Dawn of the dinosaurs demonstrates early diversification bursts and a potential role for neutral dynamics during major evolutionary radiations.

Major radiations have given rise to huge swathes of life on Earth, and understanding their dynamics reveals the process of biodiversity generation itself. The rise of dinosaurs and other archosaurs during the Triassic was a spectacular example of such a radiation; it set the stage for Mesozoic faunas and gave rise to the largest land animals on Earth. Previous attempts to understand its dynamics have relied on phylogeny, necessitating the exclusion of much data. We use the Bayesian occurrence-based approach PyRate, allowing speciation and extinction dynamics to be estimated without recourse to phylogeny. We analyse stratigraphic range-based data for all archosauromorphs from the late Permian to the Early Jurassic, and test for association of body size with diversification using a body size proxy. We find an 'early burst' pattern of diversification, with initial high speciation giving way to increased extinction. Falling speciation is consistent with niche-filling following extinction, but increasing extinction may indicate a role for neutral dynamics during radiation. Cryptic diversification may be evidenced by a peak earlier than inferred using phylogenetic estimates. Body size is not statistically associated with diversification, but the positive direction of association potentially reflects the concurrent passive 'trend' of increasing size.