that the heat of the earth is not necessarily due to an original molten state and that a steady rate of cooling cannot be ascertained4 ,any figures based on such procedure1 therefore1 are discredited today4 ,the amount of salt in the oceans1 and the time required for its concentration there by natural processes1 offers another way of attacking the problem4 ,it is a well known fact that salt is being added to the seas at a fairly constant rate2 sea water1 then1 must become saltier from year to year4 ,the salt comes from rocks exposed on land surfaces and is transported by the rivers which drain these areas4 ,by analyzing the river waters it is possible to estimate the amount of salt annually dumped into the oceans and1 also by chemical analysis1 it is a comparatively simple matter to figure the total amount now present in the oceans4 ,some recent calculations indicate that thirty-five million tons of salt are being added each year1 and this figure divided into the total amount for all the years places the age of the oceans at three hundred sixty millions of years4 ,however1 there are certain other factors which complicate the problem4 ,for instance1 it is known that land areas exposed to surface drainage have not always been of their present size1 and the annual production of salt by the different types of rocks exposed at various times in the history of the earth has not always been as it is now4 ,the rocks also must be older than the oceans1 but how much older cannot be determined by means of figures obtained in this way4 ,until the beginning of this century there was little anticipation of a better measuring stick than one in use at the time which placed its reliance on the total thickness of the sedimentary deposits and the length of time required to produce this great accumulation of material which is known as the geological column4 ,since the total thickness1 or height of the column1 was not accurately known1 and with recognized time gaps to bridge1 there was little hope of working out a complete chronology by this device1 but it has supplied highly desirable and reliable information concerning parts of the record4 ,the system has been somewhat improved since its earliest use1 and one of its latest applications gives us an age1 for known sedimentary rocks1 of nearly half a billion years1 this being based on a total thickness of one hundred miles and an average rate of #hhj years for the building up of one foot of sediments4 ,its greatest weakness is due to the absence of a reliable factor to take care of long stretches of time in which the sedimentary rocks are known to have been subjected to destructive processes4 ,a yardstick of this character cannot be applied to rocks that have been destroyed1 and there are excellent reasons for believing that these interruptions may account for several times the lapse of years indicated by the amount of rock remaining in the column which has been pieced together4 ,following the discovery of radium1 however1 the present century provided a new field of knowledge which has contributed greatly to the measurement of geologic time4 ,the penetrating rays produced by radium and other radioactive substances are due to extremely slow but violent disintegration of the material4 ,uranium and thorium are radioactive elements which occur in the rocks of many parts of the world4 ,there is little or no loss of material as the so-called disintegration proceeds2 instead there is a complicated series of transformations in which other elements are produced1 radium itself being one of these4 ,helium and lead eventually take the place of the less stable elements and the known rate at which these products accumulate provides the highly desired key to the age of the rocks4 ,part of the gas1 helium1 may escape1 but except in rare instances where chemical alteration might occur1 there probably is no loss of lead4 ,fortunately1 when this metal is produced by radioactivity it differs slightly in atomic weight from ordinary lead2 otherwise the presence of the latter would introduce a misleading factor4 ,since the speed at which the change goes on cannot be increased or decreased1 it is assumed that throughout past ages it has never been faster or slower4 ,the amount of such change that has been completed in any body of radioactive minerals may be measured by techniques employed in physics and chemistry4 ,if it is found that the amount of helium or lead present requires a hundred million years for its production at the working speed of the parent elements1 the mineral deposit must be at least that old4 ,certain conditions of course complicate the problem seriously3 knowing the age of a piece of rock which happens to contain some radioactive element is of small service in historical studies unless the rock can be definitely associated with a flora or fauna1 or some outstanding event disclosed by geological investigations4 ,but there have been a few instances in which most of the necessary conditions were present1 and more and better opportunities to apply this method will no doubt appear4 ,other elements1 or their radioactive isotopes1 are already being employed with good results4 ,some of these1 such as carbon #ad1 are more sensitive indicators for the accurate dating of events in comparatively recent time4 ,when it can be used1 this type of measurement is far less subject to uncertainties than any other4 ,it promises to eliminate all need for guessing1 and comes close to a degree of accuracy which is satisfactory to the scientist1 a person who thoroughly dislikes uncertainties of any kind4 ,if suitable material can be found in just the right places it should accomplish what the preceding method cannot do--the accurate measurement of the great time breaks which interrupt the geological record in many places4 ,something along this line already has been accomplished1 for radioactive material has been found in some of the oldest of the rocks4 ,regardless of the destruction going on in other localities1 these rocks have continued to register the passing of time1 and a tremendous antiquity for the earth and some of its first inhabitants has been indicated4 ,tests made on radioactive minerals from ,gilpin ,county1 ,colorado1 have established the age of late ,cretaceous or early ,cenozoic rocks at sixty million years1 providing a convenient and reasonably accurate date for the beginning of the ,age of ,mammals4 ,in ,russia1 one of the oldest mineral deposits yet studied in this way and regarded as early ,pre-,cambrian1 produced the astonishing figure of #a1hej1jjj1jjj years2 what we commonly refer to as geological history may therefore be regarded as covering a range of approximately two billions of years4 ,the earth1 in some form or other1 has in all probability passed through an earlier history of another billion years or more4 ,wherever we may roam1 a portion of the prehistoric record is to be found in the rocks underfoot and not far from the surface4 ,formations as already mentioned may be regarded as the pages--often torn and badly scattered--of nature's own book1 in which the geological periods are chapters4 ,but instead of numbering these pages and chapters we have named them1 in order to get the parts reassembled in orderly fashion and restored to a condition which makes the book legible4 ,however1 the names cannot render the service intended except in connection with a time chart and an outline of earth history4 ,illustration3 ,,geological ,,time ,figures to the left denote millions of years that have elapsed up to recent time ,,cenozoic ,age of ,man ,,recent ,man and his ,culture #a ,,pleistocene ,last of ,mammoths ,mastodons ,age of ,mammals #g ,,pliocene ,horses modernized #bj ,,miocene ,grasses and ,grazing ,animals ,three-toed ,horses1 ,rhinos1 ,camels #ce ,,oligocene ,specialization of ,primitive ,ancestors #fj ,,eocene ,decline of archaic types ,mammals flourishing ,,mesozoic ,age of ,reptiles #abe ,,cretaceous ,last of ,great ,reptiles ,specialization of ,dinosaurs #afj ,,jurassic ,bony ,fishes thriving ,flowering plants advance ,cycads ,birds and ,flying ,reptiles #bjj ,,triassic ,few small mammals of lower orders ,dinosaurs become prominent ,,paleozoic ,age of ,amphibians #bbe ,,permian ,reptiles advancing ,amphibians dominant insects #cjj ,,carboniferous ,dense forests of spore-bearing plants ,age of ,fishes #cej ,,devonian ,shark-like ,fishes ,land floras established #cge ,,silurian ,first land animals 7scorpions7 ,armored ,fishes prominent ,age of ,invertebrates #dbe ,,ordovician ,corals and ,bryozoa ,progress among ,mollusks #ejj ,,cambrian ,brachiopods gaining ,trilobites dominant ,advance of shelled animals ,,proterozoic ,,earliest ,,life #ajjj ,,upper ,,pre-,,cambrian ,small marine invertebrates ,lowest ,forms of ,plant and ,animal ,life ,few ,fossils ,,archeozoic #bjjj ,,lower ,,pre-,,cambrian ,some chemical evidence of life ,no fossils ,such aids have been devised and revised from time to time4 ,no figures have been offered as final or absolutely 8right0 since the beginning of scientific investigations4 ,time divisions have been proposed that are not yet in common use while others have been abandoned or modified4 ,sources of information are so numerous that appropriate credit cannot be given fairly for anything that is up-to-date4 ,the combined chart and outline here provided is based on time calculations of recent date but with figures slightly rounded off for the sole purpose of making them easier to remember4 ,in view of the still existent probability of error it is felt that the slight alteration of figures may justify itself4 ,it need not be regarded as misleading if the present purpose be considered--the stimulation of a natural history interest which is not vitally concerned with the little difference between a thousand million years and nine hundred ninety-nine million years4 ,,explanation ,,of ,,the ,,time ,,chart ,the whole of geological time has been divided and subdivided according to varying practices4 ,the development of life is perhaps the one outstanding feature of the time divisions1 but for the most part the changes in floras and faunas have been gradual rather than abrupt1 and this makes it very difficult to draw sharp lines or to visualize beginnings and endings of the various stages of development4 ,occasionally there is good excuse for drawing a line1 where the record is broken and resumed again after a long lapse of time4 ,the principal cause of such breaks is the elevation of great land masses1 which brings on an interval of erosion and surface destruction for the areas uplifted4 ,these movements of parts of the earth's crust have been exceptionally pronounced at certain times1 often culminating in the production of mountain systems1 and because of the extreme changes they introduce are known as revolutions4 ,the major divisions of prehistoric time have been established1 at least in part1 by such revolutions2 crustal1 climatic1 or other disturbances1 on a smaller scale and recurring with greater frequency1 may be regarded as establishing boundaries for the minor divisions4 ,hence we have five great ,eras of geological history1 and these are divided again into ,periods4 ,the time chart shows an arrangement commonly used in ,america4 ,in the first column the names of the ,eras are stated in technical form4 ,closely coinciding with these terms are the popular names of the ,ages which appear in the second column4 ,these names1 describing the dominant life of each age1 are very convenient4 ,the more scientific terms used for the eras1 while serving essentially the same purpose1 are a little more systematic and generalized in that they refer to ancient life 7,paleozoic71 middle life 7,mesozoic71 and recent life 7,cenozoic71 without being specific as to any class of animals or plants for any one division of time4 ,the period names1 in the central column1 have been derived from miscellaneous sources1 some of them from geographical districts1 some from descriptive references to prominent features of the rocks1 others indicating a degree of approach to recent time4 ,in paleontology 7fossil study7 it has long been a practice to cut the periods into lower1 middle1 and upper divisions1 and in a few cases it has been found desirable to make two periods out of an old one4 ,what was once known as the ,lower ,carboniferous is now commonly recognized as the ,mississippian period while the upper portion has become the ,pennsylvanian4 ,the ,lower ,cretaceous is now the ,comanchean of some authors4 ,both old and new practices are responsible for a little confusion at the present time4 ,a former division into ,primary1 ,secondary1 ,tertiary1 and ,quaternary eras has been partly abandoned1 but the term 8,quaternary0 still applies to the ,age of ,man1 while 8,tertiary time0 remains in good usage for the balance of the ,cenozoic era4 ,among the newer introductions may be mentioned the use of a ,paleocene period which precedes the ,eocene4 ,geologists are not entirely in agreement as to the necessity for this addition and many would not give it equivalent rank with other periods4 ,in the interest of simplicity these modern refinements have been omitted from the chart4 ,the figures appearing in the third column1 between the ,ages and ,periods1 indicate the millions of years that have elapsed up to present time4 ,they denote the age of the rocks at the beginning of each period4 ,the age of a plant or animal which lived in ,eocene time would be1 according to this scale1 somewhere between #ce million and #fj million years4 ,in practice it is usually possible to determine whether a fossil was embedded in the rocks during an early or late portion of the period1 and thus its age may be established within a shorter range1 but it is impossible to be exact1 even in terms of millions of years1 with regard to anything as far back in prehistory as the ,eocene period4 ,the period in which we are living today is known as ,recent4 ,it began at the close of the ,ice ,age or ,pleistocene period about ten thousand years ago and represents so little of earth history since the beginning of life that a chart many times the length of this page would be required to show the rest of the periods in proportion4 ,the ,cambrian period is an early chapter in which the story of prehistoric life suddenly becomes clear and richly varied4 ,it is1 however1 much farther from the beginning of the record than it is from the present1 and the ,pre-,cambrian eras would require a great deal more space in order to show their relative lengths4 ,the ,archeozoic and ,proterozoic eras have to some extent been divided into periods1 but the great antiquity of the rocks has obscured much of their history1 and divisions established for one locality have been of little service elsewhere4 ,consequently1 the period names are in less general use and the common practice is to refer to all this great stretch of time as ,pre-,cambrian4 ,in the last column1 at the right of the chart1 some of the historical features are indicated4 ,this column should be read from bottom to top in order to get the proper development of the story1 and at best this sketchy outline of events can be no more than suggestive of the progress and decline through which the earth's inhabitants have passed4 ,rocks of every period except probably the ,silurian are known to have been deposited somewhere in the ,colorado area1 although in most cases the record for each period is far from complete4 ,formations are too numerous and too varied locally to be shown on a chart of this type4 ,,the ,,geological ,,section ,in the study of fossils there are two important field aids usually available4 ,for any locality there should be a geological map and a section showing the sequence and character of the strata4 ,on a small-scale map many of the local details have to be omitted1 but the position of the larger exposures is indicated and1 with this information at hand1 the fossil-bearing strata may be located with the help of a geological section4 ,the latter is frequently obtained from technical reports published by ,state and ,national ,geological ,surveys4 ,frequently1 however1 it is possible to obtain only a general plan for a given locality1 and a great deal of literature may have to be scanned in order to get that4 ,excellent geological maps of ,colorado have been published by the ,colorado ,geological ,survey and the ,united ,states ,geological ,survey4 ,it often happens that a formation is not where we expect to find it1 this being due to several possible factors4 ,the sediments may not have been deposited there1 or they may have been removed by erosion4 ,where the structure has been disturbed by folding and faulting1 a multitude of complications is introduced4 ,the expected sequence is sometimes inverted and repeated through a series of folds4 ,formations also may be moved miles out of place by faulting4 ,both thickness and character of sediments may vary considerably within a formation4 ,in some regions the geology is very simple1 in others extremely difficult to understand4 ,illustration3 ,,formations of the ,,denver-,,foothills ,,region ,a ,,generalized ,,section ,,showing ,,some ,,of ,,the ,,surface ,,features ,,red ,,beds ,,hogbacks ,,table ,,mountain ,,denver ,,district ,,region ,,of ,,mountain-,,making ,,uplift ,formations bordering the mountains have been bent into upright positions4 ,,periods ,,recent ,,pleistocene ,,pliocene ,,miocene ,,oligocene ,,eocene ,,cretaceous ,,thickness ,,soft ,,sandstones ,,grits ,,clays ,,denver ,,arapahoe #bjjj ft4 ,,sandstones1 ,,shales ,,lignite ,,laramie #ajjj ft4 ,,yellowish ,,sands ,,shales ,,fox ,,hills #ajjj ft4 ,,soft ,,dark ,,gray ,,or ,,rusty ,,shale ,,pierre #ejjj ft4 ,,limestones ,,shales ,,niobrara #ejj ft4 ,,dark ,,shales ,,lime ,,benton #djj ft4 ,,gray ,,or ,,buff ,,sandstones ,,clays ,,dakota #cjj ft4 ,,shales1 ,,sandstone ,,lime ,,morrison #bjj ft4 ,,jurassic ,,triassic ,,permian ,,deep-,,red ,,sandy ,,shales1 ,,lime1 ,,gypsum ,,lykins #gjj ft4 ,,carboniferous ,,massive ,,pink ,,or ,,white ,,sandstone ,,lyons #bjj ft4 ,,red ,,or ,,brown ,,sandstone ,,fountain #aejj ft4 ,,conglomerate ,,devonian ,,silurian ,,ordovician ,,cambrian ,,pre-,,cambrian ,,metamorphic ,,intruded ,,rocks ,,idaho ,,springs ,,schist1 ,,gneiss1 ,,quartzite 7,,part7 ,,basement ,,rocks of ,,igneous ,,origin ,a generalized section for the western part of the ,denver ,basin is introduced here for the use of local students4 ,the formations normally present in this region are shown in their usual position4 ,they are briefly described on the chart1 and their thickness is indicated by figures which may be regarded as near the maximum for the district4 ,the section will apply to most of the foothills area between ,morrison and ,boulder though surface features and thickness of beds will vary considerably from place to place4 ,certain of the formations are known to be fossil bearing1 others barren or nearly so4 ,when fossils are present they are usually restricted to certain localities1 and these may be widely scattered4 ,the following remarks apply to the possibilities for finding fossils in the formations named4 ,denver and ,arapahoe4 ,leaf impressions of palms1 ferns1 and numerous species of well-known trees and shrubs are common in many localities4 ,petrified wood is fairly abundant1 and a few scattered bones of reptiles and mammals have been found4 ,the two formations are treated as a unit because the ,arapahoe is neither conspicuous nor sharply defined4 ,denver beds are well exposed on the slopes of ,table ,mountain at ,golden2 fossils1 however1 have been obtained from several localities nearer the city of ,denver1 notably from the hills just west of ,overland ,park4 ,laramie4 ,plant material is locally abundant1 principally the leaves of familiar deciduous trees1 palms1 and ferns4 ,many of the clay pits being worked near ,golden are in this formation4 ,oysters and a few other mollusks may be found in some places4 ,fox ,hills4 ,better exposures of this formation are located to the north of ,denver4 ,marine mollusks are most frequently found4 ,pierre4 ,in addition to the characteristic dark shales1 this formation includes some limy material and sandstone beds1 both of which are fossiliferous in places4 ,two types of marine mollusks are characteristic3 ,inoceramus1 generic name for several species of clam-like bivalves readily identified by concentric elevations which produce a rippled effect on the shell surfaces2 and ,baculites1 cephalopods with straight1 chambered shells which often break at the suture lines1 where the fossil is weakened by the chamber walls4 ,small oyster shells are fairly common also4 ,the formation is to be found some distance to the east of the prominent hogback where it weathers into smooth surfaces in the form of broad valleys and flats1 with rounded contours on the few elevations that may be present4 ,it forms a soft1 flaky soil when dry1 is a sticky 8gumbo0 when wet4 ,the clay is generally of a rather dark grayish color when freshly exposed but it takes on a rusty appearance after weathering4 ,at various levels there are numerous iron-cemented concretions1 many of which contain fossil shells4 ,illustration3 ,marine ,beds of the ,benton ,formation1 ,northeastern ,colorado ,a stream channel has cut deeply into the formation1 uncovering and partly destroying a plesiosaur skeleton which was found at the level where the men are standing4 ,illustration3 ,plesiosaur ,bones in ,place ,benton formation4 ,surface rubble has been cleared away1 and several vertebrae are partially uncovered in the area at the right of the hammer4 ,niobrara4 ,the formation contains fossils rather similar to those of the ,pierre4 ,shark's teeth have been found in some of the lower beds4 ,limestone is a prominent feature1 often forming a well defined ridge near the foot of the eastern slope of the main hogback4 ,the limestones commonly have a chalky character4 ,benton4 ,the formation is not especially productive in this region4 ,marine shells are numerous in some localities1 and bones of marine reptiles have been found at various places4 ,as usually seen1 it is almost entirely composed of impure clay shales1 very dark1 brownish-gray to almost black1 and commonly interbedded with thin patches of white bentonite1 yellow ochre1 gypsum1 and limestone4 ,dakota4 ,this formation produces the high hogback which is usually present some distance east of the ,red ,rocks4 ,there are generally two or three layers of massive1 light-colored sandstone separated by clays which are used extensively in the making of bricks and pottery4 ,leaf impressions and some fish scales are found in the clays and occasionally in the sandstone4 ,the hogback is a good marker from which to locate other formations1 because of its prominence in the foothills landscape4 ,morrison4 ,good dinosaur material has been taken from the ,canon ,city and ,morrison districts4 ,the formation is to be found on the lower west slope of the ,dakota hogback4 ,it consists of continental deposits of the stream and lake types4 ,there is considerable sandstone in this formation and a little limestone is to be found here and there1 but the most characteristic feature is in the shales4 ,when freshly exposed1 the shales are delicately tinted with gray1 green1 and maroon1 a bronze-green being rather prominent4 ,this formation is highly variable in character1 with much of the clay often buried under the valley floor4 ,in addition to the bones of reptiles1 there are plant fossils1 usually of poor quality1 and fresh-water gastropods more or less abundant in some localities4 ,lykins4 ,outcrops are not prominent1 owing to the small amount of weather-resisting materials4 ,the sandy clays are commonly of a deep red color mottled with spots of light gray4 ,a white limestone is sometimes present near the middle of these deposits1 and gypsum beds are included locally4 ,the formation is often indicated only by red soil in the depressions between ridges4 ,few fossils have been reported4 ,lyons4 ,this formation is usually prominent as the eastern wall of the uplifted ,red ,rocks series4 ,in some localities it forms a ridge of pink or white sandstone distinctly separated from the older sediments to the west4 ,very few fossils are found4 ,fountain4 ,exposures usually are brown to red in color1 though sometimes a dirty white4 ,the prominent rocks are rather coarse sandstone1 commonly with a gritty texture due to the angular character of the sand or gravel from which they were made4 ,these are the westernmost of the ,red ,beds and the oldest of the uplifted sedimentary rocks bordering the foothills in most of our area4 ,fossils have been found in the formation1 but it is practically barren for the territory here considered4 99 99 99 99 99 99 99 99 ,this geological section also illustrates a method of dating crustal movements and the birth of mountain ranges1 for the folding of the strata along the flanks of the ,rocky ,mountains has a great deal of significance in this connection4 ,the sedimentary layers were originally deposited over much of the present mountain area in a horizontal position1 and only those formations in existence at the time could be distorted by the upheavals which produced the new elevations4 ,of the series generally involved in the movement the ,laramie beds are the youngest4 ,since these beds had not been formed until near the close of the ,cretaceous period it is to be assumed that the mountains must be of more recent date1 younger than the topmost of the deformed beds and at least as old as the lowermost of the undisturbed formations overlying them4 ,some disturbance is evident also in the ,arapahoe and ,denver beds which overlie the ,laramie1 but this is believed to have occurred sometime after the occasion of the first great uplift4 ,volcanic materials in these beds lead to the belief that the sediments were deposited during a period of volcanic activity brought on by the crustal folding which terminated the ,mesozoic era4 ,hence the conclusion arises that the age of the ,denver and ,arapahoe beds must coincide closely with some of the earlier stages in the history of the mountain system4 ,this interval is often referred to as ,post-,laramie time4 ,,before ,,the ,,age ,,of ,,reptiles ,,the ,,pre-,,cambrian ,,complex ,the rocks of ,pre-,cambrian time have been buried deeply under the accumulation of younger sediments1 and the resulting pressure in many places has been tremendous4 ,in addition to the effects of pressure there also is recorded in these ancient formations the repeated movements of the materials since they were first deposited4 ,vertical and side adjustments of parts1 with relation to other parts1 have distorted the original arrangement of the rock particles to such an extent that ordinary fossils would eventually become unrecognizable4 ,these crushing1 grinding1 and kneading forces working through millions of years alone would account for the absence of fossils from the older deposits4 ,frequently the rocks have become so changed in form that their original character can only be conjectured1 and because of this change they are known as metamorphic rocks4 ,a few beds of ,archeozoic age remain in nearly their original condition1 but they are either without fossils or they have produced very questionable and unsatisfactory specimens4 ,the existence of life during these early stages of earth history is indicated largely by chemical rather than fossil evidence4 ,much of the ancient limestone has been converted into marble1 but it is not unreasonable to believe that plants and animals were instrumental in the production of this type of rock as they are today4 ,certain varieties of iron ore deposits are now being built up by the aid of plants1 and similar ores in the ancient rocks may have had a like origin4 ,the presence of great quantities of carbon1 in the form of graphite1 may be regarded also as a sign of life1 for this substance is accumulated on a large scale by living plants1 and may be retained in a solid form after the partial decay of the plant tissues4 ,so far as the direct evidence goes1 there is no sign of any creature of large size or of such complicated structure as the common plants and animals of today4 ,the chemistry of the mineral deposits is not entirely convincing as to the presence of life1 but it is regarded as highly probable that microscopic1 single-celled plants and animals1 comparable to modern algae and protozoa1 were in existence during ,archean time4 ,throughout later eras there is unmistakable evidence of gradual development from simpler to more elaborate life-forms and the ,archeozoic is commonly regarded as a time of preparation during which simple organisms of some kind were becoming adapted to early conditions which could not support life on a higher plane4 ,the importance of the work done by such lowly creatures in the preparation of suitable environments for more advanced modes of living is overlooked almost entirely4 ,during the next era1 the ,proterozoic1 the record of life becomes somewhat clearer4 ,fossils are hardly to be regarded as abundant but there were several well-defined types of animals which left shells and other parts composed of mineral matter4 ,among these may be mentioned the ,radiolaria1 ,foraminifera1 ,bryozoa1 and ,sponges4 ,radiolaria produced delicate1 often lace-like shells of many patterns adorned with the radiating filaments or spines which have suggested the name for this group4 ,foraminifera produced minute shells1 sometimes many chambered1 and often bearing a confusing resemblance to the work of snails4 ,common chalk is composed almost entirely of such shells and fragments of them4 ,sponges and ,bryozoa are animals of slightly higher organization4 ,they are many-celled instead of one-celled and the cells have special work to perform1 which is a most important step in the direction of the specialization which characterizes the structural and life pattern of later arrivals4 ,the ,bryozoa lived in moss-like colonies which have been important rock-makers2 the fossil forms bear some resemblance to corals4 ,sponges are too well known to require description although the familiar article of commerce is merely the framework of once-living animals4 ,they represent the earliest organization of true animal bodies even though in appearance they may have a resemblance to plants4 ,actual plants of this era were of the algae class1 aquatic in habit as were their animal neighbors1 the first to leave a record in the form of fossils4 ,this record1 obscure and distorted1 has long been a source of perplexity to investigators4 ,without well-defined floras and faunas to guide them1 and with rocks frequently in chaotic relationships1 early geologists were content to regard it all as a 8,pre-,cambrian complex40 ,recent studies have contributed a great deal of information not available some years ago4 ,it is quite possible that more advanced types of life were in abundance before the close of the second era1 but material on which to base sound opinion is still scarce4 ,rocks of ,pre-,cambrian age are plentiful in the foothills region west of ,denver4 ,the schists1 gneisses1 and quartzites exposed for some miles immediately beyond the red-beds are part of this great complex4 ,the ,idaho ,springs formation is known to be one of the oldest in this district1 although its exact age has not been determined4 ,other formations are recognized among the metamorphic rocks of the region but none has contributed to our knowledge of early life4 ,,cambrian ,,life ,there can be no mistake as to the prolific development of life in ,cambrian seas1 for fossils of this age are to be found in many parts of the world1 where ancient sea bottoms now form part of the land surface4 ,invertebrate animals appear to have made much progress1 but plants were either scarce or too small and delicate to be productive of fossils4 ,it is probable1 however1 that seaweeds and other algae were flourishing along with the invertebrates1 because animal life is directly or indirectly dependent on the existence of plants4 ,the latter sustain themselves by taking carbon and nitrogen from air1 water1 and soil1 but animals must obtain their requirements by eating plants or eating each other4 ,they cannot obtain what they need from the inorganic world without this help from the vegetable kingdom4 ,one group of animals stands out prominently above all its contemporaries4 ,known as the trilobites they were by far the most distinguished and most characteristic of ,cambrian invertebrates4 ,trilobites inhabited the warmer seas of this period and several later ones1 but were extinct by the end of the ,paleozoic era4 ,hundreds of species have been described1 most of them under four inches in length4 ,well-known distant relatives now living are the shrimps1 and other crustaceans4 ,the name ,trilobite has reference to the three lobes which are apparent in the form of the upper surface1 the central lobe forming a broad ridge extending along the back4 ,beneath the outer lobes on each side there was1 during life1 a row of short1 jointed legs used for swimming and walking1 but these delicate appendages are seldom preserved in the fossils4 ,second in importance among the animals of the period were the brachiopods or lamp-shells1 not true mollusks although they were provided with similar shells composed of calcium phosphate or calcium carbonate4 ,shells are of two parts 7bivalved7 as in the case of clams1 but the valves are above and beneath the body instead of on the right and left sides1 which is the arrangement among mollusks4 ,although abundant as individuals1 there were only a few species during the earlier part of the period2 the number of species increased1 however1 and the race became very persistent4 ,about seven thousand species have been described1 and the race is not yet extinct although the number of living species is relatively small4 ,cambrian life evidently included representatives of all the great divisions of invertebrates2 sponges1 jelly-fishes1 worms1 and primitive corals have been reported4 ,at the end of the period there was an elaborate molluscan fauna4 ,the closing of the period in ,north ,america was apparently a gentle elevation of continental areas and a consequent withdrawal of the sea4 ,illustration3 ,invertebrate ,fossils ,only a few prominent types have been selected from thousands of invertebrates known to zoologists4 ,the forms illustrated are of frequent occurrence as fossils4 ,,crinoids ,,cephalopods ,coiled types ,ammonite ,scaphite ,straight-shell type ,baculite ,,trilobite ,,brachiopods ,,bivalves ,inoceramus ,oyster ,,gastropods ,snail-like ,univalves ,,protozoa ,,unicellular ,,forms ,radiolaria 7,microscopic7 ,fusulina limestone ,foraminifera 7,enlarged7 ,,multicellular ,,forms ,cup coral ,reef coral ,sponge ,bryozoa ,,the ,,ordovician ,,record ,extensive land areas must have subsided again early in the ,ordovician period for marine sediments were laid down over a large part of the ,north ,american interior1 and three epochs or subdivisions of the period have been based on as many invasions of the sea4 ,in these ancient deposits the record of life continues to show new forms4 ,nothing of a very spectacular sort is recorded other than a great increase in the number of species among types that were established in earlier periods4 ,trilobites were at their best1 brachiopods continued to flourish1 and the mollusks made new progress1 especially the cephalopods1 a group which includes our cuttle fishes and squids4 ,some of these predatory creatures attained large size and were no doubt masters of the sea4 ,typical forms were provided with tapering chambered shells that occasionally reached a length of twelve or more feet4 ,most of the shells were straight and trumpet-like or but slightly curved4 ,some were closely coiled and in this respect more like the well-known nautilus now in existence4 ,the bryozoans became very common in the later part of the period and corals made slight advances4 ,somewhat of a novelty at this time were the crinoids1 commonly known as 8stone lilies0 although not plants at all4 ,they have been described as starfishes with back turned downward and a thick stem attached beneath4 ,where they lived in great abundance the limestone deposits may consist almost entirely of their stems4 ,crinoids continued to produce a variety of forms throughout several of the succeeding periods4 ,the brachiopods were commonest of all animals representing this period1 however1 and their wide distribution1 together with their known preference for warm waters1 is taken to be an indication of mild temperatures prevailing