Monday, September 20, 2021

[Paleontology • 2020] Lewisuchus admixtus • New Specimen Sheds Light on the Anatomy and Taxonomy of the Early Late Triassic Dinosauriforms from the Chañares Formation, NW Argentina

Lewisuchus admixtus Romer, 1972

in Ezcurra, Nesbitt, Fiorelli & Desojo, 2020.

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. This has generated a contentious debate during the last 20 years about the synonymy 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 preserves previously unknown anatomical regions for the dinosaur precursors of this unit (e.g., premaxilla, inner ear, anterior zeugopodium) and allows comparisons with other dinosauriform specimens. CRILAR-Pv 552 is referred to Lewisuchus admixtus because it possesses a proportionally large skull, a laterally projected, shelf-like ridge on the jugal, and recurved, finely serrated middle–posterior maxillary and dentary teeth ankylosed to the bone, and the absence of 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. 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 a subjective junior synonym of Lewisuchus admixtus. A preliminary phylogenetic analysis with updated scorings for Lewisuchus admixtus found this species at the base of Silesauridae. 
Keywords: Lewisuchus admixtus, Pseudolagosuchus major, Archosauria, Dinosauriformes, Ischigualasto-Villa Unión Basin, Late Triassic, South America

Fig. 3. Skeleton of Lewisuchus admixtus.
(A) Preserved bones of CRILAR-Pv 552 in approximate anatomical arrangement.
(B) Skeletal reconstruction of Lewisuchus admixtus based on the holotype and referred specimens (see Discussion). 
Scale bar equals 10 cm in B. Skeletal reconstruction drawn by Lucas E. Fiorelli.

Martín D. Ezcurra, Sterling J. Nesbitt, Lucas E. Fiorelli and Julia B. Desojo. 2020. New Specimen Sheds Light on the Anatomy and Taxonomy of the Early Late Triassic Dinosauriforms from the Chañares Formation, NW Argentina. The Anatomical Record. 303; 1393–1438. DOI: 10.1002/ar.24243