A new morphotype of Traversodontidae (Cynodontia) from the Candelária Sequence (late Triassic), Brazil

Abstract

Traversodontids are gomphodont cynodonts from the Triassic, discovered in almost all continents, being most diverse and abundant in Gondwanan landmasses. They are characterized by their labioa lingually enlarged postcanines with a quite precise occlusion and enlarged cranial structures (e.g., zygomatic arches, sagittal crest) for powerful muscles related to mastication. Among traversodontids, Gomphodontosuchinae includes the most derived taxa, composed by six genera: Gomphodontosuchus, Menadon, Protuberum, Exaeretodon, Scalenodontoides and Siriusgnathus. We present here the specimen UFRGS-PV-1419-T that was collected in Santa Maria city (Rio Grande do Sul state) in 2003, in an outcrop located on the margins of the Cancela stream. It was associated with dozens of postcranial bones of cynodonts, as well a skull and post-cranial bones of at least two rhynchosaur specimens, including a maxilla (UFRGS-PV-1458-T) assigned to Hyperodapedon sp. Due to this associated material, the taphocoenosis of this outcrop is referred to the Hyperodapedon Assemblage Zone (Candelária Sequence): UFRGS-PV-1419-T was identified as Gomphodontosuchinae based on the presence of three large upper incisors, reduced lower canine and lack of a central cusp in transverse crest of postcanines. Comparisons were made with Exaeretodon riograndensis (CAPPA-UFSM-0030, МСР-1522-PV), Exaeretodon argentinus (MACN-PV-18063) and Siriusgnathus niemeyerorum (CAPPA-UFSM-0032) as follow: UFRGS-PV-1419-T shares with Exaeretodon three upper incisors, while only two are present in Siriusgnathus; the postorbital bar of UFRGS-PV-1419 is more posteriorly positioned than in Siriusgnathus and anteriorly than in Exaeretodon; the orbital floor in UFRGS-PV-1419-T is smooth as in Exaeretodon, but with a crest in Siriusgnathus; the dorsoventral crest in the zygomatic process of the squamosal is absent in UFRGSPV-1419-T and Siriusgnathus but well pronounced in Exaeretodon species. Thus, comparisons indicate that UFRGS-PV-1419-T cannot be referred to any known species and it can represent a new morphotype, closely related to Exaeretodon and Siriusgnathus. This new morphotype, as Siriusgnathus, has a morphologic resemblance to the Argentinean taxon ‘Proexaeretodon', which was considered synonymous of Exaeretodon. Additional comparisons and a revision of 'Proexaeretodon' specimens are needed to elucidate the validity of this genus and the possible affinity with the Brazilian taxa/specimens. This new material increases the knowledge of traversodontids as well as its diversity in the Hyperodapedon AZ. [CAPES, CNPq]