First report of Neotherium mandible from the Miocene Shimane, Japan: Review of basal odobenids in the western North Pacific

ABSTRACT

The present study describes a new specimen of a pinniped mandible from the lower Miocene Irino Sandstone Member of the Takami Formation (approximately 16.5 Ma), Shimane, western Japan, as a basal odobenid Neotherium mirum Kellogg, 1931. Our review of the basal odobenid records from Japan reveals that it is the first record of N. mirum and the western- and southernmost record in the western North Pacific. In addition, the present specimen is one of the oldest records of N. mirum around the world. The records clearly showed that Japan was one of the major habitats for the basal odobenids. The early evolution of odobenids occurred around the North Pacific; however, there is no record of basal odobenids with a trans-Pacific distribution. Our study demonstrates that N. mirum was widely distributed in the North Pacific, and suggests that the basal odobenids already had the mobility to expand their distribution.

KEYWORDS:

• North Pacific
• odobenids
• palaeobiogeography
• pinnipeds
• shimane

Acknowledgments

We would like to thank E. Kakuya and M. Okubo (Ohnan town board of education), and S. Inokuchi, Y. Inokuchi, and M. Inoue (Takaumi Fossil Research Society) for supporting the investigation of the specimen and the field survey. We would like to also thank T. Kimura and Y. Takakuwa (GMNH); L. G. Barnes, J. Velez-Juarbe, and V. R. Rhue (LACM); Y. Tajima (NMNS); H. Morioka (OHFM); N. D. Pyenson and D. J. Bohaska (USNM) who facilitated access to museum collections of extant and extinct pinnipeds. We are grateful to the editor of the journal and two anonymous reviewers for providing thoughtful comments and recommendations.

Author contributions

WT examined the whole specimen, is responsible for the taxonomic assessment, and wrote the manuscript. SK organized the study, prepared the specimen, conducted the field survey, and wrote the geological part of the manuscript. TI investigated the geological background and wrote the geological part of the manuscript. All authors reviewed the manuscript.

Disclosure statement

The authors have no conflicts of interest to declare.

Additional information

Funding

This study was supported by a donation from the W. T. Yoshimoto Foundation, Hawaii to WT.