Abstract

A new eschrichtiid mysticete was recovered from the Late Pliocene Yuchi Formation, Teshio Town, Hokkaido, Japan. The specimen is relatively well preserved, comprising a posterior part of the skull, periotics and tympanic bullae, several incomplete cervical and thoracic vertebrae, ribs, and forelimb elements including scapula, humerus, ulna, and several phalanges. Its small size, unfused epiphyses of vertebrae and long bones (humerus, radius, and ulna) indicate that the individual is physically immature. It is identified as eschrichtiid based on diagnostic characters, including paired occipital tuberosities, large occipital condyles, the posteriorly concave exoccipital lateral to the occipital condyle, the cochlear portion of the periotic extending anteromedially, the relatively small and somewhat triangular anterior process of the periotic, and the proportionally large vertebral foramen relative to the vertebral body. Because all verified eschrichtiids are of Pleistocene age, the Teshio eschrichtiid significantly extends the temporal range of the group.

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