Part 6

As in other eras, the invertebrates fluctuate with the periods. Characteristic forms appear, become more or less prominent, then in many cases decline or disappear. Variations among the mollusks are particularly helpful in the identification of rocks which originated in the Cretaceous seas. Clam-like bivalves of the genus _Inoceramus_, the straight-shelled ammonids known as _Baculites_, and oysters, are locally common in some of the formations exposed a few miles west of Denver.

The ammonids, or “ammonites,” were extremely abundant throughout the world during Mesozoic time. Their shells were chambered like those of the pearly nautilus, a related cephalopod mollusk inhabiting tropical seas at the present time. While only four species of the Nautilus tribe are living today, thousands of species of ammonids swarmed the prehistoric seas. Many new forms came into existence in Triassic time but few survived the period. A pronounced revival occurred in the Jurassic, only to be followed by a decline and eventual extinction at the close of the Cretaceous. Ammonites measuring three or four inches across the diameter of the coiled shell were about average size, but diameters up to three or four feet were not uncommon. Externally the shells were ornamented with ribs, knobs, and spines; inside was a pearly lining. The partitions were thin and composed of the same pearl-like substance as the lining. Each partition becomes wavy as it approaches the shell, and the line of union has a distinct pattern which is seen in specimens which have lost the outer shell layer. This wavy suture line becomes more complicated in the later members of the race, and the peculiar markings developed by the repeated partitions provide a convenient method of identification.

The belemnites or ink-fishes, regarded as ancestors of the cuttlefishes now living, comprise another group of carnivorous mollusks. These, however, had lost the external shell, and the usual fossil is part of an internal shell or “skeleton,” known as the guard. This limy structure has the form of a pointed cigar, and is seldom over a foot long although the total body-length of the larger animals was commonly about six to eight feet. Several hundred species have been described, the majority being of Jurassic age. They declined rapidly toward the close of the era.

The invertebrate life of the Mesozoic was strongly dominated by mollusks, with cephalopods in first place, the bivalve pelecypods and the single-valve gastropods or snail-like forms sharing subordinate positions. The dominating trilobites, sea-scorpions, and tetracorals of the Paleozoic had disappeared, while the brachiopods and crinoids were greatly modified and more like the forms which live today.

Crinoids became moderately abundant at various times, but in many ways different from their relatives of the preceding era. Some of the largest known species, with stems estimated as fifty feet long, have been found in lower Jurassic rocks. A great abundance of microscopic life is indicated by the frequent occurrence of chalk in the Cretaceous formations. Corals of the modern reef-building type (hexacorals) were common in the warm seas of a large part of the world.

The insects of the early Mesozoic are represented by few fossils although it is evident some new forms were becoming established. The warm climates prevailing throughout much of the world appear to have been a favorable factor in the progress of insect life. In addition to the older cockroach and dragon-fly types may be mentioned the arrival of grasshoppers, cicadas, caddis-flies, beetles, and ants.

Several hundred species are found in Jurassic rocks, and by the end of the Cretaceous period most of the insect families now known to us were probably in existence. The record is seriously obscured by the erosion of rocks which so frequently marks the end of a period, also by the small size of the subjects, and by the exceptional conditions required for the production of such delicate fossils. Among the last of the familiar insects to appear were the bees and butterflies. These evidently came in with the more advanced types of flowering plants that produce the nectar on which many insects feed. It is probable, too, that without the arrival of these insects and their service in the pollination of flowers, the floras of today would be rather different from what they are.

EXTINCT BIRDS

It is not surprising to find that birds made their first appearance in the Mesozoic era, for of all animals they are most like the reptiles as a class. Feathers are about the only dependable characteristic of the entire group, nearly every other feature being matched by some reptilian creature of great antiquity. The nesting habit, which includes care of the young as well as the eggs, is a matter of progress which relates to flight and to warmer body temperature. It appears to have been developed by forest-dwelling types living among trees and nesting there in comparative safety from enemies prowling on the ground.

