An additional remain of Pliocene Rhinocerotidae from Ajimu, western Japan

ABSTRACT

A rhinocerotid mandibular remain from the Upper Pliocene Tsubusagawa Formation (around 3.5 Ma) was described and discussed its taxonomic status and paleobiogeographical implication. Compared with Late Miocene to Early Pleistocene and living rhinocerotid species, the studied specimen resembles the mandible of the Pliocene Eurasian taxa of the subtribe Rhinocerotina, Dihoplus and Pliorhinus in having a straight ventral margin, constant mandibular height and probably reduced lower second incisor. This specimen, however, lacks detailed diagnostic characters of these genus. In addition, the taxonomic validity of these taxa has been argued so far. Therefore, this specimen was identified as Rhinocerotina gen. et sp. indet. Japanese Pliocene rhinocerotid records and the present study suggest that the Rhinocerotina was distributed in the central to southwestern part of proto-Japan during the mid-Pliocene. If the studied specimen is truly a species of Dihoplus or Pliorhinus, its close relative might have migrated from northern China as in the case of the ancestor of Japanese Pliocene proboscidean, Stegodon miensis. Further discovery of Japanese Pliocene rhinocerotid specimens could resolve this issue.

KEYWORDS:

• Ajimu
• mammal
• Perissodactyla
• Pliocene
• Rhinocerotidae
• western Japan

Acknowledgments

We wish to thank Kikuchi K. (Saitama Industrial Technology Center, Kawaguchi) for his access to CT scanning facilities. We wish to thank Editor-in-Chief Gareth Dyke and two anonymous reviewers, whose comments improved the original manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the JSPS (Japan Society for the Promotion of Science) KAKENHI [18H01330].