A new skull of an early diverging rorqual (Balaenopteridae, Mysticeti, Cetacea) from the late Miocene to early Pliocene of Yamagata, northeastern Japan

The family of rorquals and humpback whales, Balaenopteridae includes the largest living animal on Earth, the blue whale Balaenoptera musculus. Many new taxa have been named, but not many from the western Pacific, except Miobalaenoptera numataensis from Japan. Here we describe an early balaenopterid, cf. M. numataensis from a late Miocene to early Pliocene sediment in Yamagata Prefecture, northeastern Japan. The species has a straight and sharp lateral ridge of the fovea epitubaria at the ventral surface of the periotic, and a dorsoventrally thin pars cochlearis. The new specimen provides knowledge of supposed ontogenetic variation and periotic morphology in poorly known fossil balaenopterids.