The first record of Probainognathus Romer,1970 (Therapsida, Cynodontia) in the Triassic of Rio Grande do Sul, Brazil

SOARES, M. B.; ABDALA, F.

Ribeirao Preto

Simposio; VI Simpósio Brasileiro de Paleontologia de Vertebrados; 2008

The non-mammaliaform cynodont Probainognathus is recorded for the first time in the Triassic Santa Maria Formation, southern Brazil. The taxon, represented by a fragment of a right mandibular ramus with a relatively complete sectorial postcanine tooth, was discovered in the locality of Santa Cruz do Sul. The only tooth preserved isinterpreted as the last postcanine, since the coronoid process begins to rise behind the tooth. Three preceding postcanines are represented only by their crown bases. The morphological pattern of the crown, dimensions and low degree of wear of the tooth show strong similarities with juvenile specimens of Probainognathus from the Chañares Formation, Argentina. Like these, the postcanine of the Brazilian specimen is sectorial, bearing five longitudinally aligned cusps: a large main cusp, two mesial and two distal accessory cusps. The accessory cusps closer to the main cusp are larger. The top of the larger mesial accessory cusp is broken, however, this cusp seems to be smaller than the larger distal accessory one. Mesial and distal cingular cusps are observed on the lingual view, with the distal being wider and taller than the mesial. The roots of the postcanines are single. The fauna of Santa Cruz do Sul is dominated by traversodontid cynodonts (e.g. Santacruzodon hopsoni), which are also diverse, being represented by at least four different taxa. The assemblage also records carnivorous chiniquodontid cynodonts and proterochampsid archosaurs. Some traversodontids of this fauna show a mixture of features linking them to forms from the Ladinian Chañares Formation and the Carnian Ischigualasto Formation, both in Argentina. These Brazilian traversodontids also allow a clear correlation with a Late Ladinian/Early Carnian fauna from Madagascar. In addition, four isolated tiny postcanines are similar to those of a traversodontid from the Late Ladinian/Middle Carnian of North America. Chiniquodontid cynodonts are known in all the Gondwanan faunas previously mentioned. Probainognathus was until now only represented in the Ladinian Chañares Formation, as well as a dubious taxonomic record of a juvenile specimen from the Carnian Ischigualasto Formation. The new record of this taxon in Santa Cruz do Sul provides additional evidence that tightens the correlation of this association with the Ladinian fauna from Argentina.