中国科学院南京地质古生物研究所机构知识库

We describe new material of Hipparion (Hippotherium) chiai from Fugu, northern Shaanxi, China. This collection represents the greatest number of skulls and mandibles of H. chiai that have been reported from a given locality. The late Miocene (7.8 Ma) occurrence of this species is an important chronological and palaeoecological reference for the Lamagou fauna. However, its slender limb bones and prototypical "spring foot" are derived traits that are more suited to rapid running and are a standard sign of adaptation to an open habitat. This also means that H. chiai was a grassland dweller that was well adapted to a cursorial lifestyle. Based on the ontogenetic sequence, we establish a new peer comparison framework for fossils of the same ontogenetic age. Using this peer comparison framework, we find that, compared with holotype specimens, H. chiai of the Lamagou fauna has a wider anterostyle, shorter protocone, weaker plications, rounder metastylid, and a deeper ectoflexid. These intraspecific variations reveal the evolutionary transition between the late Miocene and Pliocene Hipparion in the Old World, and these fossils can be seen as the final phase of H. chiai. These findings and the new peer comparison framework have broad applications to the systematic paleontology of hipparionine horses. This study supports the assertion that H. chiai is the most stable time indicator of the late Miocene Bahean age in northern China and has important stratigraphic significance.