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.