The oldest Asian hesperornithiform from the Upper Cretaceous of Japan, and the phylogenetic reassessment of Hesperornithiformes

Abstract

Asian hesperornithiforms are extremely rare in contrast to the much more abundant record from North America. In Asia, these fossil birds are only known from fragmentary materials from Mongolia. Here we describe the skeletal remains of a new hesperornithiform Chupkaornis keraorum gen. et sp. nov. from the Upper Cretaceous Kashima Formation (Coniacian to Santonian) of the Yezo Group in Mikasa City, Hokkaido, Japan. This is the best-preserved hesperornithiform material from Asia and it is the first report of hesperornithiforms from the eastern margin of the Eurasian continent. Chupkaornis has a unique combination of characters: finger-like projected tibiofibular crest of femur, deep, emarginated lateral excavation with a sharply defined edge of the ventral margin of the thoracic vertebrae, and the heterocoelous articular surface of the thoracic vertebrae. Our new phylogenetic analysis revises the phylogenetic relationships of Hesperornithiformes. In contrast to previous studies, Enaliornis is assigned as the most basal taxon and Baptornis is positioned as more derived than Brodavis. Chupkaornis is a sister taxon to the clade of Brodavis and higher taxa. Parahesperornis and Hesperornis are positioned within Hesperornithidae, the derived Hesperornithiformes. Many of the skeletal character changes are concentrated at the base of Hesperornithidae (Parahesperornis and more derived taxa), and involve the modification of the pelvic girdle and hind limb morphology (e.g. dorsal directed antitrochanter of pelvis, short and sprawled femur, including probable lobe-toed feet suggested by the specialized distal articular surface of first digit of fourth toe, and predominantly robust digit IV phalanges). These skeletal modifications are likely adaptations for foot-propelled diving behaviour.

http://zoobank.org/urn:lsid:zoobank.org:pub:FB783237-E565-4B74-9386-EADF8E12DFD4

Keywords:

• Hesperornithiformes
• diving birds
• fossil birds
• Late Cretaceous
• new species

Acknowledgements

We thank R. Takasaki, M. Iijima, C. Tsogtbaatar (Hokkaido University) and T. Ando (Ashoro Museum of Paleontology) for their time to discuss and review the draft. We are indebted to T. Tokaryk (RSM) and his family who allowed TT to examine bird specimens. We also thank Y. Kera and Y. Miyashita for helping with fieldwork and preparing the invertebrate materials referenced in this study, M. Eda (Hokkaido University Museum) who allowed us to examine his personal specimens of extant birds, and A. Bell (Dinosaur Institute, Natural History Museum of Los Angeles) and C. Mehiling (AMNH) who allowed us to use their photographs for this paper. Several people helped us during collection visits: M. Norell and C. Mehling (AMNH), Z. Zhou and H.-L. You (IVPP), M. Desui and D. Burnham (KUVP), L. Wilson (SMNH), G. Corner (UNSM), S. Shelton and D. Pagnac (SDSM), M. Riley (CAMSM), S. Chapman (NHMUK), T. Tokaryk (RSM), D. Brinkman (YPM), D. Field (Yale University), W. Simpson (FMNH), A. Kramarz (MACN) and C. Acosta Hospitaleche and M. Reguero (MPL). TT is grateful to his family for all of their support. This work was supported by Sasakawa Scientific Research Grant from the Japan Science Society [grant number 27-524].

Supplemental data

Supplemental material for this article can be accessed here: https://doi.org/10.1080/14772019.2017.1341960.

Additional information

Funding

This work was supported by Sasakawa Scientific Research Grant from the Japan Science Society [grant number 27-524].