The connecting link with Hyracotherium was formed by Pachynolophus (Pro palaeotherium), and the line apparently terminated in Palo plotherium, which is also Oligocene.
A, Hyracotherium (Eocene).
Outer columns of upper molars similar, the hinder ones not flattened; ridges of lower molars oblique or directly transverse, a third ridge to the last molar in the earlier forms. The Lophiodontidae, which date from the Eocene, come very close to Hyracotherium in the horse-line; and it is solely on the authority of American palaeontologists that the division of these early forms into equoids and tapiroids is attempted.
The final stage, or rather the initial stage, in the series is presented by Hyracotherium (Protorohippus), a mammal no larger than a fox, common to the Lower Eocene of Europe and North America.
These two genera may be regarded as forming the earliest stages in the evolution of the horse, coming below Hyracotherium (see Equidae).
The palaeotheres, which range in size from that of a pig to that of a small rhinoceros, are now regarded as representing a family, Palaeotheriidae, nearly related to the horsetribe, and having, in fact, probably originated from the same ancestral stock, namely, Hyracotherium of the Lower Eocene (see Equidae).