The postcranial skeleton of Ennatosaurus tecton (Synapsida, Caseidae)

The postcranial material referable to the Russian caseid Ennatosaurus tecton from the middle Permian is described. Although the cranium has been recently restudied in detail, the descriptions currently available for the postcranial skeleton are essentially limited to the original short account on the holotype provided by Efremov in 1956. The postcranium of Ennatosaurus is informative with respect to both taxonomy and phylogeny, with autapomorphic characters present particularly in the vertebral column. In addition to the anatomical description of the elements presented here, eight principal component analyses were conducted to investigate the position of the various osteological elements of Ennatosaurus within caseid morphospace. The inclusion of members of all major groups of ‘pelycosaurs’ and selected outgroup taxa allowed us to make more extensive preliminary inferences regarding postcranial morphospace occupation of these basal synapsids for each individually considered bone. The analysis revealed a major decoupling among the morphospaces of stylopodial and zeugopodial elements, with femora and humeri showing a shared common pattern, and a wider overlap in their respective morphospace. Conversely, the ulnae, radii, tibiae and fibulae show well-separated regions of morphospace in the different clades, indicating their potential importance, not only for functional and biomechanical studies, but also for taxonomic differentiation. Finally, a 3D photogrammetric model of the mounted specimen at the Paleontological Institute in Moscow forms the basis for the first in vivo reconstruction of Ennatosaurus tecton, providing a potentially realistic picture of the Russian caseid in life.