A short report of the first record of an Odontoceti (toothed whale) from the Korematsu Formation, Bihoku Group (latest Early to earliest Middle Miocene) in Japan

For toothed whales, the Early Miocene was a time of transition between archaic and modern clades. In Japan, there are horizons that include whale fossils from the Early to early Middle Miocene, such as the Ichishi Group in Mie, the Mizunami and Iwamura groups in Gifu, the Morozaki Group in Aichi, and some formations in Akita, Iwate and Hokkaido. The Bihoku Group in Hiroshima has abundant baleen whale (Mysticeti) fossils, but only one possible toothed whale (Odontoceti) fossil exists. Here, we report the first unambiguous fossil record of Odontoceti from the Korematsu Formation of the Bihoku Group (latest Early to earliest Middle Miocene), a pair of fused middle parts of mandibles with two teeth. Although this specimen is fragmentary, it is evident that odontocetes lived in this region during the time of deposition of the Korematsu Formation. However, questions remain as to why there are so few odontocete specimens relative to mysticetes.