A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 138.—Horned †toxodont (†Leontinia gaudryi), Deseado stage. Restored from a skull in the Ameghino collection.

The †Entelonychia, those strange toxodont-like animals with claws instead of hoofs, were much more numerous and varied than they were afterward in the Santa Cruz, when they were on the verge of extinction, and included both very small and very large species. The †Pyrotheria, a suborder which is not met with in the Santa Cruz or later formations, likewise included some very large forms. The typical genus, †Pyrotherium, included large, relatively short-legged and very massive animals; the upper incisors formed two pairs of short, downwardly directed tusks, and in the lower jaw was a single pair of horizontally directed tusks; the grinding teeth were low-crowned and had each two simple, transverse crests. These grinding teeth and the lower tusks so resemble those of the ancestral Proboscidea in the Oligocene of Egypt, that the †pyrotheres have actually been regarded as the beginnings of the †mastodons and elephants, but this is undoubtedly an error. The †Astrapotheria, another group which became extinct at or soon after the end of the Santa Cruz, were relatively abundant in the Deseado and counted some very large species. Finally, the †Litopterna were represented by the same two families as continued through the Pliocene and one of them far into the Pleistocene. The horse-like †proterotheres were present, but not enough of them has been obtained to show whether or not they were in a notably less advanced stage of development than those of the Santa Cruz. The †macrauchenids were quite similar to those of the latter formation, though considerably smaller. In addition, there were a few genera, survivals from earlier times, which were not referable to either of these families.

The large number of genera, especially among the †toxodonts and †typotheres, which had high-crowned, cement-covered teeth, may be taken as an indication that grazing habits had already begun to be prevalent.

Of this wonderful assemblage of hoofed animals, divisible into six separate groups, whether of ordinal or subordinal rank, not a trace remains to-day. Not only are all the species, genera and families extinct, but the suborders and orders also. Further, this was a very strictly autochthonous fauna, so far as the hoofed animals were concerned, and no member of any of the six groups has ever been found outside of the Neotropical region.

4. Eocene

North America.—In the western interior of North America the Oligocene followed so gradually upon the Eocene, that there is great difficulty in demarcating them and much difference of opinion and practice obtains as to where the boundary line should be drawn. Not to depart too widely from the scheme used by Professor Osborn, the Uinta stage is here treated as uppermost Eocene, though this is a debatable procedure. For several reasons, the extraordinarily interesting and significant Uinta fauna is far less completely known than that of the preceding Bridger and succeeding White River stages. For one thing, it has been much less thoroughly explored, and it may be confidently expected that future exploration will greatly enlarge our knowledge.

The smaller mammals of the Uinta are particularly ill-known. No Insectivora have yet been found, though this gap will assuredly be filled; rodents are scanty in the collections and include only two families, one the †ischyromyids, which were still common in the White River, the other of doubtful position, but not improbably to be considered as the beginning of the pocket-gophers (Geomyidæ). The archaic flesh-eaters, or †Creodonta, were represented by two families, one comprising smaller animals with somewhat cat-like, shearing teeth (†Oxyænidæ), the other, very large beasts with crushing teeth (†Mesonychidæ), neither of which continued into the White River. As compared with the middle and lower Eocene, the †creodonts had greatly diminished and, to replace them, the true Carnivora were beginning to come in. As yet, however, only small and very primitive dog-like forms are known and no trace of †sabre-tooths or mustelines has been found. Indeed, it is very doubtful whether members of these families ever will be found in the Uinta, for their presence in the succeeding White River was probably due to immigration.

The Perissodactyla were the preponderant type of hoofed animals, and ancestral forms of most of the White River genera have already been identified. The †titanotheres (†Diplacodon, †Protitanotherium) were much smaller and lighter than those of the lower White River and had much shorter horns. The †hyracodonts, the lightly built, cursorial rhinoceroses, were represented by a genus (†Triplopus) which was smaller and more slender than the White River form (†Hyracodon) and its teeth were of distinctly more primitive character. The heavy, massive and presumably aquatic †amynodonts (†Amynodon) were likewise smaller and less specialized than their descendants of the Oligocene. No member of the true rhinoceros series has yet been identified in the Uinta, but there is some reason to think that they were nevertheless present. Tapirs are distinctly indicated by certain fossils, but they are still too incompletely known to make possible any statement as to their degree of development. The horses (†Epihippus), like the other families mentioned, were much smaller and distinctly more primitive than their successors in the Oligocene.

The Artiodactyla were, for the first time in the history of North America, as numerous and as varied as the perissodactyls and, with the exception of the peccaries and †anthracotheres, representatives of all the White River families are known. The finding of the peccaries is merely a question of further exploration, but the †anthracotheres were migrants from the Old World, and there is no likelihood that they will be discovered in the Uinta at any future time. Fairly large, pig-like animals, probably referable to the †giant-pigs or †entelodonts, occurred, but nothing has yet been found which can be considered as the direct ancestor of the White River genus. As was true of the perissodactyls, the Uinta artiodactyls were nearly all much smaller and more primitive than their Oligocene descendants and the differences are most interesting from the evolutionary point of view. The ancestral camel (†Protylopus) was a little creature no bigger than a fox-terrier, though the †hypertragulids (†Leptotragulus) were as large as †Leptomeryx and †Hypertragulus of the White River. The most ancient known members of the †oreodonts (†Protoreodon) and the †agriochœrids (†Protagriochœrus) are found in the Uinta.

The middle Eocene fauna, Bridger stage, though it passed upward very gradually into that of the Uinta, was yet, on the whole, very different from the latter. It was exclusively indigenous and so radically distinct from the mammals of corresponding date in Europe as to preclude the possibility of a land-bridge with that continent. In the lower Eocene, as will be shown in a subsequent page, the communication between the two continents was broadly open and the faunas of the two continents were much more closely similar than they have ever been since. It is really remarkable to see with what comparative rapidity the two regions, when severed, developed different mammals under the operation of divergent evolution. Had the separation continued throughout the Tertiary and Quaternary periods, North America would now have been as peculiar zoölogically as South America is, a result which has been prevented by the repeated renewal of the connection.