Locomotor adaptations of 7.4 Ma Hipparionine fossils from the middle reaches of the Yellow River and their palaeoecological significance

Abstract

ABSTRACT New fossil remains of limb bones referred to Hipparion (Hippotherium) chiai from 7.4 Ma in the middle reaches of the Yellow River, Fugu, Shaanxi, China are herein described and analysed. Although some of the primitive features remain on the phalanges, e.g.: less well-developed V-scars on the phalanges III; the suspending extent of the side toes is limited. But the functional morphology of these limb bones also suggests that: 1) the metapodials are gracile; 2) the distal keel of the metapodial is strong; 3) the absolute digital length of side toes are reduced dramatically; (4) it has basal ‘spring foot’. These characters are characteristic of equines with superior locomotor abilities (faster and more agile), indicating that H. chiai was well adapted to an open environment. Together with the evidence of palaeozoogeography and isotopic palaeodietary proxies, we suggest a temperate C3 steppe environment for the middle reaches of the Yellow River at 7.4 Ma, instead of arid or semiarid C4 savannas as previously believed. We also propose a novel hypothesis of evolutionary process of ‘spring foot’ on Eurasian hipparionine horses.