FIRST CRANIAL FOSSIL OF A NEONATE CHAMPSOSAURUS FROM THE PALEOCENE OF NORTH AMERICA

Champsosaurus is an extinct genus of aquatic diapsid reptiles that lived throughout the Western Interior of North America during the Late Cretaceous and Paleocene. Multiple species of Champsosaurus have been described, including two Paleocene taxa from fluviolacustrine deposits in western North Dakota, C. gigas and C. tenuis. Both species are known from adult specimens, some of which grew to relatively large sizes (ca. 3 m total length). Little is known about juveniles of either taxon, as none have thus far been documented. Here, we report the first cranial fossil of a neonate Champsosaurus from the Paleocene of North America. This specimen, previously misidentified as a lizard, was recovered from the ca. 60 Ma Wannagan Creek fossil site in western North Dakota in the 1970s and is housed in the paleontology collections of the Science Museum of Minnesota (SMM). We used X-ray computed tomography (CT) scanning to capture the morphology of the specimen at high resolution. We then compared our CT scan of the neonate specimen to adult specimens of C. gigas and C. tenuis in the SMM paleontology collections, as well as other Champsosaurus specimens described in the literature. The neonate specimen consists of a right postfrontal + postorbital, with visible sutures for the missing frontal, parietal, jugal, and squamosal bones. The posterior margin of the right orbit is preserved, as well as partial margins of both the inferior and superior temporal fenestrae. The specimen is too incomplete to allow for more specific taxonomic assignment beyond Champsosaurus. However, the neonate specimen may very well belong to C. gigas, given the occurrence of this taxon at Wannagan. We used cranial measurements from adult Champsosaurus specimens (n = 3) at SMM to estimate a skull length of 86–252 mm for the neonate specimen; this is somewhat to substantially smaller than the smallest documented Champsosaurus skull, belonging to C. lindoei from the Late Cretaceous of Alberta. The occurrence of a neonate Champsosaurus at the Wannagan Creek fossil site is somewhat surprising, given the great abundance of potential predators (e.g., crocodylians) that inhabited the same local paleoenvironment. Nevertheless, the presence of a neonate implies that suitable conditions for breeding populations of Champsosaurus existed at Wannagan.