over a large portion of the earth4 ,land plants are indicated by spore-bearing forms related to the ferns and mosses4 ,impressions of such plants have been found in ,europe but1 since most of the known rocks of this age were formed in seas1 the marine algae are more abundant as fossils4 ,in the ,colorado area1 rocks of ,ordovician time are exposed only in mountainous areas where they have been lifted high above their original levels4 ,they are not especially rich in fossils although they have produced some fish remains which are of interest in that they suggest an age of vertebrates which is just ahead4 ,,silurian ,,events ,since land floras and faunas had not yet become conspicuous the fossil record for this period is limited to those areas which were invaded by the sea4 ,apparently there was no such invasion of the present ,colorado region1 for rocks of this age are not in evidence4 ,if they exist at all they are restricted to localized districts which are deeply buried under sediments of later periods4 ,there may have been no ,silurian deposition in this area1 or such rocks may have been produced only to be destroyed by elevation and consequent subjection to weathering and erosion during a long interval of time1 in which they were completely removed4 ,in the region of the ,colorado ,rockies there is no evidence of returning seas until late ,devonian time4 ,in other parts of the world1 however1 there was extensive deposition of rock-making sediments in seas which were inhabited by algae and invertebrates of the types previously described4 ,among the common animals of the time there were still numerous species of brachiopods1 trilobites1 corals1 crinoids1 and bryozoans4 ,in addition to the primitive cone-shaped1 cup corals there were several advanced types but the habit of building large reefs was not yet established4 8,sea scorpions10 really large crustaceans1 flourished during ,silurian time1 and late in the period there appeared a race of true scorpions which lived on dry land or between high and low tides along the seashore4 ,these were smaller and much like modern descendants but probably they did not wander far from the ocean shores where an abundance of food was available4 ,these little scorpions1 the largest measuring only two and a half inches in length1 are the oldest air-breathing land animals of the fossil record4 ,it was not until the period was well advanced that fishes became numerous1 and much of our knowledge of the beginning of an 8,age of ,fishes0 has been obtained from ,european fossils4 ,although fishes are classed with the vertebrate or backboned animals there are large groups which do not have bony skeletons but are provided instead with a simple framework of cartilage4 ,among the earlier and more primitive types were the ostracoderms or bony-skinned fishes with no internal bones and only a small amount of bony substance in the armor-like plates and scales which covered the forward portion of the body4 ,the ostracoderms comprise a small group of fishes about which very little is known4 ,they appear to have been inhabitants of fresh-water streams as well as lagoons bordering the seas1 and may have been related to the small sharks of the time4 ,they lived during the ,ordovician1 ,silurian1 and ,devonian periods1 and left no descendants now recognized among living creatures4 ,a much larger type of armored fishes is known as the arthrodires1 a name which refers to a pointed neck and an arrangement of the armor plates to permit a movement of the head4 ,these were the most ferocious fishes of the ,silurian and ,devonian seas1 some of them reaching a length of twenty feet though most were much smaller4 ,their jaws were provided with formidable shearing and crushing plates instead of teeth4 ,,devonian ,,progress ,the ,devonian is one of the most outstanding of all periods from the viewpoint of life development4 ,dominance of the fishes is its greatest achievement1 the invertebrates remaining about as they were and the higher vertebrates barely in evidence1 but life on a large scale was no longer confined to the seas4 ,fresh-water fishes became prominent and land plants well established4 ,the first forests appeared1 with fern-like plants predominating although woody trees of several types and considerable size were included4 ,it is quite possible that extensive land areas had been well supplied with vegetation during earlier times1 but the delicate tissues of plants are far less likely to be preserved than the limy parts of animals4 ,the fossil record1 therefore1 cannot be expected to reveal more than a suggestion of the progress made at this level of living4 ,the story of plant life becomes much clearer in the next period when conditions were more favorable for the production and preservation of plant fossils4 ,land animals of the time are almost unknown4 ,a few snails and scorpions have been found1 and some footprints made by early amphibians4 ,insects probably were in existence although the evidence is not quite clear on this point4 ,the increasing number of fresh-water fishes1 however1 may be regarded as a sure indication that inland conditions were becoming more favorable for plant and animal inhabitants of all kinds4 ,the extent of development among the fishes cannot be accurately indicated by naming a few types1 for it is mainly in the number of species and genera within the larger groups that progress is seen4 ,in general it may be stated that the fishes of the period had not yet acquired the bony skeleton and typical form of familiar modern species4 ,skeletons were of cartilage1 partly hardened in some instances by lime4 ,armor plates were customary with certain races but were not present among all fishes4 ,neither were these armored forms exceptionally large1 as compared with living sharks4 ,although occasional giants appeared1 the majority were small4 ,many were sluggish creatures with poorly-developed jaws1 living as scavengers on sea and stream bottoms4 ,tail fins were usually unbalanced as in the sharks1 or pointed and rounded rather than evenly forked4 ,illustration3 ,modernized ,types of ,fishes from ,eocene ,shales of ,southwestern ,wyoming ,the great tribe of true bony fishes1 such as the cod and perch1 which includes more than ninety percent of the fishes living today1 was not yet in existence4 ,about one-third of the many kinds of fishes then living were related to the sharks1 a group which is relatively insignificant in recent years4 ,nearly one-fourth of the total belonged to a tribe of enamel-scaled fishes1 now represented only by a few sturgeon and gar-pike4 ,lung fishes have never been a large group but it is noteworthy that they have had existence since ,middle ,devonian time4 ,living members of the race1 inhabitants of ,africa and ,south ,america1 make a practice of burrowing into the mud of stream channels during dry seasons and are provided with lungs which enable them to breathe air in the manner of higher vertebrates4 ,they survive the complete drying-up of the streams and live for months without water4 ,other forms1 with less development of lungs1 frequent stagnant pools and come to the surface occasionally for a breath of air4 ,all are provided with gills also1 which enables them to obtain their oxygen as other fishes do4 ,they are believed to be a connecting link between the fishes and the early amphibians4 ,more accurately1 perhaps1 they should be regarded as holding an intermediate position without being directly ancestral to any higher type of vertebrate animal4 ,still dominant among the invertebrates were the brachiopods1 on the whole averaging a little larger in size1 and otherwise indicating congenial times for that type of organism4 ,they reached the peak of their development during this period4 ,trilobites were declining although a few new and strangely ornamented varieties made a brief appearance4 ,crinoids apparently found living conditions less favorable during ,devonian time1 but in a later era they again became prominent4 ,corals were favored only at times and in certain localities4 ,along with the crinoids they appear to have suffered from the presence of an unusual amount of mud in the waters of their customary habitats4 ,both had a preference for clear water as indicated by the absence of fossils from limestones containing more than a very small percentage of muddy sediments4 ,crustaceans1 similar to the sea-scorpions and better known as eurypterids1 became prominent among fresh-water animals4 ,some were unusually large for creatures of this class1 lengths of several feet being recorded from fragments4 ,gastropod mollusks came into prominence in localities where living conditions were favorable4 ,bivalves continued to thrive but the cephalopods had a rather meager development considering the heights they were to achieve in subsequent periods4 ,in western ,north ,america the large expanse of territory known as the ,great ,plains was evidently well above sea level during this entire period1 for no beds of this age are found in eastern ,colorado4 ,west of the ,front ,range1 however1 there was some deposition of marine sediments during late ,devonian time4 ,formations of this age are exposed near ,salida and ,glenwood ,springs1 on the ,white ,river ,plateau1 and in the ,san ,juan region4 ,the ,carboniferous period gets its name from the vast deposits of coal which were developed during that time in many parts of the northern hemisphere4 ,depressed land surfaces bordering the continents1 and extending well into the interior of present boundaries1 supported dense growths of vegetation and provided the swampy conditions most favorable to coal production4 ,varieties of plants which are now of small size and lowly position in the botanical world acquired the proportions of large trees4 ,,carboniferous ,,forests ,best-known fossils of the period are carbonized portions of the larger trees1 and impressions left in the muds and sands of ancient bogs4 ,forest trees of several kinds reached the height of a hundred feet1 with a trunk diameter of two to six feet4 ,this size often is exceeded in modern forests1 but by trees of an entirely different type4 ,considering the amount of development among the plants of earlier periods1 ,carboniferous forests provide an outstanding spectacle of advancing life4 ,quite common among the larger trees were two varieties of club-mosses1 also known as scale trees4 ,they were cone-bearing evergreens with only slight resemblance to modern conifers4 ,instead of seeds they produced spores1 a method of reproduction which is practiced among ferns4 ,the trunks were marked from bottom to top with uniform patterns of cushions and scars indicating the points at which leaves were attached during the earlier stages of growth4 ,in the ,lepidodendrons the rows of scale-like cushions wind spirally upward while among the ,sigillaria there is a vertical arrangement of leaf-scars which resemble the imprints of a seal1 these impressions being in straight and parallel rows on a surface which may be either ribbed or smooth4 ,the leaves of scale trees were stiff and slender1 and arranged in grass-like tufts at the top4 ,calamites1 related to our horsetail rushes1 were somewhat smaller than the scale trees4 ,their trunks consisted of a thin1 woody cylinder with a pithy interior1 and were marked at intervals by nodes which gave them the 8jointed0 appearance of a bamboo stem4 ,leaves were arranged in circles around the nodes of main stem or branches4 ,spore-bearing cones appeared at the tips of the stems4 ,illustration3 ,prehistoric ,plants ,some of the larger and better known plants of past ages are shown as reconstructed by artists4 ,finer details of the reconstructions often have to be neglected because of uncertainties due to the scattered and fragmentary character of the fossil record4 ,,leaf ,,impressions ,carboniferous ,ferns ,strap-leaf ,conifer 7,cordaites7 ,,modernized ,,types ,sequoia ,cone and foliage ,miocene ,fossils 7,florissant ,shales7 ,maple ,willow ,eocene palm 7,denver ,beds7 ,,horse ,,tail ,,rushes ,restoration 7,calamites7 ,fossils ,leaves and ,stem ,,cycads ,restoration ,fossil ,trunk ,,club ,,mosses ,restoration 7,sigillaria7 ,fossils ,trunk ,impressions ,,scale ,,trees ,restoration 7,lepidodendron7 ,fossil ,leaf scars ,also included among the larger trees were the ,cordaites or large-leaved evergreens1 tall and slender1 seed-bearing but not true conifers as yet4 ,leaves were strap-shaped or grass-like1 the larger ones having a length of six feet and a width of six inches4 ,trunks were woody1 resembling pine1 but with a central pith4 ,the flowers were small and resembled catkins in form4 ,ferns and fernlike plants were so numerous that the period has been known as an age of ferns4 ,earlier knowledge of these forests was based on fossils of a fragmentary character from which an accurate association of parts could rarely be obtained4 ,a general relationship with the ferns was apparent1 but careful study of additional material has given us a rather different view of ,carboniferous plant life and we note a highly diversified array of forms with many suggestions of modern tendencies4 ,the flora as a whole may be regarded as highly specialized for the conditions which prevailed at the time and were not to continue through subsequent periods4 ,warm temperatures and abundant moisture were essential especially to spore-bearing types1 and the cold1 arid conditions of the next period put an end to many of the groups1 or greatly reduced their prominence4 ,this could be regarded equally well as an age of insects1 for some of these invertebrates acquired the greatest size they have ever had1 particularly the dragonflies with a wing-spread of more than two feet in one of the largest fossils so far discovered4 ,cockroaches numbering upwards of five hundred species have been named4 ,though large they are hardly to be regarded as giants1 lengths of three or four inches being about the limit4 ,some of the insect types of today quite evidently existed among the inhabitants of ,carboniferous forests1 but it is apparent that there were also some antiquated forms which may have descended from the trilobites4 ,although some authorities regard this as the period in which insects originated1 there are others who maintain that definite beginnings are not established so readily on present evidence4 ,spiders are believed to have made their appearance at this time4 ,four-footed vertebrates resembling salamanders were prominent animals of the ,carboniferous swamps4 ,at first adapted to a life in water and later to land conditions1 they are known as amphibians1 the name being based on the ability to live in two different kinds of environment4 ,common living representatives of this group are the toads and frogs1 but these tailless forms are not known among fossils of the ,paleozoic era and are almost unknown throughout the ,mesozoic4 ,the ,age of ,amphibians1 as we apply that phrase1 was definitely not an age of toads and frogs4 ,these primitive land animals were of different types1 ranging from much smaller sizes up to the length of a crocodile4 ,most of them had short legs1 and feet which were suitable for locomotion upon land1 but many of the creatures probably spent most of their lives in the water4 ,tails were usually high and flattened as if for swimming1 sometimes long1 at other times greatly shortened in proportion to the body4 ,heads were generally large1 jaws long1 and mouths wide4 ,before the close of the period true reptiles appear1 but this race of animals is destined to make a more spectacular advance than the amphibians and will be discussed in connection with ,mesozoic life4 ,the amphibians1 however1 are regarded as being the ancestors of the reptiles as well as the higher quadrupeds which follow them4 ,although living reptiles are readily distinguished from living amphibians there is a different situation with regard to these primitive forms1 for among the fossils it becomes increasingly difficult to separate the two groups as new material is investigated4 ,invertebrates had their ups and downs during the period4 ,trilobites became scarce1 and brachiopods for a time were the most abundant of the shelled animals but later declined rapidly4 ,in favorable localities the crinoids established a wonderful record for new species before the period had advanced very far4 ,hundreds of species of ,carboniferous invertebrates are known1 and in many of the rocks of the period they are the only fossils to be found1 for the vertebrates were still unable to venture far from the swampy districts1 and much inland territory was too well drained to support either the floras or faunas then existing4 ,in the ,colorado area there are both marine and continental formations but the great coal-making forests and their inhabitants were limited to other localities4 ,as a consequence this region is not famous for ,carboniferous fossils4 ,,permian ,,hardships ,for a time there was no great change in ,north ,america following the opening of the ,permian4 ,then began a series of mountain-making movements and continental uplifts which drained the swamps1 lakes1 and inland seas4 ,with the passing of the vegetation which had established itself in and around these areas much of the animal life followed4 ,it is probable that a considerable proportion of the marine life survived1 much more than is indicated by the fossil record1 but the receding seas carried the survivors into territory which is now inaccessible to fossil hunters4 ,after ,middle ,permian time the climate everywhere seems to have been cold and dry4 ,by the end of the period there had been accomplished more geographical change throughout the world than at any time since the beginning of the ,paleozoic era4 ,traces of the crustal movements which produced new mountain ranges can be followed in ,europe1 ,asia1 and ,north ,america4 ,the ,appalachian region was raised to a great height1 possibly in excess of three miles4 ,a major disturbance of this character is known among geologists as a revolution1 and to this particular one the name 8,appalachian ,revolution0 has been given4 ,the elevation of continents necessarily changes their coast lines4 ,this1 in turn1 influences ocean currents which have an important bearing on climatic conditions4 ,in addition to this1 the elevation of