The mandibular anatomy of the non-stahleckeriine kannemeyeriiforms (Synapsida, Dicynodontia) from the Ladinian?early Carnian Chañares Formation of northwestern Argentina, and its taxonomic implications

27. ESCOBAR J.A., MARTINELLI A.G., EZCURRA M.D., FIORELLI L.E., VON BACZKO M.B., DESOJO J.B.

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

According to the most recent taxonomic revision, all non-stahleckeriine kannemeyeriiforms from the Ladinian?early CarnianChañares Formation (Ischigualasto-Villa Unión Basin, northwestern Argentina) belong to Dinodontosaurus brevirostris andthose from the Pinheiros-Chiniquá Sequence (Santa Maria Supersequence, Brazil) to D. tener, bolstering thebiostratigraphical link between both units. However, the study of these forms has been historically biased towards cranialanatomy and poorly detailed osteological descriptions. Until recently, the only relatively complete mandible of theArgentinean species belonged to the specimen PULR 03 (Museo de Ciencias Antropológicas y Naturales, UniversidadNacional de La Rioja), which is the holotype of Dinodontosaurus ?platygnathus?. Published data about its anatomy has beenfocused on a few features to justify different taxonomic schemes, but a detailed revision is still lacking. Here we presentnovel and updated observations on the mandible of Dinodontosaurus brevirostris from the Chañares Formation on the basisof PULR 03 and a novel specimen referable to this species, CRILAR-Pv 94 (Centro Regional de Investigaciones Científicasy Transferencia Tecnológica, Anillaco), from the Massetognathus-Chanaresuchus Assemblage zone of El Torcido locality. Thedorsal margin of the mandible is relatively straight, lacking a robust dentary coronoid eminence and the sigmoidalappearance frequently found in Permian dicynodonts and a few Triassic forms (e.g., the kannemeyeriid Acratophorusargentinensis). The reflected lamina is reinterpreted in PULR 03 as widely separated from the retroarticular process, unlikestahleckeriine stahleckeriids (Stahleckeria potens, Ischigualastia jenseni, Sangusaurus parringtonii), the placeriine stahleckeriidMoghreberia nmachouensis, and a few kannemeyeriids (A. argentinensis, Kannemeyeria lophorhinus), but very similar to most?shansiodontids? (e.g., Shansiodon wangi), most kannemeyeriids (e.g., Kannemeyeria simocephalus), the placeriine stahleckeriidPlacerias hesternus, and at least some specimens of D. tener. On the dorsal surface of the dentary, the mid-dentary grooveis wider and deeper than most kannemeyeriiforms, but similar to the placeriine stahleckeriid M. nmachouensis. Thesepreliminary results, although pendant of integration with the analysis of the skull, are congruent with the position ofDinodontosaurus as more closely related to Stahleckeriinae than to other kannemeyeriiforms, but outside the former cladebecause of the morphology of the reflected lamina. Thus, our revision bolsters the presence of both stahleckeriinestahleckeriids, based on recently published specimens, and non-stahleckeriine kannemeyeriiforms in the ChañaresFormation, as occurs in the Pinheiros-Chiniquá Sequence of southern Brazil.