Ziphiid and chaeomysticete tympanic bullae from the upper Middle Miocene Taga Group of Ibaraki, Japan, and their significance

Abstract

 Ziphiid and chaeomysticete tympanic bullae from the upper Middle Miocene Taga Group of Kitaibaraki, central Japan, are described. The left tympanic bulla of Ziphiidae is the first description of tympanic bulla of this family from the North Pacific. Present ziphiid tympanic bulla has several derived characters, unlike Peruvian ziphiids of the Middle to Late Miocene. The left tympanic bulla of Chaeomysticeti is similar to several cetotheriids in semi-circular involucrar ridge. However, the main ridge of the present specimen is very weak. Therefore, this specimen is tentatively regarded as Chaeomysticeti fam. gen. et sp. indet. Two tympanic bullae add new data to the late Middle Miocene cetacean fauna of the Pacific coast of Japan. The late Middle Miocene to early Late Miocene fossil cetacean assemblages from North Kanto area including the Taga Group would be an important contribution to our understanding of cetacean faunal turnover in the end of the Middle Miocene.