The marine invertebrates so resemble those of the Eocene that any general statement of the differences is difficult; these differences are, for the most part, of species only.
The Uinta contains large and numerous crocodiles, their last appearance in the northern interior, and a highly interesting mammalian fauna, which, however, is only partially known and demands further exploration. The great Uintatheres, which dominated the Upper Eocene, have disappeared, and the Peris-sodactyls have begun to decline in relative importance, though not absolutely; the small three-toed Horses continue to develop steadily; Rhinoceroses and Tapirs are abundant, and the Titanotheres increase notably in stature and in the prominence of their horns. The most characteristic feature of the Uinta fauna, however, is the great increase in the Artiodactyls, which then began to assume the place they have ever since held as the most numerous and important of the hoofed animals. In the Uinta the Artiodactyls mostly belong to a great and typically American group (Tylopoda), of which the camel and llama are among the few modern survivors. The most primitive known ancestor of the camels and llamas is found in these beds (Protylopus) associated with a curious extinct family, the Oreodonts, which were extremely abundant and varied throughout the American Oligocene and Miocene, and with other families of small, graceful animals, which throve also in the White River and John Day. This large assemblage of the Artiodactyls distinguishes the Uinta fauna very sharply from that of the Bridger.
The Creodonts are still common, though distinctly less so than, they had been in the Eocene, and the true Carnivora are beginning to replace them.
In the White River, or Middle Oligocene, the Crocodiles have become extremely rare, and only a dwarf species is known, but Lizards are much more numerous. The Mammals, which are preserved in astonishing numbers, resembled those of the Uinta, but had made great progress since that time. The Creodonts had almost disappeared, leaving only one or two curious genera (e.g. Hycenodon), while the Carnivora became abundant, Dogs, Sabre-tooth Cats, Weasels, and primitive Raccoons, being represented. The Lemurs and Monkeys have vanished from North America. The Perissodactyls continue to be abundant; the Horses are represented by the little three-toed Mesohippus, about as large as a sheep, the Tapirs by Protapirus, and Rhinoceroses by three very distinct series: thus, Metamynodon was a heavy, short-legged, aquatic animal, not unlike a hippopotamus in appearance; Ccenopus a more slender, terrestrial animal with the proportions of a tapir, and Hyracodon was a long-necked, long:limbed, lightly built, running type, yet still a rhinoceros.
The Titanotheres culminate in the massive, elephantine Titano-therium and its allied genera, which developed huge nasal horns (see Fig. 306) and died out early in the White River.
The Artiodactyls continue to increase; the native stock which came over from the Uinta age shows a distinct advance in development; the Camels (Poebrotherium) and allied families are very common, among them Protoceras, a very curious animal, the male of which had four horns, and a pair of tusks in the upper jaw, while the Oreodonts must have covered the plains in great herds, so abundant are their remains. The Peccaries, or American representatives of the pigs, are not yet known from the Uinta, but occur in the White River (Perchcerus), and the extraordinary, long-limbed, two-toed, pig-like Elotherium may have descended from Uinta ancestors, or may have been a migrant from the Old World, as certainly were the members of the European family of Anthracotheres (Anthracotherium and Hyopotamus) which appear in the White River beds; there is nothing like them in the Uinta. The Rodents of the White River are much more numerous and varied than they had been before; Marmots, Squirrels, Beavers, Mice, Pocket-gophers, and Rabbits were already well established.
Fig. 306. - Drontotherium dolichoceras Scott and Osborn. Restoration by C. R. Knight, under the direction of Prof. H. F. Osborn.
(Copyright by the American Museum Natural History, N.Y).
The Mammals of the John Day are much like those of the White River, but are more advanced and modernized, and some ancient groups have vanished, among them the Creodonts, the aquatic and cursorial Rhinoceroses, the immigrant Anthracotheres, and the huge Titanotheres. On the other hand, the Carnivora, especially the Dogs and Sabre-tooth Cats, greatly increase in numbers and diversity, and the same is true of the Rodents. The Horses and true Camels are larger than those of the White River, as are also the Oreodonts, but the Rhinoceroses are reduced to the two-horned Diceratherium.
The Oligocene Mammals of Europe have much in common with those of North America, but there are many local differences. In Europe, the Weasels were much more varied and common than in America, and the Civet-cats, a family which never reached this continent at all, were well represented. Some families of Peris-sodactyls, such as the Palaeotheres, and a host of Artiodactyl Tamilies, some extinct, like the Anoplotheres, Cainotheres, and Xiphodonts, and the true Ruminants, were then peculiar to the Old World.
The disappearance of the crocodiles from the northern interior seems to show that the climate had grown rather cooler, though on the Atlantic coast warm-water conditions still continued, and the vegetation shows that Europe still had a subtropical climate, palms growing up to the north of Germany. The Kenai beds of Alaska contain a temperate vegetation, and probably the leaf-bearing beds which are distributed so generally around the Arctic Sea and have yielded similar plants, should be referred to the Oligocene, though they are usually called Miocene.