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

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 <i>Styxosaurus snowii</i>, and an indeterminate species of <i>Styxosaurus</i> (‘<i>Alzadasaurus pembertoni</i>’). The other two specimens are Cenomanian: the holotype of <i>Thalassomedon haningtoni</i>, and an elasmosaurid specimen from Nebraska previously referred to <i>Thalassomedon haningtoni</i>. We find that the Nebraska specimen is referable to <i>Styxosaurus</i>. We therefore refer the Nebraska material to the new species <i>Styxosaurus rezaci</i>, 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 <i>Styxosaurus browni</i> and AMNH 1495 (Elasmosauridae indet.). We find that <i>Thalassomedon</i> and the species of <i>Styxosaurus</i> do form an endemic clade. However, the long-necked taxa <i>Terminonatator</i> and <i>Albertonectes</i> are not recovered as members of this clade, due to missing data for both taxa. We recovered <i>Styxosaurus</i> 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 <i>Styxosaurus</i> 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