Hipparion crassum, History
Hipparion crassum was recognized as a new species by Gervais (1859, 1869) because of its massive limb bones and the rounded protocone of its upper cheek teeeth.
Depéret (1890) gave a more detailed description insisting on the functional reduction of the lateral metapodials, the relatively big size of the dentition, the atrophy of the protostylids on the lower adult cheek teeth, and the lack of ectostylids on the lower lacteal ones. For Depéret, all these characters suggested that H. crassum occupied an intermediate position between the Miocene true tridactyl H. gracile and the monodactyl Equus.
At that time, Marie Pawlov (1888) had already noted the likelihood of a North American filiation for Equus, and stated that hipparions could not be Equus ancestors because of their more derived cheek teeth (isolated proptocones and complicated enamel) in association with more primitive teeth.
Depéret, however was very much impressed by what he saw as an evolved condition of the limbs and lower dentitions in H. crassum.
It let him wonder if the importance attached to the protocone in equid phylogeny was not exagerate. In addition to the american lineage of Equus, he accepted the possibility of another, european. At the very least, wrote he, there was a striking convergence in the locomotion and in the lower dentition between H. crassum and Equus.
Shortly after, Marie Pawlov (1891) commented on Depéret publication. Beside pointing at an error in the captions on figure 1 (see also Eisenmann and Sondaar, 1989, text-figure 1), she objected to the idea of a polyphyletic origin of Equus in which Hipparion would play an ancestral role, reiterating her previous argumentation. She was, however, embarrassed by the apparent simplicity of the lower cheek teeth referred to H. crassum by Depéret, wondering even if these teeth did not actually belong to Equus. That last point was clearly refuted by Depéret (1891).
Thus, from the very beginning, H. crassum puzzled the paleontologists by what appeared unusual associations : big teeth associated to relatively small bones, complicated (even more than usual in Hipparion) upper cheek teeth associated to simple (like in Equus) lower cheek teeth, and reletively evolved limbs.
Gromova (1952) infirmed the alleged big size of teeth relative to bones and the functional reduction of the lateral digits mentioned by Depéret. According to Gromova, the strong enamel plication of the upper cheek teeth and the limb bone proportions of H. crassum evidence an adaptation to a very humid environment.
Forsten (1968) remarked that H. crassum resembles very much H. primigenium. The latter is a forest hipparion with plicated upper cheek teeth, robust metapodials, and relatively long proximal segments. In H. crassum the metapodials are even shorter and broader. According to Forsten (p. 60) the limb proportions would seem even heavier than in H. primigenium if the Perpignan hipparion had a radius and tibia similar to those found at Gödöllo, Hungary, and referred to H. crassum by Mottl (1939).