Morphology and function of the Axial Skeleton of Australerpeton Cosgriffi (Temnospondyli, Rhinesuchidae)

Abstract

Australerpeton cosgriffi is a temnospondyl taxon found in the Upper Permian Rio do Rasto Formation (Parana Basin, Brazil). Studied specimens, an adult (UFRGS-PV-0319-P) and a juvenile (UFRGS-PV-0320-P) are known by fully articulated axial skeletons which allowed to analyzed their locomotors capabilities. The vertebral column is composed by rhachitomous vertebrae, consisting of large crescent-shaped intercentra, two small dorso-posterior pleurocentra and the arch neural. The ribs are well preserved in the anterior presacral region and some of them have very well developed distal uncinate processes. They are mainly developed anteriorly in the adult specimen. The neural spines are straight and posteriorly displaced relative to the neural arches. The spines have longitudinal ridges that run on each side. In the juvenile, the longitudinal ridges are more evident in the anterior part of the column. The zygapophyses are well-developed and inclined approximately 45°. The structure of the axial skeleton of A. cosgriffi suggests a strong swimmer. The presence of ridges on the lateral sides of the spines indicates a well-developed epaxial musculature, combined with the presence of tall and straight neural spines, implies a reduce capability of column torsion. This condition combined with the presence of the overlapping uncinate processes would indicate that lateral movements of the trunk during swimming were limited. Therefore the trunk was mainly rigid during progression and was probably the tail the propulsive element. Another possibility according to the structure of the neural spines and zygapophyses is the presence of dorsoventral movements of the axial skeleton. This combined with the strong epaxial musculature, seems to have allowed dorsoventral movements, particularly of the head. In the juvenile, the spine longitudinal ridges and the uncinate process are less developed and diminished in size backward. This structure of the axial skeleton would suggest lateral movements of the body and thus the possibility of undulated swimming in the juveniles. In contrast, the adults had a more restricted presacral axial skeleton with a relatively stiff trunk which allowed mainly dorsoventral movements that could be related to a more active diving in larger individuals.