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.
Skulls of the clumsy, six-horned uintathere and the early, hornless titanothere form part of this Eocene display. In the mural these animals and the little “three-toed” Eohippus, smallest of horses, are pictured with a contemporary turtle and crocodile.
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.
Moropus (Moropus cooki)
Though its teeth clearly indicate a diet of plant material, this strange animal had claws on its toes, like the carnivores. It is probable that the claws were used in digging for roots, as indicated by the artist.
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.
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 periods. Ancestral titanotheres, dating back to the Eocene, were hornless animals of much smaller size. These splendid specimens were obtained in Weld County, Colorado.
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.