Fordyce RE

Introduction Beaked whales (Ziphiidae) are among the least understood cetacean families. Within this family, the genus Mesopolodon consists of approximately 14 species that are morphologically similar, and many remain relatively unknown (Pitman, 2009; Ellis & Mead, 2017). Species within this genus have a pelagic distribution and a preference for deep waters.

In the past, the structure of the canalis triosseus in plotopterids was unknown because of the poor preservation of this region of the bone. The role of the m. supracoracoideus has not been discussed despite the general assumption that plotopterids performed penguin-like underwater propulsion with penguin-like wings, although the large size of the muscle could have been inferred by the elongate processus acrocoracoideus of the coracoids which would have increased the size of the triosseal canal.

There are no direct age estimations for the Nupinai, Hikatagawa and Perupune Formations. The overlying Taiki Group is reportedly in the Thalassiosira yabei to Neodenticula kamtschatica diatom zone (11.5–5.5 Ma) ( Yamaguchi, 1989 ). Fission track age of the underlying Green-Tuff Rekifune Formation was reported as 14.2 ±1 Ma ( Koshimizu & Kim, 1986 ). Here, we have 15.2–11.5 Ma interval (Langhian to Serravallian; Middle Miocene) as the maximum possible range for the Hikatagawa Formation.