Traversodontidae is a group of Triassic herbivorous/omnivorous cynodonts that represents the most diversified lineage within Cynognathia. In southern Brazil, a rich fossil record of late Middle/mid-Late Triassic cynodonts has been documented, with Exaeretodon riograndensis Abdala, Barberena, and Dornelles, 2002 and Siriusgnathus niemeyerorum Pavanatto et al., 2018 representing two abundant and well-documented traversodontids. The present study provides a comparative analysis of the morphology of the nasal cavity, nasal recesses, nasolacrimal duct, and maxillary canals of both species using computed tomography, highlighting the changes that occurred in parallel to the origin of mammaliaforms. Our results show that there were no ossified turbinals or a cribriform plate delimiting the posterior end of the nasal cavity, suggesting these structures were probably cartilaginous as in nonmammaliaform cynodonts. Both species show lateral ridges on the internal surface of the roof of the nasal cavity, but the median ridge for the attachment of a nasal septum is absent. Exaeretodon riograndensis and S. niemeyerorum show recesses on the dorsal region of the nasal cavity, which increase the volume of the nasal cavity, potentially enhancing the olfactory chamber and contributing to the sense of smell. On the lateral sides of the nasal cavity, the analyzed taxa show a well-developed maxillary recess. Although E. riograndensis and S. niemeyerorum have roughly similar nasal cavities, in the former taxon, the space between the left and right dorsal recesses of the nasal cavity is uniform along its entire extension, whereas this space narrows posteriorly in S. niemeyerorum. Finally, the nasolacrimal duct of S. niemeyerorum is more inclined anteroposteriorly than in E. riograndensis.
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New postcranial material of a dicynodont from the ChaÃares Formation (Triassic).
The new material and an historical specimen are both referred to Stahleckeriidae.
Stahleckeriids from Argentina probably date back beyond the Ladinian-Carnian boundary.
Abstract
The kannemeyeriiforms of the latest Ladinian-?early Carnian ChaÃares Formation (Ischigualasto-Villa UniÃn Basin) exemplify the historical taxonomic controversies of several South American dicynodonts. Three taxa were originally recognized based on skull anatomy: "Chanaria" platyceps and two species of Dinodontosaurus (genus otherwise known in the Pinheiros-Chiniquà Sequence, Brazil), but in a recent taxonomic revision they were all synonymized in a single valid species (i.e., Dinodontosaurus brevirostris). The only postcranial record includes some undescribed specimens referred to D. brevirostris, an indeterminate shoulder girdle (MCZ 3459), and an ulna recently referred to Stahleckeria sp. Here we describe a new, large dicynodont specimen (CRILAR-Pv 82) from the Tarjadia Assemblage Zone (AZ) of the lowermost levels of the ChaÃares Formation, consisting of a right scapula, left ilium, sternum, and a few indeterminate fragments. The tall, dorsally expanded scapular blade with anteroventrally inclined dorsal margin and the anterolaterally directed acromion process broadly match the stahleckeriid morphotype. CRILAR-Pv 82 shares with Stahleckeria potens a sharp, vertical scapular spine on the anteroventral margin of the scapular blade, markedly convex in anterior view. The "hook-shaped" curvature of the preacetabular iliac process also resembles Stahleckeria potens and Eubrachiosaurus browni. These features differ at least with the Brazilian specimens of Dinodontosaurus (D. tener). The available data shows that MCZ 3459 also fits the stahleckeriid morphology, becoming the first historical record of the clade for the unit. A rigorous comparison with D. brevirostris will await the examination of more complete specimens. The dicynodont postcrania described so far from the ChaÃares Formation represent the oldest record of Stahleckeriinae in the Ischigualasto-Villa UniÃn Basin, expanding back its Argentinian record very likely beyond the Ladinian-Carnian boundary.