Ontogenetic and Intraspecific Variation in the Skull Morphology of the Late Miocene Bovid Hezhengia bohlini

Hezhengia bohlini is one of the extinct bovids with the most abundant material, especially skulls. To date, skulls have been described, and the phylogenetic relationships and dietary habits have been discussed, but the ontogenetic processes of this special bovid with robust horn cores and a heavily built cranium have not yet been explored. Intraspecific variation in H. bohlini is high, but the variation ranges of different parts of the skull are unknown. In this study, we took 28 measurements and distinguished five age stages from sub-adult to senile. We described the ontogeny of the horn core, dentition, and skull and compared allometric growth and intraspecific variations of different parts of the skull. Although horn core growth and cranial roof arching are supposed highly related, horn core robustness is not related to cranium shape, basioccipital size, nor skull length or width. The rapid horn core growth of H. bohlini occurs in a very short period, from sub-adult to adult, whereas minor modifications and the deposition of exostoses are lifelong processes. The ontogenetic trajectory of H. bohlini differs from that of the closely related urmiatherine, Urmiatherium intermedium , in its late horn core development and cranial roof elevation, but characteristic features of basicranium development are only slightly different in both species. This study not only provides a good model for the study of ontogenetic and intraspecific variations in fossil bovids but also provides a framework for classifying other bovids known from fewer materials.