A new species of Miophyseter (Cetacea, Odontoceti, Physeteroidea) from the Miocene of Japan

Three fossil physeteroids have been recovered from the Lower to Middle Miocene of Japan. Here, we describe and diagnose these specimens as a new species of the genus Miophyseter, M. gunmaensis, sp. nov. The holotype, GMNH-PV-3889, preserves a cranium and a nearly complete postcranial skeleton, including vertebrae from the atlas to near the posterior-most caudal vertebrae. The hyoid elements consist of a large, flattened basihyal and thyrohyals, as well as robust stylohyals, providing extensive surface area for muscle attachment to generate sufficient gular suction pressure via lingual retraction. This morphology implies that M. gunmaensis employed suction-assisted feeding. Together with the inferred deep-diving capabilities of Miophyseter, M. gunmaensis may represent an early stage in the evolutionary acquisition of the ecological and functional adaptations observed in modern physeteroids.