Norisdelphis annakaensis, A new Miocene delphinid from Japan

ABSTRACT

Two well-preserved delphinid crania have been recovered from the Haraichi Formation, Annaka Group, Gunma Prefecture, Japan (earliest late Miocene, Tortonian; 11.29–11.25 Ma). The specimens represent a new genus and species of delphinid, Norisdelphis annakaensis. The new species is characterized by transversely narrow and anteroposteriorly elongated external bony nares, and a wider proximal extremity of the left premaxilla. Phylogenetic analysis suggests that Norisdelphis is the most basal member of the Delphinidae. The previous oldest known described delphinid, Eodelphinus kabatensis, is from the upper Miocene (ca. 9 Ma), and other reliably dated and described fossil delphinids are no older than Pliocene; thus, N. annakaensis is the oldest convincingly dated and well-diagnosed fossil delphinid species yet described. Norisdelphis annakaensis expands the geological range of the family Delphinidae.

ACKNOWLEDGMENTS

We gratefully acknowledge the efforts of H. Nakajima in discovering and preparing the specimens of this study. We thank D. J. Bohaska, N. D. Pyenson, M. R. McGowen, J. J. Ososky, and D. P. Lunde of the National Museum of Natural History, Smithsonian Institution; C. Kemper, D. Stemmer, and I. Tomo of the South Australian Museum; K. Armstrong of the University of Adelaide; H. Iwaoka, M. Takagi, and H. Koike of the Matsumoto City Shiga Fossil Museum; I. Hayashi of the Tsukuba University; and Izumida Museum (Kosenji temple) for facilitating access to the specimens in their care. Constructive comments and suggestions from R. E. Fordyce of the Otago University, G. Bianucci of the Università di Pisa, and editor O. Lambert of the Institut royal des Sciences naturelles de Belgique considerably improved the quality of this work. We also extend our thanks to N. Kohno of the National Museum of Nature and Science, M. Murakami of the Shumei University, Y. Takakuwa of GMNH, S. Miyazaki, and K. Nagasawa for their help. This work was supported by a Japan Society for the Promotion of Science (JSPS) KAKENHI grant (no. JP18K01110) to T.K.