mountain systems not only rearranges the distribution of hot and cold winds over the land areas but it may produce barriers to the migrations of floras and faunas1 confining them to areas in which it is no longer possible to live4 ,when the effect upon plants and animals is considered1 it is easier to understand why a line is drawn across the geological time chart at such a point and an era of prehistory is regarded as closed4 ,during the ,permian period there was recorded in the rocks more widespread glacial action than ever before or since4 ,with less inland water to provide the necessary evaporation there was a marked decrease in rainfall1 and arid or semi-arid conditions replaced the hospitable climate that had been such an important factor in the prolific life of the ,carboniferous4 ,the struggle for existence became intense1 but hardier types of plants and animals1 with greater ability to adapt themselves to adverse conditions1 established themselves here and there1 as ancestral forms became extinct4 ,most of the large spore-bearing trees died out and seed-producing varieties began to acquire prominence1 among them the coniferous evergreens4 ,ferns1 however1 proved their adaptability by producing some new forms which became prominent in ,permian floras4 ,the prehistoric amphibians have been divided into three orders1 one of which includes all the larger forms4 ,this group1 known as the labyrinthodonts1 continued on through ,permian time but began to show backward tendencies1 with dwindling limbs and a return to life in the water4 ,among the larger land varieties are typical fossils ranging from about fifteen inches to five feet in length4 ,in outward appearance they differed from ,carboniferous amphibians4 ,one of the other orders1 including a great diversity of smaller forms1 became extinct during this period1 leaving no known descendants4 ,the third order is regarded as the oldest1 and probably the ancestral group from which the modern newts and salamanders originated4 ,the most successful of ,permian land animals were the peculiar reptiles that learned to live in drier regions4 ,like the horned toad and ,gila monster of our arid southwestern ,united ,states1 the larger ,permian reptiles were four-footed animals4 ,in size and shape they were not greatly different from amphibians then living4 ,an exception to this rule1 among some of them1 is the development of long1 bony spines above the vertebrae of the back4 ,a fairly common fossil of this type1 found in ,texas and known as ,dimetrodon1 had a total length of six feet1 about half of this being in the tail4 ,the tips of the spines adorning the back reached a height of three feet or more and there was probably a covering of skin over these bones1 which would produce a sail-like structure or 8fin0 of large size4 ,its use has not been explained but it provides an easy name for these odd creatures--the 8fin-back lizards40 ,rock deposits produced in arid regions usually have characters which are not difficult to recognize4 ,gritty texture1 irregular bedding1 red color1 and gypsum are common features4 ,formations of ,permian age are to be found in ,colorado but better fossil deposits have been discovered in ,kansas and ,texas4 ,illustration3 ,marine ,reptiles3 ,plesiosaur 7,lower ,skeleton7 and ,mosasaur ,the ,mesozoic ,era produced many types of reptiles besides the dinosaurs4 ,two of the marine forms are shown in this illustration1 both from ,cretaceous beds of the western ,united ,states4 ,plesiosaurs were the giants of the seas in their time1 lengths of forty to fifty feet being not uncommon4 ,a long flat tail provided locomotion for the mosasaurs whereas the plesiosaurs resorted to the peculiar limb structures known as flippers or paddles4 ,,the ,,age ,,of ,,reptiles ,the ,mesozoic1 or era of middle life1 was a long stretch of time during which there was marvelous development among the reptiles4 ,many strange types were produced and most of them became extinct before the end of the era4 ,the reptilian stock branched out in many directions4 ,types emerged which differed from one another so widely that their mutual relationships have become obscure4 ,hideous and fantastic creatures suggesting sea serpents and dragons were worldwide in distribution4 ,reptiles of the air and seas acquired large size and weird forms1 but greater advances were made upon land4 ,the flying reptiles or pterosaurs flourished in ,jurassic times with some of the larger varieties surviving until near the close of the ,cretaceous4 ,although these winged lizards were the first of the vertebrates to fly they are not to be confused with birds4 ,they were without feathers1 and the earlier forms were provided with long tails bearing a flattened rudder-like tip4 ,one of the best known of this type had a length of about eighteen inches4 ,its jaws were long and provided with sharp teeth4 ,the wings were membranes attached to body and legs1 stretched and manipulated by means of greatly elongated fingers4 ,in later types there was a reduction in tooth equipment and length of tail4 ,pteranodon1 found in ,kansas1 had a wing spread of twenty-five feet1 a large toothless beak1 a short body1 and a mere stub of a tail4 ,it was one of the last of these winged monsters4 ,several types of marine reptiles appeared during this era1 among them the plesiosaurs which first appeared in ,triassic seas4 ,these peculiar animals were serpent-like with regard to the character of head1 neck1 and tail1 but in other respects were quite different1 the short barrel-shaped body being provided with four large paddles corresponding to the usual limbs of quadrupeds4 ,fossil remains of these animals are common in many ,jurassic and ,cretaceous deposits1 some of the largest exceeding forty feet in length4 ,mosasaurs1 also marine carnivores1 inhabited shallow ,cretaceous seas throughout the world and are especially abundant as fossils in the ,kansas chalk beds4 ,these were elongated forms with a resemblance to salamanders in some respects but provided with long pointed jaws and sharp teeth4 ,swimming was accomplished largely by the tail though probably aided to some extent by four webbed paddles or flippers4 ,the ichthyosaurs were more fish-like in construction1 as the name implies4 ,the limbs were short and broad1 and there was usually present a well-developed tail-fin as well as a large fin on the back4 ,they were especially abundant in ,jurassic time4 ,fossils are fairly common in marine deposits of western ,north ,america4 ,mosasaurs and ichthyosaurs were about half as long as the plesiosaurs4 ,,dinosaurs ,most spectacular of the prehistoric reptiles were the dinosaurs1 a large group of animals varying greatly as to size1 form1 and habits4 ,they were adapted for a life on land though many of them probably spent much of their time partly submerged in the waters of lakes and streams4 ,there is little that can be said of the group as a whole other than that all of them were reptiles4 ,further than that it is necessary to regard them as belonging to several different subdivisions of the ,reptilia4 ,classification has been difficult and the names used for the various subdivisions are often misleading to the layman who tries to understand the terminology4 ,ancestral reptiles were five-toed and five-fingered but among the dinosaurs there were many departures from the standard formula4 ,three or four of the digits were commonly well developed1 the others when present being shortened or reduced to mere rudiments4 ,early in the history of dinosaurs there was a division of the stock into two main branches1 each of which includes a variety of types and sizes1 and is again subdivided4 ,the two main groups are best recognized by the construction of the bony framework which comprises the pelvic girdle or hip region of the skeleton4 ,in order to avoid technical difficulties1 however1 the remaining discussion of these interesting reptiles will be confined to a few names and descriptions which serve to illustrate roughly the great amount of variation that developed from the comparatively simple ancestral pattern4 ,the plan according to which the dinosaurs are usually classified is barely suggested by the types described4 ,the meat-eaters were active creatures provided with powerful jaws and teeth4 ,they were unarmored1 moved about on their hind feet1 and during their time were the most highly advanced of all animals4 ,tyrannosaurus with a length of forty-five feet or more1 and ,deinodon1 nearly as large1 were among the greatest of these4 ,both lived in the ,cretaceous period4 ,their teeth were simple but strong1 knife-like1 curved1 and finely serrated4 ,skulls were large and the forelimbs were reduced almost to a state of uselessness4 ,large carnivores lived also during ,jurassic time and even as far back as late ,triassic4 ,early ,triassic forms were of smaller size4 ,more primitive flesh-eating dinosaurs of the ,triassic and ,jurassic periods were delicately proportioned and lightly built bipeds bearing some resemblance to birds4 ,struthiomimus1 which means ostrich-resembling1 was about the size of the bird which provides the name4 ,it was slender in the limbs1 three-toed1 long necked1 long tailed4 ,the skull was small1 forelegs long for a biped4 ,unlike most dinosaurs it was toothless4 ,all these bird-like carnivores were small as compared with other contemporary forms4 ,compsognathus1 of ,germany1 and one of the smallest of all dinosaurs1 had a length of less than three feet1 including the long tail4 ,illustration3 ,one of the ,large ,jurassic ,dinosaurs 7,diplodocus longus7 ,this magnificent specimen1 exhibited by the ,denver ,museum of ,natural ,history1 has a length of seventy-five feet six inches4 ,two years were required to complete the task of removing the bones from the matrix rock and preparing them for mounting4 ,diplodocus was one of many large reptiles which inhabited western ,north ,america a hundred and fifty million years ago4 ,the skeleton was obtained from the ,morrison beds of eastern ,utah4 ,the same formation is exposed in many ,colorado localities1 including the foothills west of ,denver1 where it acquired its name from the town of ,morrison4 ,in ,jurassic time there became prominent a group of large dinosaurs which were more equally developed as to fore and hind limbs4 ,they were sluggish creatures1 quadrupedal in their manner of locomotion1 vegetarians in regard to their diet4 ,some of them reached enormous proportions and it is believed that they resorted to life in the water in order to get part of the weight off their feet4 ,diplodocus and ,brontosaurus are the names of well-known giants in this group4 ,they had long necks and tails1 very small skulls1 were the largest of all land animals and are known to have reached a length of eighty feet or more4 ,some estimates1 based on measurements of incomplete skeletons1 have exceeded one hundred feet1 but these extremes are somewhat questionable4 ,diplodocus was the more elongated of the two1 with much of its length in the whip-like tail4 ,our mounted skeleton has a length of seventy-five feet six inches1 measured along the vertebrae4 ,its height at the pelvis is twelve feet six inches4 ,the teeth of these large quadrupeds are of a slightly broadened and blunted form which has caused some speculation as to their possible use4 ,it has even been suggested that the animals were fish-eaters but this seems impossible in view of the great size and general characteristics of the group4 ,although they differ extremely in some respects1 they are regarded as being more closely related to the carnivores than to the herbivores of the second great branch of the tribe4 ,the unquestioned herbivores1 constituting this second branch of the dinosaurian race1 also include both bipeds and quadrupeds4 ,the better known plant-eaters were large animals but not such monsters as ,tyrannosaurus or ,brontosaurus4 ,of the bipeds1 ,trachodon is perhaps best known4 ,it is one of the duck-billed dinosaurs which had an average length of about thirty feet4 ,the duckbills were unarmored1 active animals1 good swimmers as well as runners4 ,they were prominent and widely distributed during late ,cretaceous time4 ,many skeletons have been found in western ,north ,america4 ,natural casts and impressions of mummified remains indicate that the hides were scaly and the feet provided with webs between the toes4 ,the bill was broad1 flat1 and toothless1 but the sides of the mouth were provided with a large number of simple teeth closely arranged in parallel rows4 ,the fine skeleton exhibited in our hall is thirty feet six inches in length4 ,near relatives of ,trachodon1 such as ,corythosaurus had hollow1 bony crests1 combs1 or tubular structures on top of the head4 ,these may have been of some service in connection with breathing while feeding under water4 ,illustration3 ,a ,duck-billed ,dinosaur of the ,cretaceous ,period 7,trachodon mirabilis7 ,illustration3 ,stegosaur 7,stegosaurus stenops7 ,among the quadrupedal vegetarians an interesting family is represented by ,stegosaurus1 a late ,jurassic dinosaur having a length of about twenty feet4 ,these creatures had heavy limbs1 all used in walking1 an arched back1 and almost no brain at all4 ,a double row of large flattened plates standing upright and extending from the rear of the skull nearly to the tip of the tail provided some protection for the back of the animal1 but otherwise there was no defensive armor4 ,several long spikes at the end of the tail probably served as weapons4 ,the mounted skeleton in our collection was obtained from ,garden ,park1 near ,canon ,city1 ,colorado1 a district which has long been famous for dinosaur remains4 ,the ankylosaurs were more completely armored with closely set bony plates fitting neatly over the body4 ,they were of about the same size as the stegosaurs but the body was broad and somewhat flattened4 ,these armored quadrupeds apparently lived only during the ,cretaceous period1 after the disappearance of the stegosaurs4 ,their tooth equipment was very poor and in a few cases entirely lacking4 ,ankylosaurus and ,nodosaurus are good examples of the type4 ,they have been described as animated tanks and are sometimes referred to as having the appearance of enormous horned toads4 ,among the last of the dinosaurs to come and go were the horned quadrupeds known as the ,ceratopsia4 ,their entire history appears to have been confined to the ,upper ,cretaceous and the closing stages of the reptilian era in ,america4 ,triceratops and ,monoclonius are well-known representatives of the group4 ,besides the horns1 which appeared above the eyes or near the center of the nose1 there was a broad1 flattened1 backward extension of some of the skull bones which produced a great frill or collar reaching over the neck as far back as the shoulders4 ,this frill1 combined with the large skull1 gave the animal the appearance of being nearly one-third head4 ,triceratops had three horns1 ,monoclonius only one4 ,the average length of the animals was slightly under twenty feet4 ,although very little is known about the ancestry of the horned dinosaurs a valuable discovery in ,mongolia may throw some light on the subject4 ,a small dinosaur with a well-developed frill1 but no horns1 once inhabited the region of the present ,gobi desert1 and in recognition of the apparent relationship it has been named ,protoceratops4 ,in addition to numerous skeletons1 several nests of eggs were found in association with the bones4 ,until this discovery was made1 dinosaur eggs had been practically unknown4 ,a reproduction of one of these nests is among our exhibits4 ,illustration3 ,a ,sea ,turtle of ,cretaceous ,time 7,protostega gigas7 ,this marine animal belongs to a group which became extinct near the close of the great reptilian era1 but a few related forms still survive4 ,their weight is greatly reduced by the peculiar construction of the shell1 and the front feet are enlarged for use as oars1 an excellent illustration of the manner in which a land type can become adapted to life in the sea4 ,with the possible exception of a very few short-lived survivals dinosaurs were extinct before the opening of the ,age of ,mammals1 many of them for millions of years4 ,along with them went other types of ancient reptiles1 and the cause of their extinction is a problem which may never be solved4 ,conditions remained favorable for the turtles1 which made their first appearance during ,triassic time1 and for the crocodiles1 which date back to the ,jurassic period4 ,snakes were only at the beginning of their history as the era closed4 ,the survival of these modern forms suggests that they were favored to a greater extent than the dinosaurs during a prolonged period of changing conditions the full details of which are unknown to us4 ,in general it is to be expected that disaster would first overcome the highly specialized creatures1 such as the dinosaurs1 which had become more delicately adjusted to the particular environments in which they lived4 ,it appears that some of them had been too progressive up to a certain point1 but not sufficiently adaptable to get beyond that stage1 or fortunate enough to make their advances in directions that could be followed1 through fluctuations in the matter of food supply1 predatory enemies1 climate1 and other factors which bear upon success and failure4 ,the reptilian era closed with exceptional volcanic activities in many parts of the world1 but these cannot account for the disappearance of the highly diversified and abundant reptilian life4 ,the eruptions were merely incidental to movements and readjustments in large masses of rock comprising the earth's crust or surface4 ,such crustal folding and elevations always have been of serious consequences to both plants and animals because of their effect upon drainage and climate4 ,there were disturbances of this kind in western ,north ,america in late ,jurassic time1 with folding and uplift in the region of the ,sierras and probably extending from ,mexico to southern ,alaska4 ,a great trough to the east of this elevated district was produced in the course of these movements and provided access to the sea from south to north4 ,during the ,cretaceous period there were repeated invasions and retreats of the sea by way of this great depression1 consequent upon