The Eocene tapirs are still very imperfectly known; all that can be said of them is that they become successively smaller as they are traced backward in time, and that in them the premolar teeth were all smaller and simpler than the molars. The Wasatch genus (†Systemodon) is the most ancient member of the series yet discovered. Dating from the Eocene immigration, the tapirs are to be regarded as a North American family, for there is here a complete continuity from the lower Eocene to the Pleistocene, while in Europe they first appeared, probably by migration from North America, in the middle Oligocene.
Fig. 170.—Head of the White River tapir (†Protapirus validus). Restored from a skull in the museum of Princeton University.
In South America the history of the tapirs is even shorter and less eventful than that of the horses; the latter, as we have seen, reached the southern continent in the Pliocene and there gave rise to a number of peculiar and characteristic genera, but the tapirs have been found only in the Pleistocene of Argentina and Brazil and only the modern genus is represented.
Wofully broken and incomplete as the developmental history of the tapirs still is, the fragments are nevertheless sufficient to show a mode of evolution differing in certain important respects from that followed by the horses or †titanotheres. Certain features are common to all three groups, such as the increase in size and in proportionate stoutness from stage to stage and the gradual enlargement and complication of the premolar teeth. On the other hand, the tapirs have been very conservative, and they underwent far less radical changes than did either of the other families. Aside from the proboscis and the modifications of the skull which the development of that organ necessitated, these animals remain to-day very nearly what they were in Oligocene times. This, then, is an example of development practically restricted to a few organs, while all the other parts of the structure changed but little.
Fig. 171.—Upper teeth, left side, of tapirs, showing comparative sizes. A, †Protapirus validus, White River Oligocene. B, Tapirus terrestris, modern. i3, external incisor. c, canine. m1, first molar.
The extinct †lophiodonts, like the tapirs, of which they would seem to have been near relatives, are known only from incomplete material, and comparatively little has been learned regarding their history. While they were abundant and varied in Europe, during the Eocene epoch, they never were a striking or prominent element among the mammals of North America, where they persisted one stage later, and they did not reach South America. In North America they are found from the Wasatch to the White River.
The White River genus (†Colodon), which is fairly well known, might almost be described as combining the characters of horses and tapirs; but such an expression is not to be interpreted as meaning that this genus is in any sense a connecting link or transition between the two families, but merely that in certain important respects its course of development ran parallel with that followed by the horses. The teeth were very tapir-like, especially those of the lower jaw, which, indeed, are hardly distinguishable from those of a tapir, and the premolars had the molar-pattern. The limbs were very light and slender and the feet long and narrow; the fore foot retained a small fifth digit; the feet, especially the hinder one, had a resemblance to those of the contemporary horses (†Mesohippus), though the median digit was not so much enlarged, nor the lateral ones so far reduced. It is highly probable that, had this family persisted till the Pleistocene, instead of dying out in the lower Oligocene, it would have eventually terminated in monodactyl forms.