The Chañares Formation crops out in the Ischigualasto-Villa Unión Basin, formed along the western margin of South America during the breakup of Gondwana. It represents one of the most continuous continental Triassic succesions in South America. These beds were explored by Alfred Romer and Jensen (1966) in their report on the geology of the Rio Chañares and Rio Gualo region.
Located in Talampaya National Park (La Rioja Province), the Chañares Formation is characterized at its base by a sandstone–siltstone fluvial facies with distinct lower and upper levels. The lower levels are composed of light olive grey fine-grained sandstones with abundant small brown carbonate concretions. The upper levels include fine-grained sandstones and siltstones that preserve vertebrate remains (Mancuso et al., 2014).
Volcanism played an important role in the generation and preservation of the Chañares Formation’s exceptional tetrapod fossil record. The diverse and well-preserved tetrapod assemblage includes proterochampsids, pseudosuchians, ornithodirans, large dicynodonts and smaller cynodonts. Almost all dinosauromorphs are preserved in diagenetic concretions that erode out of a thick siltstone interval 15–20 m above the base of the formation, and include Lagosuchus talampayensis, Marasuchus lilloensis Lewisuchus admixtus and Pseudolagosuchus major.
Analysing the ratio of U–Pb inside the zircon crystals found in the rocks assigns the Chañares Formation to the Late Triassic, specifically the early Carnian (236–234 Ma), between 5 to 10 million years younger than previous estimate. This also suggests a similarly age for the lower Santa Maria Formation in southern Brazil, because it shares with the Chañares assemblage a variety of tetrapod genera and species unknown from anywhere else. The new results provide the basis to construct a robust framework for calibrating the timing of macro-evolutionary patterns related to the origin and early diversification of dinosaurs in Gondwana (Marsicano et al., 2015). It also suggests there was little compositional difference between the Chañares assemblage and the earliest dinosaur assemblage from the lower part of the Ischigualasto succession, where dinosauromorphs (including dinosaurs) are a minority, with synapsids still dominant. Only ∼15 million years later dinosaurs begin to dominate the ecosystem.
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