The aspect of the White River fauna changes in accordance with the direction from which it is approached. If one comes to the study of it from the Eocene, it displays a very modern aspect, given by the almost complete disappearance of the archaic groups of mammals and by the great multiplication of genera and species belonging to the progressive orders. These genera, it is true, are all extinct, but many of them stood in an ancestral relationship to modern forms. On the other hand, if approached from the Miocene side, the White River mammals seem to be very ancient and primitive and very different from anything that now lives. We speak of horses and rhinoceroses, dogs and cats, in this fauna, but those terms can be employed only in a very wide and elastic sense to designate animals more or less distantly allied to those of the present day.
Fig. 133.—1. †Archæotherium. 2. Ancestral camel (†Poëbrotherium). 3. †Merycoidodon. 4. †Agriochœrus. 5. Ancestral horse (†Mesohippus). 6. †Hoplophoneus. 7. †Bothriodon. 8. †Hyænodon. 9. †Cursorial rhinoceros (†Hyracodon). 10. †Protoceras. 11. Hornless rhinoceros (†Cænopus).
Several species of opossums, some of them very small, were the only marsupials in North America then, as they are now. There was quite a variety of Insectivora; some were survivals of a family that was abundant in the Eocene, others, like the hedgehogs, moles and shrews, were probably immigrants. Here we find the last of a group (order or suborder) of ancient and primitive flesh-eaters, the †Creodonta, that had played a great rôle in the Eocene and Paleocene of North America and Europe. In White River times but a single family (†Hyænodontidæ), with two genera, remained of the Eocene host. One of these genera (†Hemipsalodon), a very large beast of prey, which was almost identical with the Old World genus †Pterodon, was confined to the lower substage of the White River beds in the Northwest Territory of Canada; the other, †Hyænodon, which was also an Old World form, was represented abundantly in the United States by many species. In size, these species ranged from a small fox to a large wolf, but they all had disproportionately large heads, and small, weak feet, with blunt claws, so that they must have been very curious-looking creatures and were probably carrion-feeders rather than active catchers of prey. The White River members of the family were migrants from the eastern hemisphere, for, though small and primitive representatives of it occurred in the North American Eocene, as well as in the corresponding formations of Europe, the family appears to have died out in America and to have been renewed by the Oligocene migration.
Fig. 134.—White River †titanothere (†Titanotherium robustum) reduced to the same scale as [Fig. 133].
Coincident with this decline of the †creodonts and, no doubt, causally connected with it, was the rise of the true Carnivora, which for the first time were numerous and were divisible into three distinct families. Small and primitive representatives of the wolves (†Daphœnus) and possibly also of the foxes (†Cynodictis) were quite common, and there were a few species of the musteline family, evidently immigrants and the most ancient yet found in America. There were several species of the †sabre-tooth cats (†Dinictis and †Hoplophoneus) all of which, except in the uppermost substage, were quite small, few of them exceeding the Canada Lynx in size. A much larger animal (†Eusmilus, also European) appeared in the latter part of the stage. None of the true cats, or feline subfamily, has been obtained. Nothing is yet known of the time and place of origin of the †sabre-tooth series, for they appeared at approximately the same date in Europe and America, and in neither continent have any possible ancestors been found in preceding formations. The problem is like that of the Proboscidea (see [p. 234]), but Egypt has given no help in the case of the †sabre-tooths, and, by a process of elimination, we reach the conclusion that these strange creatures probably arose somewhere in Asia and sent out migrants eastward and westward.
The Rodentia were fairly abundant and present a strange mixture of ancient and comparatively modern types. One very common genus (†Ischyromys), which was the last remnant of a family almost limited to the North American Eocene, was associated with the earliest American mice, arboreal and ground squirrels, beavers and rabbits; some, if not all, of these were immigrants.