The oldest known prehistoric birds were found in lithographic stone of Upper Jurassic age. _Archaeopterix_ was discovered in 1861 at Solenhofen, Germany. Sixteen years later a similar bird in a better state of preservation was found in Bavaria. The latter was named _Archaeornis_. These Jurassic fossils are regarded as true birds by some authorities, while others believe them to be more nearly related to the reptiles, the opinions being based on careful studies of the skulls and other skeletal features. Both birds had teeth of reptilian character, and it is evident that there was no beak, for the jaws were covered with scaly skin. The bony construction of the long tail would suggest lizards rather than birds, were it not for the presence of feathers which were attached at each side. Head, neck, and parts of the body were covered with scales. Wings were well provided with stout feathers but the skeletal framework indicates that the birds were gliders rather than true flyers. Claws on the wings served like fingers to aid in climbing among the branches of trees, a practice which is occasionally noted among the young of living birds. In adult birds of today, however, the claw-like appendages of the fore limbs are greatly reduced and of little service.

The next fossil birds of importance have been found in Cretaceous rocks of Kansas, both of them fishers of the seas instead of forest inhabitants. _Ichthyornis_ was a small bird, standing about eight inches in height, a powerful flyer with reptilian jaws and teeth. _Hesperornis_ was built for diving and swimming, like the loon, but was somewhat larger and provided with teeth. Its wings were too poorly developed to be of use in flying.

Toothed birds became extinct with the close of Cretaceous time, and the ancestors of modern types were in existence before the Age of Mammals, but fossil remains are few and poorly preserved. Large ostrich-like birds, however, are known to have lived in North America during the Eocene period. One of these, named _Diatryma_, stands nearly seven feet tall in the reconstructed skeleton. Its legs are heavy, wings greatly reduced, beak massive. In its relation to modern birds it is possibly nearer to the cranes than the ostriches.

Flightless birds of large size are known from many parts of the world and seem to have been prominent throughout the Cenozoic era, as they are today in the southern hemisphere. _Aepyornis_ lived in Madagascar during the Pleistocene period and may have become extinct quite recently. Its eggs are the largest known among fossils, several times the size of an ostrich egg. Also in this period the moas were living in New Zealand where their remains are still abundant. One of the largest, known as _Dinornis_, had about the same form as _Diatryma_ but the neck was longer, head and beak smaller, legs better fitted for running, height about eleven feet.

A much smaller flightless bird, the dodo, became extinct in modern time. This former inhabitant of Mauritius and other islands of the Indian Ocean was related to the doves and pigeons, and had lost its power of flight through disuse of the wings. It was a clumsy, defenseless bird weighing possibly as much as fifty pounds. Actual remains are few and incomplete, and descriptions published by the explorers who knew the bird two centuries ago are not entirely trustworthy. In the Pleistocene Rancho la Brea beds of California the largest of all prehistoric flying birds has been found, a vulture bearing the name of _Teratornis_. Re-assembled skeletons show them to be slightly bigger than existing condors.

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 fossils. Marsupials, the next higher living group, which includes the opossum and kangaroo, appeared at the end of Cretaceous time along with the placentals or higher mammals which dominate the history of the Cenozoic era. Nevertheless, there are a few teeth and jaws from rocks of Triassic and Jurassic age to indicate that small mammals, from the size of mice to slightly larger than rats, existed throughout most of the Age of Reptiles. There is no complete skeleton of any of the earlier forms, and little is known of their relationships either with living orders of mammals or with probable ancestors among the reptiles. The record becomes somewhat clearer toward the end of the era but it is obscured again by the great disturbances which followed.

