Phylogenetic and diversity analyses files

The early diversification of dinosaurs produced a major ecological change in the terrestrial ecosystems, culminating with tetrapod assemblages dominated in abundance by dinosaurs by the Triassic-Jurassic boundary (ca. 201 Ma). Therefore, studying the initial diversification of dinosaurs is crucial to understand the establishment of Mesozoic assemblages. However, the lack of stratigraphically continuous fossil data in the few geological units that preserve the oldest known dinosaurs (ca. 233–227 Ma, Carnian stage) obscure our understanding of this initial diversification. The Ischigualasto Formation in northwestern Argentina (231.4–225.9 Ma) yields a rich vertebrate assemblage and new studies resulted in an abundant and stratigraphically near-continuous fossil record that offers new insights into the early diversification of dinosaurs. Among the discoveries, we report Anteavis crurilongus gen. et sp. nov., an early-diverging theropod that supports the significant diversity of small to medium-sized dinosaurs during the late Carnian. Anteavis is recovered outside Neotheropoda, but it has features previously thought to be exclusive of that group. We show that dinosaur diversity and abundance in the Ischigualasto Formation were higher than previously recognized, particularly among small herbivores (<30 kg) and medium-sized (30–200 kg) predators. This diversification occurred in Ischigualasto during a climatic shift to semi-arid conditions, but the return of more humid conditions resulted in a gap in the dinosaur record that started at 228.91±0.14 Ma. Only 15 million years later, in the mid-Norian, the dinosaur record recovered its abundance and diversity in the basin, but now it was characterized by larger-bodied species. Our findings demonstrate an early dinosaur diversification probably punctuated by a climate-driven faunal turnover in, at least, southwestern Pangaea.