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

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.

KEYWORDS:

• Northern Shaanxi
• Chinese Loess Plateau
• Late Miocene
• vertebrate palaeontology
• palaeoecology

Acknowledgments

This work was supported by State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS) (No.193114), the National Natural Science Foundation of China (41430102, 41272020), the Strategic Priority Cultivating Research Programme, CAS (XDPB05), the Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-DQC022), and the All China Commission of Stratigraphy (2015FY310100). The authors thank Xue Xiangxu (Northwest University) for continual guidance and assistance; Shen Yi for figures; and Jiangzuo Qigao (Institute of Vertebrate Palaeontology and Paleoanthropology, Chinese Academy of Sciences), Ye Qin (China University of Geosciences) and Zhang Hanwen (University of Bristol) for constructive inputs that have improved this manuscript. The reviewers and editors are kindly acknowledged for their helpful comments on the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This work was supported by the State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology), CAS; [193114]; National Natural Science Foundation of China; [41430102, 41272020]; Strategic Priority Cultivating Research Programme, CAS; [XDPB05]; Key Research Program of Frontier Sciences, CAS; [QYZDY-SSW-DQC022]; All China Commission of Stratigraphy; [2015FY310100].