A new derived mosasaurine (Squamata: Mosasaurinae) from south-western Japan reveals unexpected postcranial diversity among hydropedal mosasaurs.

Reported herein is a largely complete mosasaurine mosasaur (Squamata: Mosasauridae) skeleton from Wakayama Prefecture, south-western Japan. It is represented by many skeletal elements including the skull, a complete cervical and dorsal vertebral series with more than 40 vertebrae, paired ribs, right and left front flippers, and the left hind flipper. The specimen is from near the Campanian/Maastrichtian boundary (c. 72 Ma) within the Hasegawa Muddy Sandstone Member of the Toyajo Formation. We assign the specimen (WMNH-Ge-1140240002) to a new genus and species based on a unique combination of characters including: jaw bones gracile; premaxilla–maxilla suture terminating above or just posterior to fourth maxillary tooth; frontal median dorsal ridge robust; frontal alae broadly rounded; frontal descending processes extending parallel to each other; jugal process of postorbitofrontal extending laterally, constituting dorsal half of posterior orbital margin; anterior and posterior carinae on marginal teeth pinched out in cross-section; cervical centra compressed dorsoventrally; zygosphenes and zygantra present at least to the 19th (= posterior) dorsal vertebrae; neural spine orientation changing from procumbent to recumbent along posterior dorsal vertebrae; front and hind flippers longer than mandible; hind flipper longer than front flipper; and hyperphalangy of up to nine. The two sets of large, wing-shaped flippers were likely selected for fast manoeuvring, as seen in the humpback whale among extant mysticetes. The presence of a dorsal fin is suggested by the sweeping arrangement of the neural spines along the dorsal vertebrae, well posterior to the presumed centre of gravity. Finally, the pubis and the ilium articulate at an obtuse angle in anteroposterior view, allowing no bony contact between the latter and the axial skeleton. [ABSTRACT FROM AUTHOR]