Deep faunistic turnovers preceded the rise of dinosaurs in southwestern Pangaea

Abstract

The Triassic period documents the origin and diversification of modern amniote lineages and the Late Triassic fossil record of South America has been crucial to shed light on these early evolutionary histories. However, the faunistic changes that led to the establishment of Late Triassic ecosystems are largely ignored 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 (uppermost Middle-lower Late Triassic epochs) of Argentina, which is older than the other vertebrate assemblages of the same basin. The new assemblage is composed of therapsids, rhynchosaurids and archosaurs, and clearly differs from that of the immediately overlying and well-known historical Chañares vertebrate assemblage. The new tetrapod association is part of a phase of relatively rapidly changing vertebrate assemblage compositions, in a time span shorter than 6 million years, before the diversification of dinosaurs and other common Late Triassic tetrapods in southwestern Pangaea.

New Specimen Sheds Light on the Anatomy and Taxonomy of the Early Late Triassic Dinosauriforms from the Chañares Formation, NW Argentina

“…Geographic position of the locality of the referred specimen of Lewisuchus admixtus CRILAR‐Pv 552 (occurrence of the specimen indicated with a red arrow) (modified from Ezcurra et al ., ).…”

“…CRILAR‐Pv 552 comes from the Massetognathus – Chanaresuchus Assemblage Zone ( M‐Ch AZ) of the lower Member of the Chañares Formation, southwest of La Rioja Province, northwestern Argentina (Fiorelli et al ., ; Ezcurra et al ., ). The specimen was found in situ during a field trip in 2013, at a low slope of the Romer Reloaded locality (Campo de Córdoba Sur; Fig.…”

“…This specimen was collected close to the dicynodont latrines‐bearing levels (Fiorelli et al ., ; Perez Loinaze et al ., ) that were dated in 236 ± 0.3 Ma (Fig. ) and approximately 7–10 m above the boundary with the Tarjadia Assemblage Zone (AZ; Ezcurra et al ., ). The CRILAR‐Pv 552 bearing level is characterized by a grey and massive fine sandstone of nearly ~3 m with fine laminations and mixture with volcanic and detrital materials (Fiorelli et al ., ; Ezcurra et al ., ; Perez Loinaze et al ., ).…”

“…The Chañares Formation of north‐western Argentina has provided one of the best sources of information about the morphology and evolution of dinosaur precursors (Romer, , ; Bonaparte, ; Arcucci, , ; Sereno and Arcucci, , ; Brusatte et al ., ; Langer et al ., , ; Bittencourt et al ., ; Ezcurra and Martínez, ). This sedimentary unit was deposited during the latest Ladinian−early Carnian (Marsicano et al ., ; Ezcurra et al ., ) and the lower member preserves at least two distinct vertebrate assemblages (Ezcurra et al ., ), the upper of which yielded three or four species of early dinosauromorphs: the lagerpetid Lagerpeton chanarensis (Romer, ; Sereno and Arcucci, ), the earliest branching dinosauriform Marasuchus lilloensis (Romer, ; Bonaparte, ; Sereno and Arcucci, ), and Lewisuchus admixtus and Pseudolagosuchus major (Romer, ; Arcucci, , ; Bittencourt et al ., ), two possible silesaurids (Nesbitt et al ., ). The anatomical knowledge provided by these early species of the avian lineage has been pivotal to shed light on the higher‐level relationships of dinosaurs among Triassic archosauriforms (e.g., Bonaparte, ; Gauthier, ; Sereno, ; Novas, ) and the relatively generalized skeletons of Marasuchus lilloensis , Lewisuchus admixtus , and Pseudolagosuchus major , in comparison to other dinosauriforms (e.g., Asilisaurus kongwe , Silesaurus opolensis , Sacisaurus agudoensis ), provide key information about the body plan of the closest members to Dinosauria, character transformations, and evolutionary scenarios (e.g., Bonaparte, ; Sereno and Arcucci, , ; Bittencourt et al ., ).…”

Supradapedon revisited: geological explorations in the Triassic of southern Tanzania

“…Both have been dated as late Carnian and are bracketed in time by younger and older deposits of the same basins, which lack the typical Hyperodapedon -rich faunas. These correspond to the early Carnian Chañares Formation (Marsicano et al, 2015; Ezcurra et al, 2017) and Santacruzodon Assemblage-Zone (Philipp et al, 2013) and the early (although not earliest) Norian Caturrita Formation (Langer et al, 2017) and upper part of the Ischigualasto Formation (Martinez et al, 2011). Indeed, until further information is available, a Carnian to earliest Norian age could be extended to the other Pangaean stratigraphic units where Hyperodapedon is highly abundant (i.e., Tiki, lower part of the Maleri, and Lossiemouth Sandstone formations).…”