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)

65. EZCURRA M.D., NESBITT S.J., FIORELLI L.E., DESOJO J.B.

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2018

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