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Abstract
Rhynchosaurs are a clade of quadruped, herbivorous stem-archosaur diapsids restricted to the Triassic Period. The group became globally distributed and the numerically dominant tetrapods of several terrestrial ecosystems before their extinction. Derived rhynchosaurs are characterized by a specialized masticatory apparatus, composed of a blade-and-groove occlusion with multiple longitudinal maxillary tooth rows. The morphology of the maxillary tooth plate has shown to be taxonomically and phylogenetically informative. So far, two rhynchosaur maxillary tooth plate morphotypes are known in Argentina, one belonging to an unnamed stenaulorhynchine from the Chañares Formation and the other to the hyperodapedontine Hyperodapedon sanjuanensis, the single rhynchosaur species currently name for the Ischigualasto Formation. Here we describe a new rhynchosaur maxillary tooth plate morphotype based on an indeterminate hyperodapedontine specimen from the Ischigualasto Formation. This maxillary tooth plate (PVL 2728) possesses a single longitudinal groove that divides symmetric lateral and medial tooth-bearing areas with relatively large tooth crowns, which is an uncommon combination of features among hyperodapedontines. These qualitative observations in addition to quantitative analyses show that the morphology of PVL 2728 differs from that of, at least, other sampled South American rhynchosaurs. Therefore, this specimen expands the morphological disparity of rhynchosaurs in northwest Argentina and southwestern Pangaea.
Acknowledgements
We thank Pablo Ortiz, curator of the Colección de Paleontología de Vertebrados del Instituto Miguel Lillo (San Miguel de Tucumán), for allowing the access to specimens and the loan of PVL 2728 for the purpose of this research. We also appreciate the access of the following curators, collection managers, and researchers to other rhynchosaur specimens: H. Morgenroth and R. Taylor (EXEMS), Ana María Ribeiro (MCN), Marco Brandalise de Andrade (MCP), Jessica Cundiff (MCZ), Sandra Chapman (NHMUK PV), Jaime Powell (PVL), Sheena Kaal (SAM-PK), and César L. Schultz (UFRGS).
