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

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.

A new silesaurid from Carnian beds of Brazil fills a gap in the radiation of avian line archosaurs

“…2 c). This groove is absent in Lewisuchus admixtus 20 , 34 . The medial articular surface is straight, whereas in L. admixtus and Asilisaurus kongwe it is rounded 19 , 35 .…”

“…kongwe , and Si. opolensis 19 , 20 , 39 , 41 . Indeed, there are faint striations on the homologous surface.…”

“…Silesaurs are the oldest dinosauromorphs reported in the fossil record 4 , 5 , providing clues on the ancestral body plan and biogeography of the group. The Middle Triassic occurrences from Brazil, Tanzania, and Zambia support a gondwanan origin of silesaurs 4 , 5 , whereas during the Upper Triassic, the group was present in both, Gondwana 17 – 20 and Laurasia 7 , 21 , 22 . Although restricted to Argentina and Brazil, the fossil record from South America is particularly rich.…”

“…On the other hand, in L. admixtus and As. kongwe the proximal tip merges smoothly with the femoral shaft, lacking the cleft 19,20,35 . Whereas the proximal tip of the anterior trochanter is not connected to the shaft in the new specimen, it is far less expanded than the wing-like trochanter of several prionodontians 10,43,44 .…”

The dawn of an Era: New contributions on comparative and functional anatomy of Triassic tetrapods

“…Surprisingly, the authors recovered the taxon as an Early poposauroid, bringing exciting implications to the biogeography of Pseudosuchia, as well as for the diversity of Middle Triassic tetrapods in South America. Finally, from the avemetatarsalian branch of Archosauria, Agnolín et al (2024, this issue) reassess the anatomy of the pelvic girdle and hindlimb of the Argentinean Middle‐Late Triassic Lagerpeton chanarensis , and challenge previous reconstructions of the gait and posture for the taxon. Instead of a parasagittal posture, the authors suggest L. chanarensis was more similar to sprawling reptiles, contrary to early interpretations made for these animals.…”