Osteology, paleoecology, and phylogenetic relationships of the carettochelyid turtle Allaeochelys crassesculpta from the middle Eocene Konservat-Lagerstätte of Messel, Germany

Allaeochelys crassesculpta is a carettochelyid turtle from the middle Eocene Messel Pit, near Messel, Germany. The species is known by one of the richest fossil records among turtles consisting of dozens of specimens, of which most represent near-complete and articulated skeletons. Despite this richness, the osteology of Allaeochelys crassesculpta is only known superficially as its anatomy was never described in detail. Here, we use micro-computed laminography and tomography to investigate the anatomy of the skull, shell, and appendicular skeleton and provide high-resolution 3D reconstructions of the cranium, mandible, and fore- and hindlimb. Allaeochelys crassesculpta exhibits an interesting mosaic of features, which are typical of early branching carettochelyids (e.g. a deeper and larger antrum postoticum, a foramen posterius canalis carotici interni located more anteriorly) and more advanced taxa (e.g. presence of a maxilla-quadratojugal contact below the jugal, complete loss of carapace and plastral scutes). We assemble and analyze a new phylogenetic matrix to test the impact of recent anatomical works on the evolutionary history of carettochelyids. We also provide evidence that Allaeochelys crassesculpta was capable of ‘synchronous rowing,’ the unique locomotor mode employed by the extant pig-nosed turtle Carettochelys insculpta and suggest that the acquisition of this peculiar trait may have occurred during the Late Cretaceous to Paleocene.