SYSTEMATICS AND PALEOECOLOGY OF FOSSIL LIZARDS FROM THE MIDDLE–LATE PALEOCENE WANNAGAN CREEK FOSSIL SITE IN WESTERN NORTH DAKOTA

The Wannagan Creek fossil site in western North Dakota provides a rich record of life from a middle–late Paleocene (ca. 60 Ma) fluviolacustrine system, particularly with regard to its reptile fauna. Despite extensive documentation of its crocodylians, choristoderes, and turtles, there are no published accounts on the lizards of Wannagan, fossils of which were collected in the 1970s but have never been formally studied or described. We remedy this issue with high-resolution computed tomography (CT) scans of all currently available cataloged lizard fossils from Wannagan (n = 21), housed in the paleontology collections of the Science Museum of Minnesota. In order to determine the systematic affinities of the Wannagan lizards, we compared our CT scans with modern and fossil lizard specimens from museum collections, the literature, and several online repositories for CT data (MorphoSource, DigiMorph, and Phenome10k). We identified cranial and mandibular elements from Wannagan representing no fewer than four distinct lizard taxa, including Provaranosaurus, Restes, Exostinus, and “Gerrhonotus,” all anguimorphs, and all of which have also been found at other Paleocene localities throughout the Western Interior. We also tentatively identified two isolated vertebrae as belonging to anguines, based on their similarity to vertebrae of extant Ophisaurus. Curiously, we did not find any definitive fossils of glyptosaurine anguids or amphisbaenians, both of which are typically present in Paleocene lizard assemblages throughout the Western Interior. Additionally, there are no iguanians, xantusiids, or varanids in the examined Wannagan lizard assemblage. Considering the overall small number of fossil lizard specimens recovered from Wannagan, the apparent low diversity and absence of some common Paleocene taxa may be attributable to sampling and/or taphonomic biases. Regardless, the Wannagan lizard assemblage helps fill in spatial and temporal gaps in the squamate fossil record of North America, which documents a slow recovery of lizards and snakes following the K–Pg extinction event 66 Ma. Our study also contributes new information to the overall paleoecology of the Wannagan Creek fossil site, and represents the very first time that any small reptile fossils from the site have ever been CT-scanned.