Two new extinct basal phocoenids (Cetacea, Odontoceti, Delphinoidea), from the upper Miocene Koetoi Formation of Japan and their phylogenetic significance

ABSTRACT

Two new extinct porpoises—Archaeophocaena teshioensis, gen. et sp. nov., and Miophocaena nishinoi, gen. et sp. nov.—are described from the upper Miocene Koetoi Formation (5.5–6.4 Ma) of Hokkaido, Northern Japan. The holotype of the former is composed of a partial skull, whereas the holotype of the latter is composed of a partial skull, right periotic, right stylohyoid, and pelvis. Both species are assigned to Phocoenidae on the basis of a unique combination of phocoenid characters: presences of frontal bosses, nasal protuberances, premaxillary eminences, and fossae for the inferior vestibule. However, they do not have dorsally developed preorbital sinus fossae or high premaxillary eminences, unlike derived phocoenids. Furthermore, their premaxillae contact the nasals (or the right premaxilla alone contacts the right nasal), unlike all other known phocoenids except Pterophocaena. A comprehensive morphological cladistic analysis indicates that the two new extinct species are the second-most basal phocoenids next to Pterophocaena and that Phocoenidae is the sister group of Delphinidae. The cranial morphology of the two new extinct species is intermediate in form between that of phocoenids and delphinids, supporting the hypothesis of a sister relationship for the two groups. Thus, these new species fill not only the morphological gap between Phocoenidae and other families of Delphinoidea but also a temporal gap in the fossil record of phocoenids in the western North Pacific.

ACKNOWLEDGMENTS

We thank T. Nishino for providing the opportunity to study the holotype specimen, and C. Argot, L. G. Barnes, G. Bianucci, D. J. Bohaska, E. Cioppi, T. A. Deméré, J. P. Dine, H. Furusawa, J. Galkin, S. J. Godfrey, P. Holroyd, H. Ichishima, N. Inuzuka, D. S. Janiger, T. Kimura, Y. Koda, N. Kohno, O. Lambert, J. G. Mead, M. Mizushima, C. de Muizon, J. J. Ososky, N. D. Pyenson, K. A. Randall, G. Raineri, C. Sarti, Y. Soeda, C. Sorbini, G. T. Takeuchi, Y. Tajima, Y. Tulu, G. B. Vai, E. Westwig, T. K. Yamada, and R. Zorzin for access to the specimens under their care and valuable suggestions, R. E. Fordyce for his important suggestions, N. Kohno for significant comments on previous drafts, the staff of the Nakagawa Museum of Natural History, and for their support during the field work. We also thank D. E. Hurlbert, B. Watanabe, the Calvert Marine Museum, and the Clarissa and Lincoln Dryden Endowment for Paleontology at the Calvert Marine Museum for their financial support. We are grateful to two anonymous reviewers and editor J. Geisler for detailed reviews that greatly improved the manuscript.

Handling editor: Jonathan Geisler