A geologically old and ontogenetically young Herpetocetus sp. from the late Miocene of Hokkaido, Japan

Abstract

An ontogenetically young fossil baleen whale from the lower part of the Horokaoshirarika Formation of Hokkaido, Japan, includes a partial skull, periotics, bullae, mandible, vertebrae, and a scapula. It is identified as Herpetocetus sp. because it exhibits a postglenoid process of the squamosal more transversely compressed than in Nannocetus, a deep and anteroposteriorly long fossa on the dorsal surface of the squamosal between the zygomatic process and the lateral wall of the brain case, and a prominent squamosal flange of the periotic. The Hokkaido specimen differs from currently described Herpetocetus species in having a large hiatus fallopii, and an angle at the anteromedial edge of the pars cochlearis, just medial to the hiatus fallopii. Because the lower part of the Horokaoshirarika Formation is late Miocene in age (approximately 7.7 to 6.8 Ma), the Hokkaido Herpetocetus specimen is the only record of Miocene Herpetocetinae from the western Pacific. Previously, Miocene Herpetocetinae were reported from the eastern North Pacific and western Atlantic. Accordingly, this new record of Herpetocetus from the upper Miocene of the western North Pacific suggests an earlier origin for both the genus and the subfamily.

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Citation for this article: Tanaka, Y., and M. Watanabe. 2018. A geologically old and ontogenetically young Herpetocetus sp. from the late Miocene of Hokkaido, Japan. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2018.1478842.

ACKNOWLEDGMENTS

 We thank K. Matsubara and T. Matsubara (both Numata Town, Hokkaido, Japan) for collecting and preparing the specimen. We also thank S. Yamashita, T. Kawashima, Y. Tsuji, K. Kosaka, M. Taniguchi, and S. Shinohara (all NFL) for their contribution to preparation. Y.T. thanks R. E. Fordyce (University of Otago) and C.-H. Tsai (National Taiwan University) for comments on an early version of the manuscript, and F. G. Marx (Institut Royal des Sciences Naturelles de Belgique) for discussion of cetotheriid systematics and morphology. We also thank reviewers P. Gol’din (Schmalhausen Institute of Zoology) and F. G. Marx for their constructive comments on the manuscript. T.Y. thanks N. Kohno (NSMT) for giving permission to examine NSMT-PV 19540. T.Y. also thanks Y. Tajima and T. K. Yamada (both NSMT), P. Holroyd (UCMP), N. D. Pyenson and D. Bohaska (both USNM), J. Nemoto (IGPS), and M. Oishi and T. Mochiduki (both Iwate Prefecture Museum) for providing access to comparative specimens, and O. Lambert (Institut Royal des Sciences Naturelles de Belgique) for providing references for this project earlier and also for his academic editorial work. Y. T. thanks M. Kimura (NFL) for his encouragement.