Reassessment of the Middle Pleistocene Japanese Rhinoceroses (Mammalia, Rhinocerotidae) and paleobiogeographic implications

A rhinocerotid fossil (NMNS-PV9600) from the lower part of the Isa Formation (Middle Pleistocene), Isa area, Mine City, Western Japan, is described. Previously, this specimen had been identified as Dicerorhinus nipponicus. However, the specimen shares the following features with the genus Stephanorhinus: molarised premolars; absence of P1; absence of metacone fold on P3 and P4; presence of crochet on the premolars and molars; presence of a crista on the molars; absence of medifossette on both premolars and molars; triangular-shaped M3. In this study, NMNS-PV9600 is redescribed as Stephanorhinus kirchbergensis based on morphological and metric comparisons with other Eurasian Pleistocene rhinocerotids. NMNS-PV9600 is close to S. kirchbergensis from Choukoutien, suggesting S. kirchbergensis could have reached the Japanese archipelago during the middle Pleistocene (before or around 0.43 Ma). NMNS-PV9600 was an adult individual around 25 years in age when it died, based on the tooth eruption and wear stage. The cheek teeth morphology suggests that S. kirchbergensis from Japan was probably a browser as the species has before been considered in occurrences from other countries.