Fig. 154.—Series of horse skulls in ascending geological order. A., †Eohippus, lower Eocene (after Cope). B., †Mesohippus, lower and middle Oligocene. C., †Protohippus, upper Miocene (after Cope). D., Equus.
Fig. 155.—Right manus and left pes of Equus.
Fig. 156.—Right manus and left pes of †Protohippus.
(7) The feet were much elongated and the median (3d) digit of each gradually enlarged until it carried the whole weight, at the same time modifying the shape of the hoof so as to fit it to be the sole support of the body. The other toes gradually dwindled and became functionless, though often retained as splints. The first digit (pollex and hallux) was first lost, then the fifth, then the second and fourth were reduced to dew-claws and finally to splints. Thus the pentadactyl horses of the lower Eocene were transformed into the monodactyl species of the Pliocene and Pleistocene.
In South America the story of the horses was a brief one, for they were among the immigrants from the north and did not reach the southern continent till the Pliocene, probably late in that epoch, for none of the three-toed genera have been found in South America. So far as known, these southern equines were small and medium sized animals, with large heads, relatively short feet and somewhat ass-like proportions. There were two well-defined groups of these animals: (1) species of the genus Equus, which thus, at one time or another, inhabited every one of the continents, Australia excepted; (2) three genera peculiar to South America and developed there from northern ancestors, probably †Pliohippus. Two of these genera (†Hippidion and †Onohippidium) displayed curious modifications of the nasal bones, which were extremely slender and attached to the skull only at their hinder ends, instead of being, as is normally the case, supported for nearly their whole length by lateral articulation with other bones. What can have been the significance and function of these excessively slender, splint-like nasals, it is difficult to conjecture. The third genus (†Hyperhippidium) was a small mountain-horse, with extremely short feet, which were well adapted to climbing.
Fig. 157.—Right manus and left pes of †Mesohippus.