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.