ANATOMY AND PALEOBIOGEOGRAPHIC IMPLICATIONS OF A NEW BAENID TURTLE FROM THE KAIPAROWITS FORMATION OF SOUTHERN UTAH

Recent discoveries demonstrate that various vertebrate clades, especially dinosaurs, exhibited basin-scale endemism during the Campanian in western North America (Laramidia). However, phylogeny-based paleobiogeographic studies are less utilized for other contemporaneous vertebrate groups. For example, baenid turtles were one of the most speciose and abundant clades of turtles during the Late Cretaceous. Additionally, this group was endemic to Laramidia, making it an ideal study system to test various biogeographic hypotheses.

In this study, I examine the anatomy and evolutionary biogeography of a new baenid taxon from the Campanian Kaiparowits Formation of southern Utah. The new taxon is highlighted by a newly discovered specimen that includes a shell, skull, vertebral column, pectoral and pelvic girdles, forelimb, and hindlimb. Anatomical description of the skeleton was augmented by the use of micro X-ray computed tomography on the articulated cranium and lower jaw to provide greater detail of anatomical features obscured by bone fusion – a common character of adult baenids – and rock matrix that could not be removed during specimen preparation. This new taxon is assignable to the subclade Baenodda based on a wedge-shaped skull and exposure of the fifth vertebral scute on the posterior carapace margin. Preliminary analysis indicates that this species is closely related to Plesiobaena antiqua from the Campanian of Alberta and Montana. However, some plesiomorphic characters – particularly large nasals – suggest that it may be basal within the clade containing the northern Laramidian taxa P. antiqua and Palatobaena. If this taxon is basal within this clade, it is consistent with the hypothesis that some latest Cretaceous (Maastrichtian) taxa from northern Laramidia were derived from earlier, southern Campanian taxa. This new species is known only from the Kaiparowits Formation, supporting the hypothesis of basin-scale endemism during the late Campanian. Other known baenid species do not exhibit the same level of endemism during this time, but do appear to support a division between northern and southern Campanian biogeographic provinces across western North America.