The first report of Champsosaurus lindoei (Choristodera: Champsosauridae) from the Campanian of the United States: anatomical, phylogenetic, and palaeoecological significance

Abstract

Although the neochoristodere Champsosaurus is well documented in Campanian deposits of western North America, species-diagnostic remains from these strata are restricted to the Dinosaur Park Formation of Alberta, Canada. Here, we describe an exceptionally well-preserved specimen of Champsosaurus lindoei from the Two Medicine Formation of Montana, USA—one of the few occurrences of the genus within the formation, and the first confirmed occurrence of C. lindoei outside of Dinosaur Provincial Park and its vicinity. This specimen preserves previously unknown aspects of this species, including the pes, tail, and integument, allowing for the first detailed postcranial description in this taxon. The integumentary impressions preserved on the ventral surface are the largest described thus far for Champsosaurus, supporting previous suggestions that Champsosaurus scales increase in size ventrally. We also conducted the first species-level phylogeny for Champsosaurus, where we recovered a pectinate tree topology with Palaeocene species as the sister taxa to older Maastrichtian and Campanian species, and provide further evidence for at least three lineages of Champsosaurus surviving the Cretaceous-Palaeogene (K-Pg) extinction. Finally, the recovery of the specimen from the semi-arid upland deposits of the Two Medicine Formation suggests that Champsosaurus may have been better able to withstand drier environments than previously thought.