The first cranium of Postschizotherium (Pliohyracidae, Hyracoidea) from the Lower Pleistocene of China

Abstract

Pliohyracidae, a group of large-bodied hyracoids, successfully colonised Eurasia during the late Cenozoic. However, their cranial morphology has remained poorly understood due to the scarcity of complete specimens in East Asia, particularly for Postschizotherium. This study presents the first nearly complete cranium of Postschizotherium cf. intermedium from the Early Pleistocene Longdan locality in Gansu Province, China. The cranium exhibits several distinctive features, including an exceptionally laterally projecting orbit, a massive maxillary sinus that significantly invades the orbital fossa, and a highly developed pre-orbital fossa system. These morphological traits, especially the first one, suggest that Postschizotherium is closely related to Pliohyrax and Kvabebihyrax, both being large pliohyracids from the upper Cenozoic of East Europe and West Asia. The morphology also suggests that Postschizotherium likely occupied semi-aquatic habitats, similar to those of extant hippos, with a diet primarily consisting of grasses. The discovery of this species at Longdan also provides evidence for a relatively humid palaeoenvironment in the region during the earliest Pleistocene, contributing to our understanding of the palaeoenvironmental conditions and evolutionary history of pliohyracids in Eurasia.