MICRO-COMPUTED TOMOGRAPHY AND 3-D RECONSTRUCTION OF A NEW BAENID TURTLE FROM THE CRETACEOUS KAIPAROWITS FORMATION, SOUTHERN UTAH

Articulated specimens provide important insights into the anatomy of vertebrate taxa; however, articulation can often obscure features only observable on disarticulated specimens. In these instances, X-ray computed tomography (CT) and 3-D reconstructions allow for digital disarticulation of a specimen.

I present information on the cranial and endocranial anatomy of a new taxon of baenid turtle from the Upper Cretaceous (Campanian) Kaiparowits Formation of Utah. In addition to a shell and well-preserved postcranial elements, the specimen includes an articulated cranium, lower jaw, atlas, and axis. This turtle possesses two distinct external nares separated by a bony septum, a character previously unknown within Baenidae and rare among Testudines. Because the mandible and anterior cervical vertebrae articulate with the cranium, many other key osteological features are obscured. To gain further insight into the anatomy of this specimen, I utilized μCT to image the skull, digitally remove matrix, digitally disarticulate the lower jaw and vertebrae, and reconstruct cranial morphology in three dimensions.

Three-dimensional digital visualization elucidates several characters that are phylogenetically significant and not visible on the specimen, including the triturating surfaces, external pterygoid processes, and basioccipital tubercles. My phylogenetic analysis found this taxon to be relatively basal within the subclade Baenodda and sister to the Maastrichtian taxon Hayemys latifrons. Enlarged olfactory lobes visible on the endocast suggest a heightened sense of smell compared to other baenids. The mandible was found to possess a prominent symphysial hook, moreso than in any other baenid. Also on the lower jaw, large muscle attachment sites and high coronoid processes are indicative of a powerful bite, as has been proposed for the distantly-related baenid taxon Palatobaena. However the new taxon lacks broad triturating surfaces, which are thought to indicate a molluscivorous lifestyle for Palatobaena. Most baenids, such as Plesiobaena and Boremys, are thought to have been generalist feeders, as they lack wide triturating surfaces and muscle scars. The hooked mandible and strong bite suggest this new taxon may have been adapted to exploit a unique food source with a tough but pierceable outer coating.