A history of land mammals in the western hemisphere

by William Berryman Scott

In brief, the whole structure of the horses is pre-eminently adapted to swift running; they are admirable “cursorial machines,” as they have been called, and every part of the skeleton has been modified and specialized to that end; the narrow, rigid hoofs fit them for walking on firm ground and they speedily are made helpless in quicksand or bog. Did we know nothing of their mode of life, we might confidently infer from their teeth that the horses were grazers, feeding principally upon grass. A long-legged, grazing animal must needs have a neck of sufficient length to enable the mouth to reach the ground easily, unless a long proboscis is developed; and so we shall find in the history of the horses that the elongation of the head and neck kept pace with the lengthening of the legs and feet.

Though it can hardly be doubted that the horses passed through most of their development in North America, yet the immediate ancestry of all the existing species must be sought in the Old World, none of the many Pleistocene species of the western hemisphere having left any descendants. In North America all of the known Pleistocene forms belonged to the genus Equus, but the True Horse, E. caballus, was not among them. The more abundant and important of these species have been sufficiently described in Chapter VII ([p. 199]); it need only be recalled that there were ten or more distinct forms, ranging in size from the great E. †giganteus of Texas to the minute E. †tau of Mexico, while the plains and forests were the feeding grounds of moderate-sized species, about 14 hands high.

In the latest Pliocene, and no doubt earlier, species of the modern genus Equus had already come into existence; and in association with these, at least in Florida, were the last survivors of the three-toed horses which were so characteristic of the early Pliocene and the Miocene. However, little is known about those earliest recorded American species of Equus, for the material so far obtained is very fragmentary. In the absence of any richly fossiliferous beds of the upper Pliocene generally, there is a painfully felt hiatus in the genealogy of the horses; and it is impossible to say, from present knowledge, whether all of the many species of horses which inhabited North America in the Pleistocene were autochthonous, derived from a purely American ancestry, or how large a proportion of them were migrants from the Old World, coming in when so many of the Pleistocene immigrants of other groups arrived. It is even possible, though not in the least likely, that all of the native American stocks became extinct in the upper Pliocene and that the Pleistocene species were all immigrants from the eastern hemisphere, or the slightly modified descendants of such immigrants; but, on the other hand, it is altogether probable that some of these numerous species were intruders. Unfortunately we are in no position yet to distinguish the native from the foreign stocks.

In the middle Pliocene, which also has preserved but a meagre and scanty record of its mammalian life, we again meet with horses in relative abundance, but of a far more primitive type. They are still incompletely known, but it is clear that they belonged to three parallel series, or phyla, of three-toed grazing horses, with teeth which, though high-crowned, had not attained to the extreme degree of hypsodontism seen in the species of Equus and had a somewhat less complex pattern of the grinding surface, though distinctly foreshadowing the modern degree of complication. One of the genera (†Pliohippus) was not improbably the ancestor of a very peculiar horse (†Hippidion) of the South American Pleistocene. These middle Pliocene genera were much smaller animals than the Pleistocene horses, aside from the pygmy species of the latter, of light and more deer-like proportions, and with three functional toes or digits. The median digit (3d of the original five) was much the largest and carried most of the weight, on hard ground practically all of it; the lateral digits (2d and 4th) which in existing horses are represented by the rudimentary metapodials, or “splints,” though much more slender than the median digit, yet had the complete number of parts and each carried a small hoof. Mere “dew-claws” as these lateral toes were, they may have been of service in helping to support the weight in mud or snow. In all parts of the skeleton there are little details which show that these species of the middle Pliocene were not so advanced and differentiated as are their modern successors, but it would be unprofitable to enumerate these details, which are of interest only to the anatomist.

In the lower Pliocene the horses were very much more numerous and varied than in the middle portion of the epoch. The same three genera of grazing animals, represented by less advanced and modernized species, are found; and, in addition, there was an interesting survival (†Merychippus) from the middle Miocene of an intermediate type, together with several species of browsing horses (†Parahippus and †Hypohippus). In these browsing forms the teeth were all low-crowned and early formed their roots, and the crowns were either without cement or with merely a thin film of it in the depressions of the grinding surface. The pattern of the grinding surface is so very much simpler than in the high-crowned, prismatic teeth of the grazers that it requires close analysis to detect the fundamental identity of plan. Such teeth imply that their possessors must have fed habitually upon a softer and less abrasive diet than grass, probably the leaves and soft shoots of trees and bushes and other succulent vegetable substances, very much in the fashion of existing deer, and must therefore have been chiefly inhabitants of the woods and groves and thickets along streams, as the grazing species were of the plains and open spaces. “This assemblage of the progressive and conservative types of horses was certainly one of the most distinctive features of Lower Pliocene time in North America” (Osborn).

Fig. 150.—Three-toed, grazing horse (†Neohipparion whitneyi) of the upper Miocene. Restored from skeletons in the American Museum of Natural History.

In the upper Miocene very much the same conditions prevailed and, for the most part, the same genera of horses, with different and somewhat less advanced species, were found as in the lower Pliocene, so that no particular account of them is needed. In the middle Miocene, however, there was a change, the typically grazing horses being very rare or absent and those with intermediate forms of teeth taking their place. Evidently, it was about this time that the horses with more plastic organization and capable of readjustment to radically different conditions began to take to the grazing habit, while other phyla, less capable of advance, retained the ancient, low-crowned type of grinding teeth and, after persisting, as we have seen, into the lower Pliocene, became extinct before the middle of that epoch. It is of great interest to observe that in the genus (†Merychippus) intermediate between the browsing and grazing types, the milk-teeth retained the older and more primitive character of low crowns without covering of cement, while the permanent grinders had much higher, cement-covered and complex crowns. In the lower Miocene, the variety of horses was much diminished and all had the low-crowned, cement-free, browsing type of teeth. Reversing the statement, we see that in the middle and still more in the upper Miocene the primitive and more or less distinctly homogeneous phylum branched out into several series, like a tree, some of the branches continuing and further subdividing through the Pliocene and Pleistocene, while others, less progressive and less adaptable, underwent but little change and had died out before the middle Pliocene.