slight changes in the elevation of the floor4 ,hence there are numerous marine formations in ,colorado and adjoining states1 some of them rich in fossils4 ,before the close of the ,cretaceous period the sea had made its final departure from this region1 and the ,mesozoic era was terminated by revolutionary disturbances which brought about the uplifting of a new mountain system4 ,the ,rocky ,mountains may be regarded as part of this system and to have had their birth at this time4 ,the ,rockies1 however1 show unmistakable signs of repeated elevation1 with intervals of erosion during which there was great reduction of their total height4 ,what we see of them today is the result of more than fifty million years of continuous geological activity4 ,,plant ,,life ,,and ,,climate ,some idea of the ,mesozoic climate is obtained from the character and distribution of the plant life4 ,triassic floras are not large and there is very little fossil evidence for the earlier half of the period4 ,it is quite possible that arid or desert conditions prevailed for a time in much of ,north ,america1 as at the close of the ,paleozoic era4 ,plant life was at first not abundant1 and conditions were unfavorable for the production of fossils4 ,in ,upper ,triassic rocks of ,virginia1 however1 there are signs of swampy conditions1 with rushes and ferns predominating4 ,adjoining forest areas were well timbered with large coniferous evergreens which show no annual growth rings1 as similar trees do in regions where cold winters alternate with warm summers4 ,this suggests1 for that time and place at least1 a uniformly warm climate1 lacking seasonal variations4 ,warm temperature or subtropical climates are indicated again by some of the ,jurassic and ,cretaceous plants1 but intervals of lower temperatures and variable climates are also apparent4 ,palms1 figs1 and other trees1 very similar to modern types now living only in warmer regions1 were widely distributed in late ,cretaceous time1 and their range was extended into regions which have since become too cold to support such growths4 ,the trend toward modern forms in the plant world was gradual1 but throughout the era there were occasional novelties that attract the attention of botanists4 ,ferns and horsetail rushes1 reminiscent of the ,paleozoic forests1 soon began to lose their prominence as the seed-bearing trees gained the ascendency4 ,mesozoic time could well be called the age of cycads1 because of the striking performance of this plant group4 ,different varieties flourished in the three periods1 with the ,jurassic standing out as the time of greatest abundance4 ,to the uninitiated1 the usual cycad fossils resemble 8petrified pineapples10 but these are merely the scarred stems or trunks of small to medium-sized trees with a tufted arrangement of leaves at the top1 and usually without branches4 ,foliage and habit of growth suggest something more like large ferns or low-growing palms1 with short1 thick trunks seldom more than fifteen feet tall and many of them under three feet4 ,the leaves are rarely found entire or attached to the trunks1 but occasional discoveries indicate a leaf-length of about ten feet4 ,although they are classed among the first and lowest of seed-bearing plants1 and in this respect are related to the conifers1 their appearance was quite unlike that of the modern cone-bearing evergreens4 ,more nearly resembling the common conifers of today were the sequoias1 of early ,mesozoic origin and far more abundant during ,cretaceous time than they are at present4 ,the maidenhair trees1 now represented by a single species of ,ginkgo which is cultivated principally in ,china and ,japan1 were never very prominent but are of interest as an ancient family that persisted throughout the ,mesozoic and down to our own time4 ,before the close of the ,cretaceous period the flowering plants had greatly outnumbered the spore-bearing groups1 such as the ferns and horsetails which were formerly so abundant4 ,we know little of early flowers1 however1 except in connection with trees1 the large gayly colored blossoms of the type now conspicuous in woodlands1 meadows1 and gardens being later arrivals and poor subjects for preservation as fossils4 ,cretaceous floras were surprisingly modern in character1 far in advance of the animal life4 ,poplars1 plane trees1 magnolias1 palms1 figs1 oaks1 and buckthorns were abundant at the close of the ,cretaceous1 as indicated by fossils of the ,laramie formation1 which is the surface rock in many localities near ,denver4 ,also abundant in various places at this time were walnut1 hazelnut1 laurel1 tulip1 maple1 beech1 birch1 breadfruit1 ivy1 holly1 and many other well-known trees and shrubs4 ,sedges and grasses1 which became so important to the herbivorous mammals of the next era1 made their first appearance in ,cretaceous time but were then inconspicuous4 ,,coal ,,and ,,fossil ,,footprints ,the abundance of plant life in the ,colorado area during the ,cretaceous period is indicated by the extent of coal deposits of this age4 ,about one-fourth the area of the state is underlain by coal seams varying in thickness from a few inches to fifty feet or more1 most of it being ,cretaceous4 ,in the northern ,colorado district the coal-bearing formation is the ,laramie4 ,near ,denver there is some coal in the ,arapahoe formation which overlies the ,laramie and is of later age4 ,coal mines often produce excellent plant fossils1 and occasionally other evidence of prehistoric life4 ,in a mine near ,canon ,city1 ,colorado1 a series of natural casts of dinosaur feet was taken from the overlying rock after the coal had been removed4 ,one of these1 in the ,denver ,museum of ,natural ,history1 is seen to consist of sandstone inside a very thin layer of dark clay4 ,flattened against the lower surface is the carbonized stem of a ,cretaceous plant which grew in the swamp where the coal deposit was formed4 ,since the shape of dinosaur feet is unmistakable we can only assume that a large reptile of this type walked over the surface of swampy ground in which a great thickness of decaying vegetation had accumulated4 ,a layer of mud settled over the top and became sufficiently firm to retain the mold of the feet as the animal moved along4 ,any plant material either on the mud or included in it was pushed to the bottom of the impressions and flattened out by the weight of the huge creature4 ,then sand was washed into the footprints from some nearby source during a heavy rainstorm4 ,following these events there was probably a subsidence of the area1 and a great thickness of rock-making sediments was built over the ancient swamp4 ,the buried vegetation gradually became converted into coal1 the sand consolidated into a firm sandstone1 and the mud produced the shales forming the roof of the present mine1 which is now at an elevation of a mile above sea level as a consequence of the general uplifting of the ,rocky ,mountain region during late ,cretaceous and subsequent time4 ,when the coal was removed1 the hard sandstone casts separated readily from the softer shales surrounding them4 ,a small amount of the shale adheres to the sandstone1 and some of the flattened vegetation1 now in the condition of coal1 still remains attached4 ,,mesozoic ,,invertebrates ,as in other eras1 the invertebrates fluctuate with the periods4 ,characteristic forms appear1 become more or less prominent1 then in many cases decline or disappear4 ,variations among the mollusks are particularly helpful in the identification of rocks which originated in the ,cretaceous seas4 ,clam-like bivalves of the genus ,inoceramus1 the straight-shelled ammonids known as ,baculites1 and oysters1 are locally common in some of the formations exposed a few miles west of ,denver4 ,the ammonids1 or 8ammonites10 were extremely abundant throughout the world during ,mesozoic time4 ,their shells were chambered like those of the pearly nautilus1 a related cephalopod mollusk inhabiting tropical seas at the present time4 ,while only four species of the ,nautilus tribe are living today1 thousands of species of ammonids swarmed the prehistoric seas4 ,many new forms came into existence in ,triassic time but few survived the period4 ,a pronounced revival occurred in the ,jurassic1 only to be followed by a decline and eventual extinction at the close of the ,cretaceous4 ,ammonites measuring three or four inches across the diameter of the coiled shell were about average size1 but diameters up to three or four feet were not uncommon4 ,externally the shells were ornamented with ribs1 knobs1 and spines2 inside was a pearly lining4 ,the partitions were thin and composed of the same pearl-like substance as the lining4 ,each partition becomes wavy as it approaches the shell1 and the line of union has a distinct pattern which is seen in specimens which have lost the outer shell layer4 ,this wavy suture line becomes more complicated in the later members of the race1 and the peculiar markings developed by the repeated partitions provide a convenient method of identification4 ,the belemnites or ink-fishes1 regarded as ancestors of the cuttlefishes now living1 comprise another group of carnivorous mollusks4 ,these1 however1 had lost the external shell1 and the usual fossil is part of an internal shell or 8skeleton10 known as the guard4 ,this limy structure has the form of a pointed cigar1 and is seldom over a foot long although the total body-length of the larger animals was commonly about six to eight feet4 ,several hundred species have been described1 the majority being of ,jurassic age4 ,they declined rapidly toward the close of the era4 ,the invertebrate life of the ,mesozoic was strongly dominated by mollusks1 with cephalopods in first place1 the bivalve pelecypods and the single-valve gastropods or snail-like forms sharing subordinate positions4 ,the dominating trilobites1 sea-scorpions1 and tetracorals of the ,paleozoic had disappeared1 while the brachiopods and crinoids were greatly modified and more like the forms which live today4 ,crinoids became moderately abundant at various times1 but in many ways different from their relatives of the preceding era4 ,some of the largest known species1 with stems estimated as fifty feet long1 have been found in lower ,jurassic rocks4 ,a great abundance of microscopic life is indicated by the frequent occurrence of chalk in the ,cretaceous formations4 ,corals of the modern reef-building type 7hexacorals7 were common in the warm seas of a large part of the world4 ,the insects of the early ,mesozoic are represented by few fossils although it is evident some new forms were becoming established4 ,the warm climates prevailing throughout much of the world appear to have been a favorable factor in the progress of insect life4 ,in addition to the older cockroach and dragon-fly types may be mentioned the arrival of grasshoppers1 cicadas1 caddis-flies1 beetles1 and ants4 ,several hundred species are found in ,jurassic rocks1 and by the end of the ,cretaceous period most of the insect families now known to us were probably in existence4 ,the record is seriously obscured by the erosion of rocks which so frequently marks the end of a period1 also by the small size of the subjects1 and by the exceptional conditions required for the production of such delicate fossils4 ,among the last of the familiar insects to appear were the bees and butterflies4 ,these evidently came in with the more advanced types of flowering plants that produce the nectar on which many insects feed4 ,it is probable1 too1 that without the arrival of these insects and their service in the pollination of flowers1 the floras of today would be rather different from what they are4 ,,extinct ,,birds ,it is not surprising to find that birds made their first appearance in the ,mesozoic era1 for of all animals they are most like the reptiles as a class4 ,feathers are about the only dependable characteristic of the entire group1 nearly every other feature being matched by some reptilian creature of great antiquity4 ,the nesting habit1 which includes care of the young as well as the eggs1 is a matter of progress which relates to flight and to warmer body temperature4 ,it appears to have been developed by forest-dwelling types living among trees and nesting there in comparative safety from enemies prowling on the ground4 ,the oldest known prehistoric birds were found in lithographic stone of ,upper ,jurassic age4 ,archaeopterix was discovered in #ahfa at ,solenhofen1 ,germany4 ,sixteen years later a similar bird in a better state of preservation was found in ,bavaria4 ,the latter was named ,archaeornis4 ,these ,jurassic fossils are regarded as true birds by some authorities1 while others believe them to be more nearly related to the reptiles1 the opinions being based on careful studies of the skulls and other skeletal features4 ,both birds had teeth of reptilian character1 and it is evident that there was no beak1 for the jaws were covered with scaly skin4 ,the bony construction of the long tail would suggest lizards rather than birds1 were it not for the presence of feathers which were attached at each side4 ,head1 neck1 and parts of the body were covered with scales4 ,wings were well provided with stout feathers but the skeletal framework indicates that the birds were gliders rather than true flyers4 ,claws on the wings served like fingers to aid in climbing among the branches of trees1 a practice which is occasionally noted among the young of living birds4 ,in adult birds of today1 however1 the claw-like appendages of the fore limbs are greatly reduced and of little service4 ,the next fossil birds of importance have been found in ,cretaceous rocks of ,kansas1 both of them fishers of the seas instead of forest inhabitants4 ,ichthyornis was a small bird1 standing about eight inches in height1 a powerful flyer with reptilian jaws and teeth4 ,hesperornis was built for diving and swimming1 like the loon1 but was somewhat larger and provided with teeth4 ,its wings were too poorly developed to be of use in flying4 ,toothed birds became extinct with the close of ,cretaceous time1 and the ancestors of modern types were in existence before the ,age of ,mammals1 but fossil remains are few and poorly preserved4 ,large ostrich-like birds1 however1 are known to have lived in ,north ,america during the ,eocene period4 ,one of these1 named ,diatryma1 stands nearly seven feet tall in the reconstructed skeleton4 ,its legs are heavy1 wings greatly reduced1 beak massive4 ,in its relation to modern birds it is possibly nearer to the cranes than the ostriches4 ,flightless birds of large size are known from many parts of the world and seem to have been prominent throughout the ,cenozoic era1 as they are today in the southern hemisphere4 ,aepyornis lived in ,madagascar during the ,pleistocene period and may have become extinct quite recently4 ,its eggs are the largest known among fossils1 several times the size of an ostrich egg4 ,also in this period the moas were living in ,new ,zealand where their remains are still abundant4 ,one of the largest1 known as ,dinornis1 had about the same form as ,diatryma but the neck was longer1 head and beak smaller1 legs better fitted for running1 height about eleven feet4 ,a much smaller flightless bird1 the dodo1 became extinct in modern time4 ,this former inhabitant of ,mauritius and other islands of the ,indian ,ocean was related to the doves and pigeons1 and had lost its power of flight through disuse of the wings4 ,it was a clumsy1 defenseless bird weighing possibly as much as fifty pounds4 ,actual remains are few and incomplete1 and descriptions published by the explorers who knew the bird two centuries ago are not entirely trustworthy4 ,in the ,pleistocene ,rancho la ,brea beds of ,california the largest of all prehistoric flying birds has been found1 a vulture bearing the name of ,teratornis4 ,re-assembled skeletons show them to be slightly bigger than existing condors4 ,,ancestors ,,of ,,the ,,mammals ,the monotremes or egg-laying types of mammals such as the duck-bill and spiny anteaters which now inhabit ,australia are almost unknown as fossils4 ,marsupials1 the next higher living group1 which includes the opossum and kangaroo1 appeared at the end of ,cretaceous time along with the placentals or higher mammals which dominate the history of the ,cenozoic era4 ,nevertheless1 there are a few teeth and jaws from rocks of ,triassic and ,jurassic age to indicate that small mammals1 from the size of mice to slightly larger than rats1 existed throughout most of the ,age of ,reptiles4 ,there is no complete skeleton of any of the earlier forms1 and little is known of their relationships either with living orders of mammals or with probable ancestors among the reptiles4 ,the record becomes somewhat clearer toward the end of the era but it is obscured again by the great disturbances which followed4 ,looking back among earlier land animals for the origin of the first mammalian stock it is necessary to go as far as ,permian or even ,carboniferous time4 ,reptiles then living had many structural features in common with mammals1 and mammal-like forms continued to flourish until late in the ,triassic4 ,an interesting group of such animals1 named therapsids1 was one of the earliest reptilian stocks to appear1 and is well known from fossils found in the ,red ,beds of ,texas and ,new ,mexico1 in ,europe1 ,south ,africa1 and ,asia4 ,quite a variety of types is included in this group1 with many advances in dentition1 and modifications of the skull1 limbs and pelvic construction which strongly suggests a relationship to the mammals4 ,illustration3 ,murals ,over ,fossil ,exhibits1 ,hall of ,mammals ,top3 ,eocene2 ,protylopus1 ,tanyorhinus1 ,patriofelis1 ,uintatherium1 ,turtle1 ,crocodile1 ,eohippus4 ,middle3 ,upper ,oligocene2 ,mesohippus1 ,merycoidodon1 ,hoplophoneus1 ,metamynodon1 ,poebrotherium1 ,trigonias4 ,bottom3 ,pliocene2 ,teleoceras1 ,turtle1 ,synthetoceras1 ,amebelodon1 ,teleoceras4 ,,the ,,age ,,of ,,mammals ,the striking feature of life development in the ,cenozoic era is the great progress and expansion over the earth of the mammalian races4 ,the division of the era into periods1 however1 was based largely on a study of fossil mollusks4 ,in the ,paris basin of ,france1 it was noticed by the geologists of a century ago