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 time. Reptiles then living had many structural features in common with mammals, and mammal-like forms continued to flourish until late in the Triassic. An interesting group of such animals, named therapsids, was one of the earliest reptilian stocks to appear, and is well known from fossils found in the Red Beds of Texas and New Mexico, in Europe, South Africa, and Asia. Quite a variety of types is included in this group, with many advances in dentition, and modifications of the skull, limbs and pelvic construction which strongly suggests a relationship to the mammals.

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 races. The division of the era into periods, however, was based largely on a study of fossil mollusks. In the Paris basin of France, 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 species. 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 series. From the percentage of recent forms among prehistoric ones it was proposed that the following division be made: Eocene, meaning _dawn of the recent_; Miocene, meaning less recent; and Pliocene, meaning _more recent_. Sometime later it was suggested that another period be added, and to this was given the name Pleistocene, meaning _most recent_. In 1854, the older Miocene formations were segregated and referred to a newly provided Oligocene period, this name meaning _little of the recent_.

Early geologists grouped the rocks in three great divisions, applying the names Primary, Secondary, and Tertiary. To these was added afterwards the name Quaternary, which applied to the youngest formations of the earth. Only two of these terms remain in common use at present: it is a frequent practice to refer to the combined Eocene, Oligocene, Miocene and Pliocene periods as the “Tertiary” division of Cenozoic time; to the Pleistocene and Recent periods as the “Quaternary” division. 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 parts. It may be that a great era was concluded at the end of Pliocene time as others have been concluded, by the usual earth disturbances and climatic changes and by the decline of animals once prominent in the faunas of the world. 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 divisions. 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 concluded.

The oldest of Eocene rocks show a great variety of mammals and a strange assortment of forms far in advance of the Mesozoic record. Ancestries and successive stages of development have been only partially worked out, though details have been better preserved for some of the groups than for others. 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 period. These animals, represented in our collection by _Uintacolotherium_, acquired large bodies and many horns, but a peculiar tooth equipment fitted them for a special diet which apparently failed to be supplied in sufficient abundance at a critical time.

On the other hand, we find in this period the ancestors of more successful groups, some of which continue on into modern times. Only a few of these histories can be traced in a brief sketch, 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 now, by a process of adjusting themselves, or by becoming better adapted to new or previously unused conditions in their respective environments. In their early stages the various types had much in common; they were generalized, rather than specialized for any particular kind of existence. What they were fitted for is best indicated by their teeth and feet, though other structural features frequently contribute valuable information. The rise of mentality is indicated by skull capacities and the increasing development of the upper lobes of the brain, as revealed by casts taken from the interior of skulls.

The creodonts were the earliest and most primitive of the flesh-eating mammals or carnivores. Many of them were small of body and brain, and equipped with teeth that indicate a mixed or largely insectivorous diet, or possibly the habit of feeding on carrion. Although there was considerable variation among them, and some tendency toward specialization, there was little to suggest the coming of more progressive groups such as the cats and dogs, with teeth perfected for the tearing and cutting of flesh, and feet especially fitted for the life of hunters. The ancestry of the cats cannot be traced farther back than the Oligocene but it probably connects somewhere prior to that time with the creodonts.

_Cynodictis_, an Oligocene carnivore slightly under two feet in length is commonly regarded as a primitive dog, but its characters are so generalized that it probably differs but little from the ancestors of many other carnivores. The skeleton of this animal suggests a slender and flexible body like that of the weasel, with somewhat shortened limbs and a long tail. It lived in forested regions and was probably more or less of a tree dweller. The more advanced carnivores required longer legs, better adapted for running and overtaking the prey, which is the dog’s way of hunting, or for stalking and springing upon the quarry, which is the method of the cat.

A prominent group of mammals today is that known as the ungulates, or hoofed animals, which includes the horses, cattle, deer, swine, rhinoceroses, tapirs, and other types both living and extinct. Their probable ancestors were the condylarths, primitive ungulates of the Eocene period. One of these, known as _Phenacodus_, serves well to illustrate the general character of the early hoofed mammals. It was about five and one-half feet long, rather large for its time, with long tail and short limbs, low elongated skull and small brain, very similar in many respects to the creodonts or ancestral carnivores. The teeth, however, were partially of the grinding type so essential to the welfare of plant feeders.

