A Pliocene rhinocerotid (Mammalia, Perissodactyla) from Ajimu, Oita Prefecture, southwestern Japan, with comments on the Japanese Pliocene rhinocerotid fossil records

Abstract

A few lower cheek teeth of rhinocerotid, probably belonging to the same individual, from the upper Pliocene Tsubusagawa Formation in Ajimu, Oita Prefecture, southwestern Japan, are described. Morphologically, the Ajimu specimen is obviously distinguished from the subtribe Teleoceratina (Brachypotherium), Elasmotheriina (Elasmotherium) and a species of subtribe Rhinocerotina, Coelodonta, but it is similar to the Late Miocene species of tribe Aceratheriini and Miocene to Pleistocene species of Rhinocerotina. The Ajimu specimen is metrically similar to the lower cheek teeth of several species of Rhinocerotina, such as species of genus Diceros, Dicerorhinus, Dihoplus, Rhinoceros and Stephanorhinus. Therefore, the Ajimu specimen is identified as Rhinocerotina gen. et sp. indet. The previous works and present result reveal that this group was presented in Japan during mid-Pliocene. We also briefly summarize fossil records of the Japanese Pliocene rhinocerotid including footprints, suggesting that Pliocene rhinocerotid distributed in Japan through about 4.2 Ma to the end of the Pliocene. Japanese Pliocene rhinocerotids would be migrated from the Asian continent during the Early Pliocene or early Late Miocene when the proto-Japan connected with the Asian continent. However, more Japanese Pliocene specimens and detailed comparison with Asian species are needed to discuss this issue.