Morfologia da cavidade nasal de cinodontes traversodontidae (Eucynodontia: Gomphodontia) do Triássico Superior do Rio Grande do Sul

Abstract

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 and Siriusgnathus niemeyerorum representing two abundant and well-documented traversodontids. The present master’s dissertation provides a comparative analysis of the morphology of the nasal cavity, recesses, nasolacrimal duct, and maxillary canals of both species using computed tomography, highlighting the changes that occurred in parallel to the origin of mammaliaforms. Three crania that represent the best-preserved specimens of these species were scanned, the tomograms were analyzed, and three-dimensional models were generated. After these procedures, the nasal morphology was compared to other representatives of Cynognathia and Probainognathia. Our results show that there were no ossified turbinals or a cribriform plate were delimiting the posterior end of the nasal cavity, suggesting these structures were probably cartilaginous as in other 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 a roughly similar nasal cavity, 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.