The condylarths were five-toed animals and evidently provided with small hoofs, but the more progressive ungulates soon lost one or more of the toes, and a division of the group into odd-toed and even-toed branches became firmly established. Consequently, the families of ungulates having one, three, or five toes are classed together as being closely related to one another, and those having two or four toes are segregated in a second lot. The odd-toed clan, known as perissodactyls, included such animals as the horses, rhinoceroses, tapirs, and titanotheres, each of these types being placed in a separate family. The even-toed clan has been treated in a similar way and named the artiodactyls. In this division are such families as swine, cattle, deer, camels, oreodonts, and others. 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 flourishing, although some of the older families have become extinct.

Among the exhibits of the Denver Museum of Natural History may be seen complete skeletons of extinct horses, rhinoceroses, titanotheres, and chalicotheres representing the perissodactyls. _Moropus_ was one of the chalicotheres, 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 America. The family is grouped with the ungulates because of many similarities found in the molar teeth, skulls, and other parts of the skeletons, but the toes were provided with claws instead of hoofs. The use of these claws is somewhat of a puzzle: possibly for defense against carnivorous enemies, for dragging down branches in order to obtain food, or for digging roots which may have been an important part of the diet.

Titanotheres are represented in our collection by the skeletons of the large, horned type which was the last of the race and destined to extinction by the middle of the Oligocene period. Smaller hornless varieties of Eocene time are illustrated by skulls. This family of ungulates had an unprogressive dental equipment, and a small brain in a flattened skull. The molar teeth readily distinguish the group from other ungulates and enable us to trace the relationship between earlier and later varieties. These teeth were of a type which is soon destroyed by wear, and it is evident that the animals survived only so long as their environment provided them with an abundance of soft vegetation.

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 continent. Some of the mounted skeletons have been restored on one side to show how these animals appeared in the flesh.

Of the even-toed ungulates there are also several types illustrated by complete skeletons. _Merycochoerus_, the subject of one of our mounted groups, represents the oreodonts, a large family of mammals whose history begins with the Upper Eocene and ends in the Lower Pliocene. The oreodonts were small animals, rather pig-like in form and quite common in the western plains region shortly after the time of the titanotheres. Ancient swine are represented in our exhibits by two mounted skeletons which were obtained from northeastern Colorado, where the bones were found associated with rhinoceros and titanothere remains. Some of these animals were of very large proportions, and the entire family is commonly known as the “giant pigs.”

Camels and closely related forms were quite abundant in North America from early Oligocene to comparatively recent time. Numerous types were developed during the course of their history, some small and delicately formed, others tall and clumsy and much like the giraffe in structure. Parts of many of these creatures have been found but the only completely prepared skeletons in our collection are of the little gazelle-camel, _Stenomylus_, from Lower Miocene deposits in northwestern Nebraska. Pleistocene bisons are represented by several complete skeletons and numerous skulls and horncores, some of the species showing an extreme development in the length of horns. With two of the bison skeletons are shown prehistoric weapon points, found with the bones and indicating that these animals were hunted by primitive men at some time near the close of the Ice Age. The artifacts first discovered near Folsom, New Mexico, by field workers of our Museum, have become known to archeologists as Folsom points.

PREHISTORIC HORSES

The past history of horses is well known from an abundance of fossil material, ranging in age from the Eocene down to the present. Modern horses have only one toe in each foot, but there are remnants of two additional toes which may be seen only in the bony structure underlying the skin. Most of their ancestral relatives were three-toed as far back as the Oligocene period. During Eocene time, however, there was a stage which may be regarded as four-toed although it was evidently a temporary condition, linking known horses with more remote forms having five toes.