The Late Miocene Hipparion (Equidae, Perissodactyla) fossils from Baogeda Ula, Inner Mongolia, China

Abstract

New material of Hipparion (Baryhipparion) tchikoicum from Baogeda Ula in Abag, Inner Mongolia, China is described. The latest Miocene (∼6 Ma) occurrence of the species is an important chronological reference for the stratigraphy of Baogeda Ula. Furthermore, the Baogeda Ula Hipparion connects the distribution of this species in the Yushe Basin and Mongolia. The deep ectoflexids of the premolars and the rounded double knots of the lower cheek teeth place the Baogeda Ula specimens in the primitive forms of Hipparion in Eurasia. The two North American species of Hipparion, Hipparion shirleyi and Hipparion tehonense with deep ectoflexids and rounded double knots in the lower premolars, may be the ancestral forms of the subgenus Baryhipparion. Furthermore, their age was much earlier than the first appearance of Baryhipparion in Eurasia, so the period and route of the immigration of North American ancestral forms to Eurasia are still unclear. Nevertheless, the Eurasian expansion of this subgenus is clear: it first appeared in North China during the latest Miocene, and then spread northward to the Mongolian Plateau and Transbaikalia; it migrated westward into Europe through Kazakhstan and Caucasus in the Pliocene; and it survived in Transbaikalia until the Early Pleistocene.

Keywords::

• Equidae
• Hipparion
• Miocene
• Inner Mongolia
• stratigraphy
• zoogeography

Acknowledgements

We thank Yuri Kimura for her invitation to contribute to this festschrift volume. Our friend and colleague Yukimitsu Tomida has participated in several field seasons in Inner Mongolia, including works on various Baogeda Ula localities, and it is thus a fitting tribute to dedicate this paper to him. We have benefited from discussions with Qiu Zhanxiang about the Hipparion fossils. We gratefully acknowledge participants in fieldwork in Inner Mongolia such as Qiu Zhuding, Gary Takeuchi, Yuri Kimura, Hou Sukuan, Shi Qinqin, Chen Shaokun, Pang Libo, Rachel Yukimura, Feng Wenqing and Shi Fuqiao. We thank Gao Wei for his photographs and Hou Yemao for his CT scans.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [grant number 41430102], the Strategic Priority Research Program of the Chinese Academy of Sciences [grant number XDB03020104], and the Ministry of Science and Technology of China [grant number 2012CB821906]. The 2009 excavation was also funded by the Society of Vertebrate Paleontology Patterson Memorial Grant, the National Geographic Society Young Explorer Grant, the Evolving Earth Foundation Grant, and the Geological Society of America Student Research Grant (to Z.J. Tseng).