that the youngest of the sedimentary beds contained the greatest number of recent or still living species4 ,successively downward into the older beds the percentage of recent species decreased until there were practically no living species represented in the oldest rocks of the series4 ,from the percentage of recent forms among prehistoric ones it was proposed that the following division be made3 ,eocene1 meaning dawn of the recent2 ,miocene1 meaning less recent2 and ,pliocene1 meaning more recent4 ,sometime later it was suggested that another period be added1 and to this was given the name ,pleistocene1 meaning most recent4 ,in #ahed1 the older ,miocene formations were segregated and referred to a newly provided ,oligocene period1 this name meaning little of the recent4 ,early geologists grouped the rocks in three great divisions1 applying the names ,primary1 ,secondary1 and ,tertiary4 ,to these was added afterwards the name ,quaternary1 which applied to the youngest formations of the earth4 ,only two of these terms remain in common use at present3 it is a frequent practice to refer to the combined ,eocene1 ,oligocene1 ,miocene and ,pliocene periods as the 8,tertiary0 division of ,cenozoic time2 to the ,pleistocene and ,recent periods as the 8,quaternary0 division4 ,the geology of some remote future may be clearer with regard to the full significance of this subdivision of the ,age of ,mammals into two parts4 ,it may be that a great era was concluded at the end of ,pliocene time as others have been concluded1 by the usual earth disturbances and climatic changes and by the decline of animals once prominent in the faunas of the world4 ,events of such character have registered their occurrence but may eventually prove to have been a series of minor events not comparable with the revolutionary changes that terminated other great time divisions4 ,the favored practice of including ourselves and our times in the ,cenozoic is based on a trend of opinion which holds that no great era has been ended since the ,age of ,reptiles was concluded4 ,illustration3 ,skulls of the clumsy1 six-horned uintathere and the early1 hornless titanothere form part of this ,eocene display4 ,in the mural these animals and the little 8three-toed0 ,eohippus1 smallest of horses1 are pictured with a contemporary turtle and crocodile4 ,the oldest of ,eocene rocks show a great variety of mammals and a strange assortment of forms far in advance of the ,mesozoic record4 ,ancestries and successive stages of development have been only partially worked out1 though details have been better preserved for some of the groups than for others4 ,some oddly shaped creatures such as the uintatheres reached their full development in a relatively short time and passed out of the picture before the end of the period4 ,these animals1 represented in our collection by ,uintacolotherium1 acquired large bodies and many horns1 but a peculiar tooth equipment fitted them for a special diet which apparently failed to be supplied in sufficient abundance at a critical time4 ,on the other hand1 we find in this period the ancestors of more successful groups1 some of which continue on into modern times4 ,only a few of these histories can be traced in a brief sketch1 but in a general way it may be stated that the successful races had modest beginnings and that they developed very slowly into what they are now1 by a process of adjusting themselves1 or by becoming better adapted to new or previously unused conditions in their respective environments4 ,in their early stages the various types had much in common2 they were generalized1 rather than specialized for any particular kind of existence4 ,what they were fitted for is best indicated by their teeth and feet1 though other structural features frequently contribute valuable information4 ,the rise of mentality is indicated by skull capacities and the increasing development of the upper lobes of the brain1 as revealed by casts taken from the interior of skulls4 ,the creodonts were the earliest and most primitive of the flesh-eating mammals or carnivores4 ,many of them were small of body and brain1 and equipped with teeth that indicate a mixed or largely insectivorous diet1 or possibly the habit of feeding on carrion4 ,although there was considerable variation among them1 and some tendency toward specialization1 there was little to suggest the coming of more progressive groups such as the cats and dogs1 with teeth perfected for the tearing and cutting of flesh1 and feet especially fitted for the life of hunters4 ,the ancestry of the cats cannot be traced farther back than the ,oligocene but it probably connects somewhere prior to that time with the creodonts4 ,cynodictis1 an ,oligocene carnivore slightly under two feet in length is commonly regarded as a primitive dog1 but its characters are so generalized that it probably differs but little from the ancestors of many other carnivores4 ,the skeleton of this animal suggests a slender and flexible body like that of the weasel1 with somewhat shortened limbs and a long tail4 ,it lived in forested regions and was probably more or less of a tree dweller4 ,the more advanced carnivores required longer legs1 better adapted for running and overtaking the prey1 which is the dog's way of hunting1 or for stalking and springing upon the quarry1 which is the method of the cat4 ,illustration3 ,moropus 7,moropus cooki7 ,though its teeth clearly indicate a diet of plant material1 this strange animal had claws on its toes1 like the carnivores4 ,it is probable that the claws were used in digging for roots1 as indicated by the artist4 ,a prominent group of mammals today is that known as the ungulates1 or hoofed animals1 which includes the horses1 cattle1 deer1 swine1 rhinoceroses1 tapirs1 and other types both living and extinct4 ,their probable ancestors were the condylarths1 primitive ungulates of the ,eocene period4 ,one of these1 known as ,phenacodus1 serves well to illustrate the general character of the early hoofed mammals4 ,it was about five and one-half feet long1 rather large for its time1 with long tail and short limbs1 low elongated skull and small brain1 very similar in many respects to the creodonts or ancestral carnivores4 ,the teeth1 however1 were partially of the grinding type so essential to the welfare of plant feeders4 ,the condylarths were five-toed animals and evidently provided with small hoofs1 but the more progressive ungulates soon lost one or more of the toes1 and a division of the group into odd-toed and even-toed branches became firmly established4 ,consequently1 the families of ungulates having one1 three1 or five toes are classed together as being closely related to one another1 and those having two or four toes are segregated in a second lot4 ,the odd-toed clan1 known as perissodactyls1 included such animals as the horses1 rhinoceroses1 tapirs1 and titanotheres1 each of these types being placed in a separate family4 ,the even-toed clan has been treated in a similar way and named the artiodactyls4 ,in this division are such families as swine1 cattle1 deer1 camels1 oreodonts1 and others4 ,the odd-toed group dominated among the larger animals of ,north ,america for a time but has been completely replaced by the even-toed division which is still flourishing1 although some of the older families have become extinct4 ,among the exhibits of the ,denver ,museum of ,natural ,history may be seen complete skeletons of extinct horses1 rhinoceroses1 titanotheres1 and chalicotheres representing the perissodactyls4 ,moropus was one of the chalicotheres1 an exceptional family which never became very prominent although it had a prolonged history and persisted in ,europe and ,asia after its extinction in ,north ,america4 ,the family is grouped with the ungulates because of many similarities found in the molar teeth1 skulls1 and other parts of the skeletons1 but the toes were provided with claws instead of hoofs4 ,the use of these claws is somewhat of a puzzle3 possibly for defense against carnivorous enemies1 for dragging down branches in order to obtain food1 or for digging roots which may have been an important part of the diet4 ,titanotheres are represented in our collection by the skeletons of the large1 horned type which was the last of the race and destined to extinction by the middle of the ,oligocene period4 ,smaller hornless varieties of ,eocene time are illustrated by skulls4 ,this family of ungulates had an unprogressive dental equipment1 and a small brain in a flattened skull4 ,the molar teeth readily distinguish the group from other ungulates and enable us to trace the relationship between earlier and later varieties4 ,these teeth were of a type which is soon destroyed by wear1 and it is evident that the animals survived only so long as their environment provided them with an abundance of soft vegetation4 ,illustration3 ,titanotheres of ,oligocene ,time ,the name of these animals refers to the large size though they were greatly exceeded in bulk by the mastodons and mammoths of later periods4 ,ancestral titanotheres1 dating back to the ,eocene1 were hornless animals of much smaller size4 ,these splendid specimens were obtained in ,weld ,county1 ,colorado4 ,the large assortment of rhinoceros material provides an idea of the great abundance and variety of forms in this family which was once prominent in ,north ,america but no longer among the inhabitants of that continent4 ,some of the mounted skeletons have been restored on one side to show how these animals appeared in the flesh4 ,of the even-toed ungulates there are also several types illustrated by complete skeletons4 ,merycochoerus1 the subject of one of our mounted groups1 represents the oreodonts1 a large family of mammals whose history begins with the ,upper ,eocene and ends in the ,lower ,pliocene4 ,the oreodonts were small animals1 rather pig-like in form and quite common in the western plains region shortly after the time of the titanotheres4 ,ancient swine are represented in our exhibits by two mounted skeletons which were obtained from northeastern ,colorado1 where the bones were found associated with rhinoceros and titanothere remains4 ,some of these animals were of very large proportions1 and the entire family is commonly known as the 8giant pigs40 ,camels and closely related forms were quite abundant in ,north ,america from early ,oligocene to comparatively recent time4 ,numerous types were developed during the course of their history1 some small and delicately formed1 others tall and clumsy and much like the giraffe in structure4 ,parts of many of these creatures have been found but the only completely prepared skeletons in our collection are of the little gazelle-camel1 ,stenomylus1 from ,lower ,miocene deposits in northwestern ,nebraska4 ,pleistocene bisons are represented by several complete skeletons and numerous skulls and horncores1 some of the species showing an extreme development in the length of horns4 ,with two of the bison skeletons are shown prehistoric weapon points1 found with the bones and indicating that these animals were hunted by primitive men at some time near the close of the ,ice ,age4 ,the artifacts first discovered near ,folsom1 ,new ,mexico1 by field workers of our ,museum1 have become known to archeologists as ,folsom points4 ,,prehistoric ,,horses ,the past history of horses is well known from an abundance of fossil material1 ranging in age from the ,eocene down to the present4 ,modern horses have only one toe in each foot1 but there are remnants of two additional toes which may be seen only in the bony structure underlying the skin4 ,most of their ancestral relatives were three-toed as far back as the ,oligocene period4 ,during ,eocene time1 however1 there was a stage which may be regarded as four-toed although it was evidently a temporary condition1 linking known horses with more remote forms having five toes4 ,illustration3 ,oligocene ,mammals ,from ,weld ,county1 ,colorado ,the giant pigs 7,archaeotherium mortoni7 at the left of the group1 and the rhinoceros 7,trigonias osborni7 were common animals of the western plains region at one time4 ,eohippus1 the 8dawn horse0 as it has been called1 is one of the oldest and best known of the ,american horses4 ,its relation to existing members of the family can be traced by means of changes in tooth structure as well as in the gradual reduction in the number of toes that is seen among intermediate forms4 ,its ancestors some day may be positively identified in that group of generalized1 primitive1 five-toed1 hoofed mammals which are known to have lived at the beginning of the mammalian era1 but such identification has not yet been established4 ,even ,eohippus bore little resemblance to the familiar horse of today4 ,its height was only eleven inches1 and in body form it had much of the appearance of a modern dog4 ,there were four toes on the front foot1 one of them decidedly shorter than the others but complete in all its parts1 and evidently capable of service in carrying a portion of the animal's weight4 ,the hind foot had three complete toes and a tiny remnant of a fourth which could not have been apparent externally4 ,as changes in the structure of the feet progressed1 the central toe of the original five continued to increase in size while the adjacent digits became relatively shorter and eventually so reduced in length that they could touch the ground no longer4 ,the smaller bones at the extremities1 corresponding to the joints of our fingers and toes1 eventually disappeared from the side toes4 ,then the longer bones of the outer digits lost the broadened supporting surface1 where the missing toes had been attached1 and became reduced to pointed remnants known as splints4 ,extreme shortening of the splint bones eventually leaves only a small knob which is often referred to as a rudimentary toe4 ,in the skeleton of a large horse the splints are readily seen1 but in some of the earlier species they are so small that they may easily be destroyed or overlooked by the collector who removes the fossilized material from the surrounding rocks4 ,even then1 the bones of the wrist and ankle may indicate in an unmistakable manner that an additional toe once was present1 for each bone is supported by another1 and at the point of attachment there is a characteristic surface whose purpose is usually obvious4 ,throughout the ,cenozoic era the changes continued4 ,among the horses of the ,north ,american ,oligocene were ,mesohippus1 approximately the size of a collie dog1 and ,miohippus which was slightly larger4 ,both were three-toed1 but the rudimentary splint of a fourth toe was still present in the front foot4 ,parahippus and ,merychippus carried on during the ,miocene period1 the latter being characteristic of the time1 and showing1 in addition to other progress1 a decided trend toward the modern structure of molar teeth4 ,there was some increase in size but the largest horse of that period was hardly more than a small pony4 ,illustration3 ,a ,pleistocene ,horse of the ,texas ,plains 7,equus scotti7 ,hipparion and ,protohippus1 living during ,upper ,miocene and ,pliocene time1 represent later stages of the three-toed condition4 ,the side toes were completely formed but greatly shortened1 only the central toe touching the ground4 ,in some of the species the outer toes had also become very slender1 approaching the splint condition4 ,by this time the molar teeth were longer and better adapted for feeding on grasses which were becoming sufficiently abundant to attract some of the forest dwellers into the open country4 ,during the ,pliocene period1 in the genus ,pliohippus and also in ,hipparion1 the feet were far advanced in structure1 with most of the species single-toed1 the side digits having reached the splint stage4 ,pleistocene horses of the genus ,equus1 like living species of that genus1 were strictly one-toed animals1 ranging over grassy areas and highly specialized for a life in that kind of environment4 ,specialization is to be noted partly in the foot and leg structures where the modifications have contributed to greater speed and travelling ability4 ,this is of great service to an animal of the plains where food and water are often scarce1 and great distances frequently have to be covered in order to obtain sustenance4 ,the horse1 as we know it1 is built for speed1 its limbs and feet being elongated to permit a greater stride1 and also modified to decrease the weight without loss of strength4 ,the ordinary ball-and-socket joint is replaced by a pulley-like construction which limits the direction of movement but provides an excellent mechanism for locomotion1 especially over flat1 open ground4 ,flexibility in other directions is sacrificed for greater strength1 and the foot incidentally becomes less suited for other purposes4 ,this is what is meant by 8specialization0--a departure from 8generalization40 ,the study of fossils provides numerous illustrations of specialized development which contributes greatly to an interest in prehistoric life4 ,any specialized structure or habit which increases fitness for a particular way of living is also known as an 8adaptation40 ,quite in line with the idea of specialization and adaptation is the change which occurred in the construction of the horses' teeth1 for the dental equipment of the modern grazing animals differs widely from that of the browsing creatures which lived on the soft leaves and other plant substances of the forests4 ,illustration3 ,the ,structure of ,molar ,teeth ,the large lower molar of a long-jawed mastodon shows worn and unworn cusps1 with the enamel layer forming a heavy border around the central dentine where the surface covering has been worn through4 ,in the grinding teeth of rhinoceroses 7illustrated at the right7 the crown pattern is quite different1 but both types are adapted for softer foods and are similar in having the protective enamel on the outside only4 ,the central tooth shows the condition after the shallow surface depressions have been removed by wear4 ,the cheek teeth or grinding equipment of the horses underwent as complete a change as the feet4 ,modification resulted in a new type of tooth which enabled herbivorous animals to take advantage of a kind of vegetation which was late in arriving and has since become the principal