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

Abstract

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.

http://zoobank.org/urn:lsid:zoobank.org:pub:E36B7A31-BFCA-47B8-9E97-AB249AC9D79B

Keywords:

• Mosasaurinae
• western Japan
• Late Cretaceous
• secondary aquatic adaptation
• dorsal fin
• paraxial swimming

Acknowledgements

We first and foremost would like to extend our gratitude to Aridagawa Town, Wakayama Prefecture, Japan, for logistical support during the collection and preparation of the mosasaur skeleton. The preparation of the specimen was funded by the Ministry of Health, Labour and Welfare, Government of Japan. We further thank Kanaya Junior High School (Aridagawa Town, Wakayama Prefecture) for accommodating fossil preparation. Y. Shigeta (National Museum of Nature and Science) provided us with valuable insights into the Campanian–Maastrichtian stratigraphy of Japan. Constructive feedback from A. Zietlow and M. Polcyn on the original version of our manuscript was very helpful and much appreciated. H.S. and M.W.C. acknowledge financial support from an NSERC Discovery Grant to M.W.C. (#23458), an NSERC Accelerator Award to M.W.C., and a Chairs Research Allowance to M.W.C.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplemental material

Supplemental material for this article can be accessed here: https://doi.org/10.1080/14772019.2023.2277921.

 

Associate Editor: Serjoscha Evers

Additional information

Funding

We first and foremost would like to extend our gratitude to Aridagawa Town, Wakayama Prefecture, Japan, for logistical support during the collection and preparation of the mosasaur skeleton. The preparation of the specimen was funded by the Ministry of Health, Labour and Welfare, Government of Japan. We further thank Kanaya Junior High School (Aridagawa Town, Wakayama Prefecture) for accommodating fossil preparation. Y. Shigeta (National Museum of Nature and Science) provided us with valuable insights into the Campanian–Maastrichtian stratigraphy of Japan. Constructive feedback from A. Zietlow and M. Polcyn on the original version of our manuscript was very helpful and much appreciated. H.S. and M.W.C. acknowledge financial support from an NSERC Discovery Grant to M.W.C. (#23458), an NSERC Accelerator Award to M.W.C., and a Chairs Research Allowance to M.W.C.