Cranial anatomy of the large aetosaur Paratypothorax andressorum from the Upper Triassic of Germany

DESOJO, J. B.; RAINER R. SCHOCH

Congreso; Society of Vertebrate Paleontology 74th Annual Meeting; 2014

The large aetosaur Paratypothorax andressorum has so far been known only by its osteoderms. Here we describe the skull of a complete, articulated specimen of this taxon that was found in the type horizon at Murrhardt, southwestern Germany. P. andressorum has the following cranial autapomorphies: (1) upper jaw margin with deep notch between premaxilla and maxilla, (2) maxilla-lacrimal suture with finger-like projection, (3) upper temporal fenestra triangular, and (4) first paramedian cervical plates narrow and oval, much smaller than second row. Apart from these features, the skull of P. andressorum. closely resembles that of the small aetosaur Aetosaurus ferratus known from the same horizons, despite major differences in the morphology of the osteoderms. Both taxa share (1) the pointed, beak-shaped premaxilla which expands only gently anterior to the nasal, (2) maxilla and lacrimal excluding jugal from margin of antorbital fenestra, (3) exclusion of squamosal from margin of infratemporal fenestra, and (4) posterior part of jugal not downturned. Phylogenetic analysis performed recovered Paratypothorax as the sister taxon of Rioarribasuchus, supported by two osteoderm characters, as a Paratypothoracisini in agreement with previous studies. The close resemblance of Paratypothorax and Aetosaurus and the immature state of all available Aetosaurus specimens suggest two alternative hypotheses: either (1) Aetosaurus is the juvenile of a close relative of Paratypothorax or (2) it is itself the juvenile form of Paratypothorax.