A NEW TETRAPOD ASSEMBLAGE FROM THE TRIASSIC CHAÑARES FORMATION AND THE DEEP FAUNISTIC CHANGES THAT PRECEDED THE RISE OF DINOSAURS IN SOUTHWESTERN PANGAEA

FIORELLI, L.E.; M. BELÉN VON BACZKO; 4.EZCURRA, M.D.; AGUSTIN GUILLERMO MARTINELLI; ROCHER, SEBASTIÁN; EZPELETA, M.; JEREMÍAS R. A. TABORDA; HECHENLEITNER, E.M.; M. JIMENA TROTTEYN; J.B. DESOJO

San Luis

Simposio; Reunión Comunicaciones de la Asociación Paleontológica Argentina 2017; 2017

The Triassic documents the origin and diversification of modern amniote lineages in the aftermath of the Permo-Triassic mass extinction. The Triassic fossil record of South America has been crucial to shed light on the early evolutionary histories of these modern amniote lineages, including the diversification of eucynodonts and adaptive radiation of archosaurs. However, the macroevolutionary faunistic changes that led to the establishment of well-known Late Triassic ecosystems, which include the oldest known dinosaurs, are largely ignored worldwide because of the global scarcity of fossils from assemblages a few million years older. Here, we contribute to fill this gap with the description of a new tetrapod assemblage from the lowermost levels of the Chañares Formation (Middle−Upper Triassic) of Argentina, which is older than the other vertebrate assemblages of the same basin. This new assemblage (named here as the Tarjadia Assemblage Zone) occurs in the first ~17 metres of the Chañares Formation and is numerically dominated by the formerly enigmatic archosauriform Tarjadia ruthae, here reinterpreted as an erpetosuchid pseudosuchian and together with other erpetosuchids as the sister-taxon of Ornithosuchidae. Tarjadia, basal paracrocodylomorph pseudosuchians, non-massetognathine traversodontids, very basal probainognathians, and stenaulorhynchine rhynchosaurids form a faunal assemblage that distinctly differs from that of the historical Chañares vertebrate assemblage. The new tetrapod association is part of a phase of relatively rapid changes in vertebrate assemblage composition, in a time span lower than six million years, before the diversification of dinosaurs and other common Late Triassic tetrapods in southwestern Pangaea.