diet of the ungulates4 ,the grasses are coarse and harsh as compared with the leaves of forest shrubbery1 requiring more thorough grinding to make them digestible4 ,in addition they contain minute particles of silica1 which is a highly abrasive mineral that quickly wears down the tooth substance1 especially the softer materials found in tooth construction4 ,an increase in the length of the tooth would offset the excessive wear but would not necessarily produce a better mechanism for grinding4 ,the fulfillment of the new requirements is to be seen in the change from what is known as the low-crowned1 browsing type of molar1 to the high-crowned1 grazing type4 ,details of the changes that may be traced through millions of years of gradual adjustment become apparent only from the examination of a great deal of fossil material4 ,as compared with earlier types of construction1 a modern molar tooth may appear extremely complicated1 but the process which brought about the improved quality is very simple4 ,a little discussion of tooth structure1 however1 is required to make this clear4 ,a tooth1 as everyone knows1 is partly imbedded in the jaw1 partly exposed outside the gum4 ,in a short-crowned tooth the exposed portion is known as the crown1 and the part imbedded in the jaw consists of one or more roots which are comparatively long4 ,the crown is nearly always protected by a thin layer of hard enamel4 ,in a grinding tooth1 the working surface has a number of more or less prominent elevations known as cusps4 ,the enamel layer completely covers this surface until wear begins4 ,as the tooth goes into service the signs of use begin to appear2 the enamel is soon worn from the tops of the cusps1 and the underlying substance1 called dentine1 becomes exposed4 ,this is far less resistant to wear1 and as the enamel continues to be reduced the tooth becomes less efficient as a grinding device1 partly because of the smoothing off of the surface1 partly because of the relative softness of the inner material which is being exposed in increasing quantity4 ,a very old molar tooth of the low-crowned type has a smooth surface from which almost the last trace of the enamel has been removed4 ,in many prehistoric animals the enamel is of a darker color than the dentine or cement1 this difference in color enabling one to see at a glance how the teeth are constructed4 ,illustration3 ,grazing ,type of ,molar ,teeth ,the side view of the bison's molar and premolar equipment illustrates the elongated construction which is common among grazing animals4 ,in the pattern of the grinding surface may be seen a cross section of the enamel layers4 ,one layer surrounds each tooth while two folded 8cylinders0 of the same material occupy the interior4 ,in a long-crowned tooth the roots are usually very short1 for much of the crown itself is imbedded in jaw bone1 and the longer roots are not required4 ,growth of the tooth is usually completed after a few years2 then as it is gradually worn away it is continuously moved upward by the production of new bone under the roots1 which slowly fills the bottom of the socket and continues to provide the necessary support4 ,an equally important difference between the two types of teeth1 however1 is to be seen in the arrangement of the enamel1 the long-crowned type being provided with this durable substance on the inside of the crown instead of having a mere protective cap on the outside4 ,the more complicated structure was developed from the simpler form by the easy method of deepening certain depressions located between cusps at the top of the tooth4 ,as the crown of the tooth increased its length these depressions remained tucked in1 and eventually became deep pits roughly cylindrical in shape4 ,in addition to the enamel and dentine1 a third tooth substance1 known as the cement1 made its appearance at about this time1 and we find that quantities of this new material were deposited outside the crown enamel and also inside the enamel walls of the pit1 in this way producing a firmly consolidated structure otherwise weakened by deep channels and hollow pockets4 ,the cement differs only slightly from the dentine but is deposited while the uncut tooth is in the gum tissues of the mouth1 the enamel and dentine elements being formed earlier in the embryonic tooth before it emerges from the jaw bone4 ,a tooth constructed by such a process1 if cross-sectioned through the crown1 will be found to consist of successive layers of hard and softer materials4 ,in living animals the top of the tooth soon wears off and the enamel layers stand in higher relief because of their greater resistance to wear4 ,a roughened surface of excellent grinding quality is thus provided1 and as long as the wear continues there remains the same relative amount of enamel to retain the roughness1 and resist abrasion4 ,among the various types of grazing animals there is a marked difference in the arrangement and form of the enamel layers4 ,within a species of genus1 however1 the complicated enamel patterns of the molar teeth are consistently similar4 ,in the case of horses especially1 these patterns provide a most helpful key to the identification of extinct forms4 ,the general pattern1 in any of the more modern horses1 may be understood more readily if the wavy enamel layers be regarded as forming a set of cylinders with deeply crinkled walls4 ,near the outer border of the tooth1 surrounded by a thin layer of cement1 is the enclosing cylinder which represents the enamel cap of the old-fashioned1 low-crowned tooth4 ,inside of this is the central mass of dentine which has been penetrated by two of the deep pits previously mentioned4 ,the original enamel cap has been depressed into these pits1 forming two inner cylinders which are filled with cement4 ,instead of being circular in outline1 when the cap is worn through at the grinding surface these inner cylinder walls are seen to be wrinkled and folded so as to produce a most irregular pattern4 ,however1 if several teeth of the same kind of horse are compared1 it will be found that the edges of these cylinders produce figures which are remarkably uniform and characteristic for that species4 ,illustration3 ,american ,mastodon 7,mastodon americanus7 ,a true mastodon of the short-jawed type4 ,,mastodons ,,and ,,mammoths ,elephant-like mammals both living and extinct are classed together in a single order bearing the name ,proboscidea4 ,living members of the group are the elephants1 of which the large ,indian and ,african species are best known4 ,among prehistoric representatives the most frequently mentioned in the popular literature of ,north ,american animals are the following3 ,the ,american ,mastodon1 an immigrant from ,siberia which ranged over nearly all of the ,united ,states and ,canada4 ,it was principally a forest dweller1 rarely found in plains regions1 was abundant during the ,pleistocene period and may have been known to the early ,american ,indians2 ,the ,woolly ,mammoth1 which was about nine feet tall4 ,it ranged over ,british ,columbia into the ,united ,states and across to the ,atlantic1 disappearing in late ,pleistocene time2 ,the ,columbian ,mammoth1 about eleven feet tall1 lived in the early half of the ,pleistocene period1 ranging over the warmer portions of ,north ,america1 including practically all of the ,united ,states and much of ,mexico2 ,the ,imperial ,mammoth1 reaching a height of more than thirteen feet1 and becoming extinct in the ,middle ,pleistocene4 ,it was a western form1 remains being found from ,nebraska to ,mexico ,city4 ,originally placed in the genus ,elephas1 the mammoths are referred to commonly as elephants1 though technically they should not be regarded as such4 ,recent explorations and researches have added greatly to our knowledge of these animals but have also caused much confusion with regard to scientific names1 for many new subdivisions of the larger group are now recognized1 and it has become necessary to change some of the older nomenclature4 ,illustration3 ,a ,long-,jawed ,mastodont 7,trilophodon phippsi74 ,one of the ,early ,american ,proboscideans ,the large mammoth exhibited by the ,museum bears the impressive name of ,archidiskodon meridionalis nebrascensis4 ,fifty years ago it might have been identified simply as a specimen of the imperial elephant and in such case would have received the old name of that species1 which was ,elephas imperator4 ,but late in the last century it was proposed that the mammoths be recognized by some other name to distinguish them more sharply from living elephants4 ,the name suggested for the new genus thus established was ,archidiskodon1 in recognition of the more archaic or primitive construction of the enamel plates in the mammoths molar teeth4 ,the specific name1 meridionalis1 had been given to a kind of mammoth which is well known from the southern part of ,europe1 and the ,latin name1 signifying 8southern10 had been applied to differentiate this species from the northern or woolly mammoth4 ,this mammoth1 however1 had disappeared from southern ,europe and for many years its subsequent history remained a mystery4 ,the late ,dr4 ,henry ,fairfield ,osborn had been engaged in an extensive study of the subject1 and when the nearly perfect skeleton from ,angus1 ,nebraska1 was brought to his attention he recognized it as being closely related to meridionalis1 and considered it to be a record of the migration of that species into ,north ,america4 ,because of minor variations from the typical mammoth of southern ,europe he regarded it as a variety or subspecies which had descended from the latter1 and the subspecific name1 nebrascensis1 was added to take care of this situation1 using a ,latinized form of the name of the ,state in which the skeleton was found4 ,with the knowledge we now have of these mammoths it becomes apparent that ,archidiskodon meridionalis nebrascensis is an ancestor of the imperial mammoth1 currently known as ,archidiskodon imperator1 and not identical with it4 ,this instance is typical of the manner in which prehistoric animals obtain their names4 ,although given a ,latin form1 these technical names are derived from many languages1 and the root words are applied with reference to anything that happens to appeal to the author as significant4 ,consequently there is seldom a name of this kind which may be translated directly into natural history or science4 ,it is a mistake to believe that these strange phrases conceal important technical information which is available only to those who are familiar with dead and foreign languages4 ,actually they contain nothing of the sort1 and the most enlightened of the ,greeks and ,romans could not find it there4 ,when a name is needed there is none better than the one provided by the specialist who is skilled in the business of naming things4 ,some technical ability is required1 to apply the name where it properly belongs1 but technical knowledge is not obtained from such sources4 ,names1 in any form1 have another purpose to serve4 ,there is no magic in them and there need be no mystery about them4 ,illustration3 ,molar ,tooth of ,mammoth ,this type of tooth is constructed for long continued use and will withstand the wear of more abrasive foods4 ,the position of the white enamel plates is seen in this view of the grinding surface4 ,these plates extend all the way to the base of the tooth1 which is of the long-crowned variety and not to be destroyed by the wearing away of a single outside layer of enamel4 ,other specimens in the ,museum collection are the long-jawed mastodonts1 so named because of the elongated jaws and protruding chin which is often mistaken for a tusk4 ,early members of this group had more cheek teeth than later types of mastodons1 and longer jaws were required for their accommodation4 ,some of them had flattened lower tusks which evidently were used for digging purposes4 ,these are popularly known as 8shovel tuskers40 ,the more modern ,american mastodon had shorter jaws and1 like the mammoths and elephants1 only one pair of tusks4 ,both the long-jawed and short-jawed types are represented by complete skeletons1 and also by tusks1 jaws1 and teeth of many individuals4 ,the ,american mastodons and mastodonts were of about the same size as the smaller mammoths4 ,the difference between mastodons and mammoths is most readily recognized in the structure of the grinding teeth1 the molars and pre-molars4 ,in the mastodon these teeth are of the short-crowned type1 while in the mammoths1 as in the modern elephants1 they are long-crowned4 ,the difference between these two types of molars has been described with reference to horses1 and the change from the older to the modern form may be regarded as coming about in the same general way1 through a series of gradual modifications4 ,in both horse and mammoth the final development shows internal enamel extending from the grinding surface nearly to the roots4 ,otherwise1 however1 there is almost no resemblance1 for the mammoth tooth is made up of flattened enamel plates1 the number of which is variable for different species4 ,in the jaws of a very young individual these plates may be seen as separate parts4 ,as the tooth continues to grow1 the plates become cemented together1 and when the ends of the plate are worn down it may be observed that each consists of a layer of enamel surrounding a flat central core of dentine4 ,the type of construction is rather more obvious in the mammoth tooth than in that of a horse1 partly because of the larger size1 and partly because of the relative simplicity of construction4 ,the earlier history of the ,proboscidea is not recorded in the rocks of ,north ,america1 for the group was of ,african origin and its migrations did not extend as far as the ,new ,world until middle ,cenozoic times4 ,the mastodons and mammoths were the largest of land animals since the ,age of ,reptiles1 but their ,old ,world ancestors were not conspicuous because of their bulk4 ,many of these ancient forms1 even in the earliest stages1 reveal some of the prominent characters that dominate the entire group4 ,none of them1 however1 should be regarded as a miniature mammoth or mastodon1 for these highly specialized types were perfected only at a comparatively recent date1 and by a process that works very slowly4 ,among the earlier forms there were also some oddities which failed to survive or to produce a successful branch of the stock such as the elephants4 ,illustration3 ,nebraska ,mammoth 7,archidiskodon meridionalis nebrascensis7 ,the earliest known member of the order was ,moeritherium1 an animal of the size of a tapir1 living in ,egypt during the late ,eocene and early ,oligocene time4 ,at this stage the characteristic specializations leading to the mastodons and mammoths were apparent but not far advanced4 ,the proboscis was probably much like the flexible snout of modern tapirs1 for the need of a long trunk had not yet arrived4 ,in upper and lower jaws the second pair of incisor teeth were becoming large and prominent4 ,the enormous tusks of the mammoths later developed from the enlargement of the same pair of upper incisors1 and in some of the long-jawed mastodonts the lower pair also produced large tusks1 though frequently the lower tusks were not prominent4 ,dinotherium had downward-growing tusks in the lower jaws1 none in the upper4 ,this genus was fairly common in the ,miocene of ,europe1 ,asia1 and ,africa4 ,in the tropics it survived throughout the ,pliocene and possibly into the ,pleistocene4 ,some of the species acquired the size of elephants1 but it is apparent that they were not ancestral to any of the more progressive types4 ,they are to be regarded rather as an offshoot from the main line of descent4 ,in #ahei only ten species of the elephant-like mammals were known1 and all were referred to a single genus4 ,at the present time eleven genera appear to be well founded1 and the number of recognized species has reached a hundred1 if it has not already passed that figure4 ,new discoveries are expected to add to the existing total4 ,with this mass of material before us we note certain definite trends among the more progressive types4 ,the increasing weight was accompanied by the development of strong1 upright limbs in which the bones have a columnar position instead of the angular assembly which prevails among most of the mammals4 ,as the tusks increased in size there was a shortening of both skull and neck to bring the weight closer to the point of support4 ,the front teeth disappeared except the second pair of upper incisors which remain as tusks in the modern elephant4 ,the cheek teeth present in the shortened jaws of the mammoth were reduced to one pair at a time in the upper set and another pair below4 ,from a simple1 low-crowned origin these grinding teeth developed into the more successful high-crowned pattern with numerous plates of enamel inside4 ,a prehensile upper lip acquired the length and usefulness of the elephants trunk4 ,illustration3 ,rancho la ,brea ,fossils ,one of the most unusual of the many animals that have been taken from the tar pits is the large ground sloth1 seen at the left in this group4 ,such sloths were very abundant during ,pleistocene time1 and some may have lived up to a few thousand years ago4 ,archaeologists have found indications that these creatures may have been hunted by cave-dwelling peoples of the ,american ,southwest4 ,other skeletons include the saber-tooth tiger1 characterized by the long curved upper canine teeth which undoubtedly were used for stabbing and slashing1 and the dire wolf1 the smaller of the two which are facing the sloth4 ,the artist's reconstruction of this scene also shows the great vulture1 ,teratornis1 which is the largest known bird of flight4 ,over-specialization in the production of tusks appears to have been the principal factor in the downfall of the mammoths4 ,the large size of the animals and the difficulties of finding sufficient food to sustain life must have been a serious handicap at times1 but their ability and inclination to travel over long distances enabled some of them to find tolerable living conditions until the end of the ,glacial ,period4 ,they are now extinct and the nearest living relatives are the elephants1 somewhat reduced in size of tusks and body but otherwise very