ANCHITHERIUM (MAMMALIA, PERISSODACTYLA, EQUIDAE) FROM THE EARLY MIOCENE HIRAMAKI FORMATION, GIFU PREFECTURE, JAPAN, AND ITS IMPLICATION FOR THE EARLY DIVERSIFICATION OF ASIAN ANCHITHERIUM

摘要

An Anchitherium specimen with a nearly complete series of the upper cheek teeth (P2–M3), from the upper part (ca.17–18 Ma) of the Hiramaki Formation, Kani Basin, Gifu Prefecture, Japan, was previously referred to Anchitherium "hypohippoides" nomen dubium. Despite that it provides one of the best examples of significant dental characters of Asian Anchitherium, it has remained undescribed and unprepared until recently. Although a paucity of materials from Asia makes the taxonomy of Asian Anchitherium difficult to assign, comparison showed that the specimen should be reassigned to Anchitherium aff. A. gobiense; it differs from A. aurelianense and is rather similar to A. gobiense from China by virtue of large size and expanded hypostyles. The Japanese Anchitherium also shows distinct features including straight (flattened) ectolophs with narrow mesostyles, rudimentary crochets, and enamel protuberances at the lingual mouth of the median valleys. This combination of accessory features has not been known in Asian Anchitherium and seems to be rarely observed among the diversified European species. The existence of Japanese Anchitherium implies early species diversification in East Asia that predates a greater diversification in Europe associated with the Middle Miocene Climatic Optimum and supports paleogeographical and paleozoological connection to the Asian mainland under a warm and humid climate prior to the formation of the Japanese archipelago (ca. 16.5 Ma).