Eocene mammals, especially those gigantic whale-like creatures called Zeuglodon ([Fig. 180]), have been found in Eastern North America, but the most remarkable discoveries have been made in the Western Territories, where vast numbers of bones are imbedded in certain ancient and wide-spread lacustrine beds. It may be well to premise here that though the division into Eocene, Miocene, and Pliocene is recognised in America as well as in Europe, the limits of these groups may not precisely correspond with those in the Old World. Still we have this certain point of departure, that the Eocene begins where the peculiar animals of the Cretaceous end, and that the drying up of the later Cretaceous sea and the establishment of the Eocene land were probably nearly contemporaneous in both continents. It is true, however, in animals as in plants, that in the successive periods of the Tertiary, America presents an older aspect than Europe, just as its modern fauna still contains such old forms as the opossum.
It would seem that as the mountain-ranges and table-lands of Western America emerged from the Cretaceous waters, they became clothed with Eocene forests and inhabited by Eocene mammals. But the waters, dammed up by surrounding ridges, formed large lake basins, which were drained only by the slow excavation of “cañons” as the land rose still higher. In the successive deposits formed in these lakes both by ordinary deposition of silt and by paroxysmal showers of volcanic ashes were entombed great numbers of the animals which fed on their banks. It appears that these deposits, which in some places are estimated at not less than 8000 feet in thickness, hold the remains of three successive faunas, differing materially from each other, and representing the Lower, Middle, and Upper Eocene. On the flanks of the elevated region supporting the beds formed in the Eocene lakes, are other later lake basins of Miocene age, also abounding in animal remains. East of the Rocky Mountains, and also on the Pacific coast, are still later Pliocene deposits holding other and more modern Mammalia. The vast area of these formations and the complete sequence which they show are scarcely equalled elsewhere.
Fig. 171.—Coryphodon Hamatus. A Lower Eocene Perissodactyl skull, greatly reduced, showing small size of brain, a.—After Marsh.
As in the Paris basin, the large Ungulates constitute the most conspicuous feature. The great group is now usually divided into those that are odd-toed (Perissodactyl) and those that are even-toed (Artiodactyl). Though these are apparently arbitrary characters, they correspond with other more fundamental differences. The first includes such modern animals as the Rhinoceros, Tapir, and Horse. The second includes two somewhat distinct assemblages—that with mammillated teeth, of which the Hog and Hippopotamus are types (Bunodonts), and that with crescental plates of enamel in the teeth, of which the Ruminants like the Deer, Ox and Camel, are examples (Selenodonts).
Fig. 172.—Fore-foot of Coryphodon. Greatly reduced.—After Marsh.
The most characteristic animals of the lowest Eocene belong to the genus Coryphodon ([Figs. 171, 172]), which so abounded in Eocene America that bones of about 150 individuals were found by the Wheeler Expedition in one year in the Eocene beds of New Mexico. These animals in their dentition approached the American tapirs, except that they had great canines like the bear, while their feet resembled those of the elephant, and some of them attained the dimensions of the ox. Coryphodon is thus, as might be expected in a primal placental mammal, a creature of somewhat generalised type. Another point in which it resembles some at least of its early Tertiary contemporaries is the small size of the brain, especially in those parts of it supposed to minister to the intelligence and higher instincts ([Fig. 171], a). It is certainly remarkable that as Tertiary time went on the successive groups of mammals were gifted with brains of larger and larger size, fitting them for higher functions; and ultimately for associating with man. Animals thus low in development of brain were probably slow and sluggish and stubbornly ferocious, and dependent on brute force for subsistence and defence; and they would have been altogether unsuitable for domestication had they lived to the present time.