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| Lewisuchus admixtus Romer, 1972 in Agnolín, Egli, ... et Novas, 2021. |
Abstract
Lewisuchus admixtus is an early dinosauriform described by Alfred Romer in 1972 on the basis of a single, incomplete skeleton, collected in lower Upper Triassic rocks of the renowned Chañares Formation, at the Los Chañares type-locality, La Rioja Province, north-western Argentina. Recent field explorations to the type-locality resulted in the discovery of two partial articulated skeletons, which provide significant novel information. The cranial bones, presacral series, femur, tibia, and proximal tarsals of the new specimens match the preserved overlapping anatomy of the holotype and previously referred specimens of L. admixtus, including the presence of unique combination of character states among dinosauriforms (anterior presacral column with additional ossification on the top of neural spines, dorsal neural spines fan-shaped, anterior surface of the astragalus with a dorsally curved groove, and an inflated area on the anterior portion of the medial surface of this bone). This new information improves our understanding of the anatomy and taxonomy of early dinosauriforms and reinforces the role of Argentinean beds on the study of the origin of dinosaurs.
Keywords: Archosauria, Dinosauriformes, Late Triassic, Lewisuchus admixtus, South America
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| Lewisuchus admixtus. preserved bones of PULR V-111. Scale bar: 2 cm. |
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| Posterior cervical and anterior dorsal vertebrae of Lewisuchus admixtus (PULR V-111) |
CONCLUSIONS:
New specimens reported here add valuable information on the anatomy of Lewisuchus admixtus. Outstanding novel features described here for this taxon include an iliac portion of the acetabulum that is medially closed, elongated ischium and pubis (representing at least 2/3 of the femoral length), fan-shaped dorsal neural spines with a spine tables, and gastralia well separated from one another. Besides, the cranial bones, presacral series, femur, tibia, and proximal tarsals of the new specimens match the preserved overlapping anatomy of the holotype of L. admixtus and some of the specimens previously referred to “Pseudolagosuchus major” (e.g., MACN-Pv 18954), including the presence of unique combination of character states among dinosauriforms. This provides stronger evidence for the association of specimens that currently form the hypodigm of L. admixtus. An improved understanding of the anatomy and taxonomy of the Chañares Formation dinosauriforms is crucial to shed new lights on the phylogenetic relationships among nondinosaurian dinosauriforms and the dawn of dinosaur evolution.
Federico Agnolín, Federico Brissón Egli, Martín D. Ezcurra, Max C. Langer and Fernando Novas. 2021. New Specimens provide insights into the Anatomy of the Dinosauriform Lewisuchus admixtus Romer, 1972 from the upper Triassic levels of the Chañares Formation, NW Argentina. The Anatomical Record. DOI: 10.1002/ar.24731
