Occurrence of Styxosaurus (Sauropterygia: Plesiosauria) in the Cenomanian: implications for relationships of elasmosaurids of the Western Interior Seaway

Abstract

The systematics of the elasmosaurid plesiosaurs from the Late Cretaceous of the Western Interior Seaway have been contentious, with studies supporting an endemic Elasmosaurinae clade. In this study four elasmosaurids from the Western Interior Seaway are examined in detail, and others discussed, to clarify the validity and ingroup relationships of the clade Elasmosaurinae. Two of the taxa examined here are Campanian in age: the holotype of Styxosaurus snowii, and an indeterminate species of Styxosaurus (‘Alzadasaurus pembertoni’). The other two specimens are Cenomanian: the holotype of Thalassomedon haningtoni, and an elasmosaurid specimen from Nebraska previously referred to Thalassomedon haningtoni. We find that the Nebraska specimen is referable to Styxosaurus. We therefore refer the Nebraska material to the new species Styxosaurus rezaci, distinguished by an extremely large caniniform tooth in the second maxillary tooth position, circular external nares, and the frontal forming the entire posterior margin of the external naris. We examined a matrix of 134 operational taxonomic units (OTUs) and 283 morphological characters with a parsimony analysis in TNT. We scored each of these elasmosaurs listed above as an individual OTU; also listed as OTUs were Styxosaurus browni and AMNH 1495 (Elasmosauridae indet.). We find that Thalassomedon and the species of Styxosaurus do form an endemic clade. However, the long-necked taxa Terminonatator and Albertonectes are not recovered as members of this clade, due to missing data for both taxa. We recovered Styxosaurus as a monophyletic clade diagnosed by features of the squamosal, the exclusion of the frontal from the orbit margin, and 60–63 cervical vertebrae. We refer the headless skeleton, AMNH 1495, to Elasmosauridae (indet.). We conclude that the genus Styxosaurus was a long-lived clade that inhabited the Western Interior Seaway for at least 12 million years.

http://zoobank.org/urn:lsid:zoobank.org:pub:7E81A26D-9E19-42FE-9CAD-D58908EDCF7A

Keywords:

• Elasmosauridae
• Plesiosauria
• cranial osteology
• Styxosaurus
• Western Interior Seaway
• Cretaceous

Acknowledgements

I (EAS) thank members of my master’s committee, Dr Herman Mays, and Dr Habiba Chirchir, for their valuable feedback on this manuscript and the ideas within. The Marshall University Department of Biological Sciences provided travel support. We thank curators Ross Secord (University of Nebraska), Chris Beard (University of Kansas), Tylor Lyson (Denver Museum of Nature and Science) and Darrin Paginac (South Dakota School of Mines and Technology) for granting access to fossil specimens. Many thanks to Joel Nielsen (University of Nebraska State Museum), Megan Sims (University of Kansas) and Kristen MacKenzie (Denver Museum of Nature and Science) for facilitating access to fossil specimens. Additional thanks to Danielle Serratos (for both collections management and consultation), R. Clark, E. Lester, C. Sidor, B. Gee, S. Olroyd and Z. Kulik. We thank the reviewers for feedback that was consequential in improving this manuscript.

Disclosure statement

We, the authors of this manuscript (Elliott Armour Smith and F. Robin O’Keefe), have no financial conflicts to disclose related to the publication of the research contained in this paper.

Supplemental material

Supplemental material for this article can be accessed here: https://doi.org/10.1080/14772019.2023.2242846.

Associate Editor: Erin Maxwell