On the occurrence of the traversodontid Massetognathus ochagaviae (Synapsida, Cynodontia) in the early late Triassic Santacruzodon Assemblage Zone (Santa Maria Supersequence, southern Brazil): Taxonomic and biostratigraphic implications

Abstract

The traversodontid cynodont Massetognathus ochagaviae is confirmed for the first time in the Santacruzodon Assemblage Zone (Carnian) of the Upper Santa Maria Supersequence of Brazil. Previously to this record, the taxon was known from the Massetognathus-Chanaresuchus AZ of the Chañares Formation, in Argentina (early Carnian) and the Dinodontosaurus AZ of the Santa Maria Supersequence, Brazil (late Ladinian-early Carnian). The studied material was collected in the Schöenstatt outcrop in Santa Cruz do Sul municipality, state of Rio Grande do Sul. The attribution to the species M. ochagaviae is based on the labial border of the upper postcanines teeth extending outward to form an isosceles triangle, one of the diagnosis features of the species, the subrectangular shape and the presence of a short lingual ridge in the lower postcanines, and others traits shared with M. pascuali and M. ochagaviae, as the general morphology of skull, with a lateral platform on the maxilla, 10 to 12 upper postcanine teeth with posterior cingulum, and up to 11 lower postcanine teeth with the transverse mesial width wider than the distal. A cladistic analysis placed the new specimen as sister-group of M. ochagaviae. The Schöenstatt outcrop is the best-studied fossil-bearing site of the Santacruzodon AZ, in which the traversodontid cynodonts are the dominant faunal components, representing 82% of the collected specimens. The Santacruzodon AZ, correlated to the Malagasy basal “Isalo II” beds of the Morondava Basin, also shares taxa with the Dinodontosaurus AZ and the Chañares and the Ischigualasto formations, representing a unique Triassic association. As a consequence, the Santacruzodon AZ represents a faunal association still not recognized in the Ischigualasto-Villa Unión Basin. New data from the Santacruzodon AZ is required to further address these questions.