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

JUAN ALEJANDRO ESCOBAR; AGUSTÍN GUILLERMO MARTINELLI; MARTÍN DANIEL EZCURRA; LUCAS ERNESTO FIORELLI; MARÍA BELÉN VON BACZKO; JULIA BRENDA DESOJO

Ciudad Autónoma de Buenos Aires

Congreso; 12° Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

According to the most recent taxonomic revision, all non-stahleckeriine kannemeyeriiforms from the Ladinian-early Carnian Chañares Formation (Ischigualasto-Villa Unión Basin, northwestern Argentina) belong to Dinodontosaurus brevirostris and those from the Pinheiros-Chiniquá Sequence (Santa Maria Supersequence, Brazil) to D. tener, bolstering the biostratigraphical link between both units. However, the study of these forms has been historically biased towards cranial anatomy and poorly detailed osteological descriptions. Until recently, the only relatively complete mandible of the Argentinean species belonged to the specimen PULR 03 (Museo de Ciencias Antropológicas y Naturales, Universidad Nacional de La Rioja), which is the holotype of Dinodontosaurus “platygnathus”. Published data about its anatomy has been focused on a few features to justify different taxonomic schemes, but a detailed revision is still lacking. Here we present novel and updated observations on the mandible of Dinodontosaurus brevirostris from the Chañares Formation on the basis of PULR 03 and a novel specimen referable to this species, CRILAR-Pv 94 (Centro Regional de Investigaciones Científicas y Transferencia Tecnológica, Anillaco), from the Massetognathus-Chanaresuchus Assemblage Zone of El Torcido locality. The dorsal margin of the mandible is relatively straight, lacking a robust dentary coronoid eminence and the sigmoidal appearance frequently found in Permian dicynodonts and a few Triassic forms (e.g., the kannemeyeriid Acratophorus argentinensis). The reflected lamina is reinterpreted in PULR 03 as widely separated from the retroarticular process, unlike stahleckeriine stahleckeriids (Stahleckeria potens, Ischigualastia jenseni, Sangusaurus parringtonii), the placeriine stahleckeriid Moghreberia 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 stahleckeriid Placerias hesternus, and at least some specimens of D. tener. On the dorsal surface of the dentary, the mid-dentary groove is wider and deeper than most kannemeyeriiforms, but similar to the placeriine stahleckeriid M. nmachouensis. These preliminary results, although pendant of integration with the analysis of the skull, are congruent with the position of Dinodontosaurus as more closely related to Stahleckeriinae than to other kannemeyeriiforms, but outside the former clade because of the morphology of the reflected lamina. Thus, our revision bolsters the presence of both stahleckeriine stahleckeriids, based on recently published specimens, and non-stahleckeriine kannemeyeriiforms in the Chañares Formation, as occurs in the Pinheiros-Chiniquá Sequence of southern Brazil.