? A new skeleton of the dinosaur precursor Lewisuchus admixtus allows reassessing the taxonomy of the dinosauriforms from the Lower Carnian levels of the Chañares Formation (La Rioja province)

EZCURRA, MARTÍN D.; NESBITT, STERLING J.; FIORELLI, LUCAS E.; DESOJO, JULIA B.

Puerto Madryn

Congreso; RCAPA; 2018

CCT CONICET-CENPAT

The lower Carnian levels of the Chañares Formation (Ischigualasto-Villa Unión basin, La Rioja province) have yielded some of the most informative dinosaur precursor species known so far. However, these species are based on partial skeletons that in several cases hamper the comparison between them because of the absence of overlapping bones. The latter has generated a contentious debate about the synonym between two of these species, Lewisuchus admixtus and Pseudolagosuchus major. Here, we describe a new dinosauriform partial skeleton (CRILAR-Pv 552) recently collected in the Chañares Formation that for the first time preserves previously unknown anatomical regions and allows comparisons with other dinosauriform specimens. CRILAR-Pv 552 is referred to Lewisuchus admixtus because it possesses a proportionally large skull with recurved teeth ankylosed to the bone and lacks a coracoid foramen. The new specimen preserves a dorsally bowed dentary with a lateroventral shelf that is identical to a dentary associated with the holotype of Lewisuchus admixtus. The latter bone has been recently reinterpreted as probably belonging to a proterochampsid, but CRILAR-Pv 552 confirms that it instead is part of the holotype of the species. Additionally, the morphology of the new specimen is completely congruent with that of specimens of Pseudolagosuchus major, bolstering the hypothesis that the latter species is the same as Lewisuchus admixtus. We concluded that the holotype of Pseudolagosuchus major is not diagnostic at a species level, thus we propose that this species should be considered a nomen dubium, but its referred specimens can be referred to Lewisuchus admixtus.