| Summary: | 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 6.8Ma), 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.
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