First CT-assisted study of the palate and postcrania of Diarthrognathus broomi (Cynodontia, Probainognathia).

Abstract

Diarthrognathus broomi is a transitional taxon between non-mammaliaform cynodonts and Mammaliaformes that occurred during the Late Triassic to Early Jurassic. All known specimens of Diarthrognathus represent juveniles, and the postcrania have not been thoroughly described. The palatal, basicranial and postcranial elements of the referred specimen NMQR 1535 are described here for the first time using 3D reconstructions generated from X-ray micro-computed tomography (μCT) data. The presence of a large interpterygoid vacuity, open medial suture between the vomers and medially unossified secondary palate all support the interpretation that NMQR 1535 is a juvenile. In addition, Diarthrognathus uniquely possesses "suborbital" vacuities, which distinguishes it from every other known cynodont. The presence of an ossified olecranon process, among other features, suggests that Diarthrognathus may have been a scratch-digger. The postcranial skeleton of Diarthrognathus appears to be more plesiomorphic than tritylodontids, Brasilodon and other tritheledontids as, among other traits, it retains amphicoelous vertebrae. However, this taxon also displays synapomorphies with the more derived cynodonts, such as the mammalian pattern of neurocentral ossification and possible absence of an ectepicondylar foramen.