Assessment of intraspecific skull variation in the Campanian baenid turtle Plesiobaena antiqua

Baenids are freshwater paracryptodiran turtles with a fossil record ranging from the Early Cretaceous to the Eocene. The baenid Plesiobaena antiqua is the most common turtle in the Campanian (Late Cretaceous) Dinosaur Park Formation in Canada, but the apparent intraspecific variation remains poorly understood. We apply micro-computed tomography scanning technology (μCT scanning) and bone-by-bone segmentation to a sample of four crania and three mandibles representing different size classes. The available material allows us to rigorously document variation to the skull, of which most can be attributed to phenotypic or genetic variation, with exception of the jugal, which appears to insert itself into the margin of the orbit during ontogeny. It is unclear if the apparent variation to the presence of an epipterygoid is controlled by ontogeny or random variation. Notable variation found in the available mandibles can mostly be attributed to ontogenetic variation, consistent with a dietary shift towards durophagy. The detailed description of the skull anatomy of Plesiobaena antiqua combined with detailed documentation of variation is expected to provide new impulses for reconstructing baenid taxonomy and phylogeny.