A humpback whale, Megaptera novaeangliae (Borowski, 1781), from the Pleistocene Kioroshi Formation of Inba-mura, Chiba Prefecture, central Japan

A humpback whale skull was discovered in the upper Middle Pleistocene sand bed of Inbamura, Chiba Prefecture. The morphological features of the skull are summarized as follows: the supra-orbital process of the frontal is elongated laterally and its posterior margin is straight; the orbit is relatively short considering the skull size; the squamosal is slender and protrudes anterolaterally at an obtuse angle; the squamosal sulcus is not developed at the base of the zygomatic process; the dorsal shape of the supra-occipital is equilaterally triangular; the basioccipital and the basisphenoid are fused, forming a wide plane on the ventral surface; the alisphenoid is not exposed in the temporal region. Based on these characteristics, the specimen was identified as Megaptera novaeangliae (Borowski, 1781). This is the first record of M. novaeangliae in the western North Pacific during Pleistocene time. No fossil has yet been described systematically as M. novaeangliae from Pleistocene strata. Therefore, there is a strong possibility that the new material is the oldest M. novaeangliae fossil in the world.