The dentition was full and highly selenodont, i.e. the premolars and molars had curved and crescent-shaped cutting edges (as in today's ruminants).

A peculiar characteristic of this group were the auditory bulla, protective structures of the bones of the ear : they were very large, like those that are found today in small mammals that live in open and dry environments.

This evolved cranial anatomy was in stark contrast to the rest of the skeleton, rather primitive: the cainotheriids still possessed four non-reduced toes (an ancestral condition of the artiodactyls), even if the lateral fingers were shorter; they ended in long claws, as in modern rabbits.

In the course of the Upper Eocene primitive genera such as Oxacron and Paroxacron developed, considered the first true cainotheriids as sister taxa in the subfamily Oxacroninae.

[citation needed] Subsequently, in the course of the Oligocene, the subfamily Cainotheriinae underwent a discrete evolutionary radiation, with the rabbit-sized genera Plesiomeryx and Caenomeryx.