similar4 ,there are many other tribes of mammals whose ancient history is partially known though broken by periods of time for which there is no fossil evidence4 ,all have undergone changes in which various forms and degrees of specialization are featured2 this general process is best revealed by the horses and elephant-like animals which have left a clearer record4 ,for other groups the story would differ but little except as to names and specific details4 ,,the ,,rancho ,,la ,,brea ,,fossil ,,pits ,the ,la ,brea tar pits1 as they are often called1 provide a remarkable record of ,pleistocene life in southwestern ,north ,america4 ,scattered over an area of about thirty acres just off ,wilshire ,boulevard in ,los ,angeles1 these bone deposits were known1 as far back as #ahge1 to contain the remains of prehistoric animals4 ,it was not until #aije1 however1 that their value was recognized by paleontologists4 ,in that year the ,university of ,california began an investigation1 and excavations were carried on at intervals by various institutions during the next ten years4 ,a great deal of material was acquired by the ,los ,angeles ,museum of ,history1 ,science1 and ,art1 where many skeletons1 skulls1 and other interesting specimens have been placed on exhibition4 ,the pits have the form of small craters formed by the seeping of oil from the underlying rocks4 ,the seeps appear to have been active during part of the ,pleistocene period but apparently not at the beginning4 ,the oil is rich in asphalt which has served as a preservative for the bones1 and owing to its sticky properties has been an effective animal trap for thousands of years4 ,the fossil beds at present are of oil-soaked earth and sand4 ,in past times there must have been a greater percentage of oil1 often concealed by a layer of dust or pools of water4 ,the large number of carnivorous animals found in the deposits suggests that they were attracted by the cries and struggles of creatures wandering carelessly into the asphalt and serving as live bait to keep the traps in continuous operation4 ,animals found there include many species still living in the locality1 some that have migrated to other territory1 and a large number that have become extinct4 ,among the latter may be mentioned species that differ but slightly from living relatives1 others that have left no descendants4 ,horses1 bison1 and wolves1 though extinct species1 were of relatively modern types4 ,on the other hand the large sloths and saber-tooth cats seem rather out of place4 ,true cats are represented by the mountain-lion1 bob-cat1 and a species of lion which is nearly one-fourth larger than any of the great cats of the ,old ,world4 ,a long-legged camel1 with a height of approximately eight feet to the top of the head1 was among the native animals of the district4 ,skunks1 weasels1 badgers1 squirrels1 rabbits1 bear1 deer1 and antelope were more or less abundant4 ,the ,la ,brea group exhibited by the ,denver ,museum of ,natural ,history includes the following species3 horse 7,equus occidentalis71 bison 7,bison antiquus71 wolf 7,aenocyon dirus71 saber tooth 7,smilodon californicus71 sloth 7,mylodon harlani74 ,horses had entirely disappeared from the ,north ,american continent by the time the first white man arrived4 ,equus occidentalis was one of the several species living during the ,pleistocene period1 this one apparently being restricted to ,california and perhaps adjacent states4 ,bison antiquus was slightly larger than the plains bison of recent times and had it horns set at a characteristic different angle4 ,the species was first described from ,kentucky and appears to have had a wide distribution4 ,the wolves in this group are about the size of timber wolves1 but have heavier skulls with less brain capacity1 massive teeth especially adapted to biting and crushing large bones1 and limbs of rather light construction4 ,they probably assembled in packs where meat was abundant and1 hunting in this fashion1 were able to attack and overcome the larger ungulates and edentates4 ,to most visitors the large ground sloth is the most interesting animal of the group4 ,this edentate animal is shown at the edge of the pool with one foot stuck in the 8tar40 ,the edentates are a group of primitive animals with very simple teeth1 if any4 ,teeth are usually lacking in the front part of the mouth1 sometimes entirely absent1 as among anteaters4 ,better known living representatives of the group are the tree sloths1 armadillos1 and anteaters of ,south ,america4 ,ground-sloths were prominent among ,south ,american mammals during much of ,cenozoic time4 ,during ,pliocene and ,miocene time there was a marked tendency to large size1 and it was principally during these two periods that they appeared in the ,united ,states area4 ,mylodon was one of the larger ,north ,american ground-sloths4 ,its teeth1 without the protective enamel which is present among higher mammals1 are restricted to the cheek region1 and have the form of simple pegs2 instead of being specialized they stand close to the extreme of generalization4 ,the construction of the entire skeleton is massive1 suggesting great strength with slow movements4 ,the hands are well developed1 provided with stout claws1 and must have served the creature well as protection against attacks by predatory neighbors4 ,we have some idea as to what caused the extermination of the ground-sloths in this particular region1 but the complete disappearance of such a large and widely distributed group at the close of the ,pleistocene period is a mystery that may never be explained4 ,illustration3 ,the ,folsom1 ,new ,mexico1 ,bison 7,bison taylori7 ,the saber-tooth cat1 sometimes referred to as a tiger1 was specialized as a meat eater though hardly as a hunting animal4 ,in the ,la ,brea region its principal food was probably the flesh of the sluggish ground-sloths4 ,the size was equal to that of the ,african lion1 with hind limbs slightly longer and the front legs more powerfully developed4 ,the most remarkable characteristic is to be found in the development of the upper canine teeth and modifications of the skull which were necessary to enable the animal to use these teeth as weapons4 ,in order to make the 8sabers0 effective it was necessary to get the lower jaws out of the way1 and this was provided for in an unusual type of hinge which enabled the mouth to open wider than is possible in the case of the less specialized carnivores4 ,judging by all the structural features of the skeleton1 ,smilodon could not have lived well on small animals1 for it was not equipped to capture that kind of prey4 ,it is evident that large mammals were preferred1 and that the method of attack was to spring upon the victim and cling there with the powerfully developed fore limbs until the kill was completed by stabbing into a vulnerable spot4 ,that the position of the large sabers near the front of the mouth interfered with normal feeding1 is a reasonable conclusion4 ,there are also anatomical features which lead to the belief that this carnivore was a blood sucker1 perhaps more than it was meat-eater4 ,if most of these conclusions are correct we have here another case of over-specialization and a possible explanation of the extinction of two species4 ,such evidence as we have is far from conclusive1 for there is no proof that ,rancho ,la ,brea was the last stand of either the saber-tooth or the ground-sloth4 ,both races were widely distributed and their living conditions could not have been exactly duplicated in other localities4 ,it has been suggested1 however1 that ,smilodon ate the last of ,mylodon1 and starved soon afterward because it had become unable to partake of other foods4 ,the conjecture is offered for what it is worth1 together with the facts on which the story has been based4 ,the geological record for ,pleistocene time is not as complete as one might imagine4 ,numerous localities have produced representative fossils but the yield is rarely large enough to solve many of the riddles which are constantly arising as investigation proceeds4 ,aside from those areas which bordered the retreating ice cap and where living conditions were far from favorable1 the sedimentary deposits of this period are not continuous over large areas4 ,many ,pleistocene fossils are found in stream channel beds which are always subject to removal by subsequent floods4 ,illustration3 ,early ,man in ,north ,america ,there is abundant evidence to indicate that the great elephants of ,pleistocene time were hunted by primitive ,americans whose only weapons were darts or spears tipped with points of stone4 ,a skull and the lower jaws of several mammoths are shown here4 ,isolated patches of fossil-bearing sediments frequently record the migration of animals in unmistakable terms1 but the details of the wanderings and the conditions encountered in the newly established habitats are often left in doubt4 ,to correlate the facts revealed at one locality with findings at other places and1 if possible1 to date all prehistoric events with a greater degree of accuracy are among the major tasks of current investigations4 ,,the ,,age ,,of ,,man ,the ,pleistocene or 8,ice ,age10 and the ,recent period in which we are living at the present moment are not sharply separated by any event readily recognized or dated1 and the two combined are of very short duration as compared with other periods more clearly established by the passing of centuries4 ,together they comprise the ,age of ,man as commonly recognized1 with about a million years representing the ,pleistocene period1 some ten to twenty thousand years the ,recent4 ,when geologists of the nineteenth century suggested that the coming of man should be regarded as the beginning of a new era1 the name ,psychozoic was proposed1 and to some extent this term has been applied to the present period4 ,more in keeping with other period names is ,holocene1 meaning entirely recent4 ,common usage1 however1 applies the simple term ,recent to this unfinished chapter which is also without a clear-cut beginning4 ,zoologically1 man is merely one of the creatures that arrived in the course of time1 along with other mammals4 ,just when he arrived and how he looked at the time of his coming cannot be determined from a study of fossils4 ,perhaps it is of no importance4 ,there is nothing to indicate his existence before the ,cenozoic1 no completely satisfactory proof of existence before the ,pleistocene period4 ,as with other inhabitants of the earth1 it is probable that he became prominent only after a great deal of competition with other creatures which kept his ancestors submerged for thousands of years4 ,the ,ice ,age1 with its check upon the progress of competing animals1 undoubtedly gave him an advantage4 ,his superior mentality enabled him to overcome adversity by methods not available to other mammals2 his inventive and mechanical genius must have been greatly strengthened by his experience during this interval4 ,at about this point1 where prehistory begins to merge into history1 the geologist and paleontologist must let other interpreters carry on4 ,archeologists and anthropologists take up the work1 and through their efforts many details have been added to our knowledge of the human race4 ,the study of biology1 which is the science of life1 has provided an instructive viewpoint that enables us to see ourselves against the vast background built up by investigations into the nature of the earth and its ancient inhabitants4 ,this science deals with living creatures as organisms--plants and animals so organized as to be capable of existence only in an environment which provides exact life requirements4 ,the ,age of ,man has been variously characterized as an age of soul1 of higher intelligence1 of culture1 and finally1 of civilization1 freedom and democracy4 ,the 8crowning glory0 of the organic world is pictured in history as a creature who has busied himself for thousands of years with the building up and tearing down of civilizations4 ,prehistory reveals this habit as something unique in the human character1 for there is no other organism that has specialized so persistently in the creation of its own environment1 no other that has had the combined power and talent to produce so much change4 ,more than anything else1 the prehistoric record is a lesson in adaptation1 which in its broadest sense means fitness for life under particular conditions1 and always subject to organic law4 ,man's efforts to bring about an adjustment between himself and his civilization have centered largely on the method of forcing himself into the mold that happens to be present1 one pattern today1 another tomorrow4 ,no creature of the past has had to adapt itself to anything so radically new or so thoroughly revolutionary4 ,the vital problem now is whether this man-made environment will prove helpful or disastrous4 ,though one of its names is 8culture10 it has grown sporadically and unevenly1 with little evidence of the cultivation that is implied and required4 ,parts have been expanded to extraordinary proportions while others equally essential have been retarded in their growth4 ,a more intelligent handling of this environment factor seems to be possible1 and the present mania for 8organization0 may become tempered with an awakening consciousness of organic requirements where organism and environment are involved4 ,once we grasp the idea that 8culture0 results from man's effort to improve his living1 by putting into his environment something that was not there before--then1 surely1 this history of a billion years of living1 and as many 8ways of life10 should teach us something we ought to know as we go into an all-out endeavor to teach a whole world how to obtain a one-and-only way4 ,we may stand at the beginning of an era for which an appropriate name has not yet been suggested4 ,civilization1 on the other hand1 may provide only a minor epoch to be added in some remote time to the story of fossils4 ,,supplementary ,,reading ,the literature pertaining to fossils is widely scattered and usually too technical for the layman4 ,it is better to use the resources of the nearest library than to feel that a specified list of books is necessary4 ,any textbook on geology1 zoology1 or botany will provide helpful information4 ,most books of this type will be found interesting and readable if used to solve definite problems suggested by the student's immediate curiosity4 ,very few can be read from beginning to end without a great deal of effort and discouragement4 ,the following have been prominent among the books consulted by the author3 ,textbook of ,geology2 by ,pirsson and ,schuchert4 ,this work has undergone several revisions and currently appears in two volumes3 ,physical ,geology by ,longwell1 ,knopf1 and ,flint2 ,historical ,geology by ,c4 ,o4 ,dunbar4 ,published by ,john ,wiley ,sons4 7,historical geology covers the entire range of prehistoric life--plant1 invertebrate1 and vertebrate47 ,historical ,geology 7,the ,geologic ,history of ,north ,america72 by ,russell ,c4 ,hussey4 ,published by ,mc,graw-,hill4 ,concise1 interesting1 and informative4 ,geology and ,natural ,resources of ,colorado2 by ,r4 ,d4 ,george4 ,published by the ,university of ,colorado4 ,contains an excellent summary of the historical geology and sedimentary formations of ,colorado4 ,vertebrate ,paleontology2 by ,alfred ,sherwood ,romer4 ,published by the ,university of ,chicago ,press4 ,this is one of the most comprehensive and up-to-date treatments of the subject for students desiring to go beyond the elementary stage4 ,a ,history of ,land ,mammals in the ,western ,hemisphere2 by ,william ,berryman ,scott4 ,published by ,the ,macmillan ,company4 ,this well-known account of living and extinct mammals is one of the favorites among students4 ,the ,age of ,mammals2 by ,henry ,fairfield ,osborn4 ,a classic in this field of literature1 but for advanced reading4 ,the book is now out of print4 ,the ,dinosaur ,book2 by ,edwin ,h4 ,colbert4 ,published by the ,american ,museum of ,natural ,history1 ,new ,york4 ,an illustrated story of amphibian and reptilian evolution4 ,down to ,earth2 by ,carey ,croneis and ,william ,c4 ,krumbein4 ,published by the ,university of ,chicago ,press4 ,an excellent popularization of the earth sciences--geology and paleontology4 ,lexicon of ,geologic ,names of the ,united ,states2 compiled by ,m4 ,grace ,wilmarth4 ,bulletin #hif 7in two parts7 of the ,united ,states ,geological ,survey4 ,a rich source of information concerning the age1 character1 and distribution of geologic formations1 with numerous references to fossil-bearing beds4 ,bibliography of ,north ,american ,geology 7including paleontology72 various bulletins of the ,united ,states ,geological ,survey4 ,where library facilities provide access to the technical literature of museums1 universities1 and scientific societies1 this is a valuable aid in locating publications dealing with original work in paleontology4 ,bulletins #gdf and #gdg cover the years between #aghe and #aiah2 ,bul4 #hbc 7#aiah-aibh72 ,bul4 #icg 7#aibi-aici72 ,bul4 #ich 7#aidj-aida72 ,bul4 #idi 7#aidb-aidc72 ,bul4 #ieb 7#aidd-aide72 ,bul4 #ieh 7#aidj-aidg72 ,bul4 #ifh 7#aidh72 ,bul #igg 7#aidi74 ,preparation is a continuous process with recent bulletins appearing at one or two year intervals4 ,ancient ,man in ,north ,america and ,prehistoric ,indians of the ,southwest2 by ,h4 ,m4 ,wormington4 ,published by ,denver ,museum of ,natural ,history1 ,city ,park1 ,denver #f1 ,colorado4 ,both volumes contain authentic and up-to-date accounts of early ,american cultures4 ,,maps ,geologic ,maps4 ,united ,states ,geological ,survey3 map of the ,united ,states 7#aicb72 map of ,colorado 7#aice74 ,geologic maps of a few other states are available2 information regarding these may be obtained from state universities or state geological surveys4 ,note3 ,bulletins of the ,u4,s4,g4,s4 are purchasable from the ,superintendent of ,documents1 ,washington1 ,d4 ,c4 ,maps are sold by the ,director of the ,geological ,survey1 ,washington1 ,d4 ,c4 ,transcriber's ,notes --,silently corrected a few typos --,restored one accidental omission in the ,table of ,illustrations --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,in the text versions only1 text in italics is delimited by underscores4