A new specimen of Australerpeton Cosgriffi Barberena, 1998 (Stereospondyli: Rhinesuchidae) from the middle/upper Permian Rio Do Rasto formation, Paraná Basin, Brazil

Abstract

Here we present a skull and a lower jaw of a temnospondyl specimen collected from the São Jerônimo da Serra Site, an exposure of the Morro Pelado Member of the Rio do Rasto Formation (Middle/Upper Permian, Paraná Basin), Paraná State of southern Brazil. The sedimentary environment in which the fossil was preserved is characterized by a massive reddish mudstone derived from the slow deposition of fine sediments from suspended load in a terminal splay or ephemeral lake setting. The comparative anatomical description of the new specimen allows its assignment to the rhinesuchid Australerpeton cosgriffi Barberena. The new specimen represents the most complete skull and lower jaw of this species found up to date. Its high degree of ossification associated with its large skull size (58 cm long) indicates that it was a mature adult. This work, besides confirming several anatomical characteristics of A. cosgriffi, reveals previously unknown information about this species: a long cultriform process whose anterior edge surpasses the level of the posterior border of the choanas, being interposed between them; and a pair of longitudinal ridges lacking denticles that run parallel along the premaxillary surface without making contact with the choanae. These features were not reported before in the smaller known skulls of A. cosgriffi because they likely develop late in the ontogeny of this species. This new find provides evidence that this unique long-snouted rhinesuchid was a common and successful faunal component of the Middle-Late Permian fluvio/lacustrine ecosystems of southern Brazil, and reaffirms the Rio do Rasto Formation of Brazil as a rich source of information concerning the initial diversification of the Stereospondyli.