Anatomy of the skull and inner ear cavity of the North American Eocene turtle Echmatemys based on high-resolution computed tomography and its implications for the deep history of Geoemydidae (Cryptodira: Testudinoidea)

Testudinoidea is a radiation of terrestrial and freshwater species that account for nearly half of all extant turtle diversity. The early fossil history of this lineage is exemplified well by the early Eocene taxon Echmatemys, whose extensive, shell-dominated record is difficult to place in a modern phylogenetic context due to a lack of cranial details. Here, we use computed tomography to describe the anatomy of two skulls, USNM 17999 and YPM PU-11525, that have long been attributed to Echmatemys and whose preservational quality bears some promise of clarifying early testudinoid history. Results largely confirm earlier predictions that these two skulls represent well the anatomical expectations of an early, plesiomorphic testudinoid. A positioning of the geniculate ganglion (cranial nerve VII) within the facial canal rather than the canalis cavernosus is a derived feature among testudinoids that brings these skulls into phylogenetic congruence with certain shells of Echmatemys as being closely related to the extant Mauremys radiation of Eurasian pond turtles. This deeply nested position within Geoemydidae further supports the inference that an extensive Cretaceous and Paleocene diversification of crown Testudinoidea awaits detailed study. The prominent vomerine ridge of these Echmatemys skulls suggests this feature is not unique to the testudinid (tortoise) lineage, thus adding to the complex nature of early testudinoid morphology.