The inner ear morphology of the ‘condylarthran’ Hyopsodus lepidus

Comparisons with the inner ear of the Eocene early diverging artiodactyl Diacodexis and perissodactyl Xenicohippus allow refining the picture of the ancestral inner ear morphology of Euungulates. These taxa are very close morphologically and mostly differ by slight differences in their semicircular canal angulations and profile.

They all present a secondary crus and a low position of the plane of the lateral semicircular canal relative to the posterior semicircular canal. These two characters, considered as ancestral features for Theria, might be symplesiomorphies of Euungulata as well. Hyopsodus and Xenicohippus share characters also observed in other basal Equoidea, which would support the close relationship between these two taxa previously proposed in the literature.