The cynodont Menadon (Traversodontidae) in the early Late Triassic of the Ischigualasto-Villa Unión Basin, NW Argentina: First record of a possible new faunal association in the Upper Chañares Formation and its gondwanan biostratigraphic implications

MARTINELLI, A.G.; EZCURRA, M.D.; FIORELLI, L.E.; ESCOBAR, J.A.; VON BACZKO, M. B.; HECHENLEITNER, E. MARTÍN; TROTTEYN, M.J.; DESOJO, J.B.

La Rioja

Congreso; 36° Jornadas Argentinas de Paleontología de Vertebrados; 2023

CRILAR-CONICET, Gobierno de la Provincia de La Rioja, Ministerio de Turismo y Culturas

We report here the first occurrence of the gomphodontosuchine traversodontid cynodont Menadon cf. M. besairiei from the upper half of the Upper Member of the Chañares Formation (Ischigualasto-Villa Unión Basin), La Rioja Province, Argentina, which until now has provided no tetrapod fossils. The new specimen includes a partial skull and jaw, plus a few postcranial elements, and was collected immediately above a level with a U–Pb age of ~233 Ma. Menadon besairiei was previously recognized in the ‘Isalo II’ beds of the Isalo Group of Madagascar and in the Santacruzodon Assemblage Zone (AZ) of the Santa Cruz Sequence of Brazil. The age of deposition of these units was inferred around the Ladinian-Carnian boundary, with a detrital zircon date of 236.6±1.5 Ma for the Brazilian sequence, which supported an older age for the underlying Dinodontosaurus AZ. However, based on their faunal contents, the Dinodontosaurus AZ has been historically considered coeval with the Massetognathus-Chanaresuchus AZ of the Chañares Formation, which has a U-Pb age of ~236 Ma. The Menadon-bearing level of Argentina has a similar age to the detrital zircon date obtained for the Hyperodapedon AZ (Candelária Sequence) of Brazil, which was traditionally interpreted as coeval to the lower third of the Ischigualasto Formation (~231–229 Ma at its base). As a result, there are important temporal inconsistences between the Brazilian and Argentinean units when the biostratigraphic and radioisotopic data is tried to be integrated. Based on the continuous stratigraphic sequence and successive faunal assemblages of the Ischigualasto-Villa Unión Basin, which have several, independent radiometric dates, here it is suggested that the Santacruzodon and Hyperodapedon AZs of Brazil have been interpreted as older than they really are because the analysis of detritical zircons provides maximum deposition ages and not absolute ages. Thus, we propose that the Santacruzodon AZ of Brazil is probably ~233 Ma. Hence, the Dinodontosaurus AZ of Brazil would have an age closer to the AZs of the lower member of the Chañares Formation and the deposition of the Hyperodapedon AZ-bearing levels, including their early dinosaurs, would have been closer to the age of the lower third of the Ischigualasto Formation of Argentina. Finally, the presence of Menadon in the uppermost portion of the Chañares Formation could represent the first taxon of a third faunal association within this unit, which are levels that definitely deserve research efforts in the coming years.