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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]