An early burst of skeletal evolution at the origin of dinosaurs

Over 230 million years of Earth's history, dinosaurs became a major terrestrial animal clade and produced one of the most species-rich living tetrapod lineages: birds. Yet, largely because of uncertainty surrounding the phylogeny of early dinosaurs, the tempo and mode of their emergence and initial radiation remain poorly constrained. Here, we reconstruct the initial diversification of dinosaurs through Bayesian tip-dating analyses. Using nine morphological datasets, we estimate that dinosaurs emerged between 250 and 240 Ma, 10 million years before the earliest unambiguous dinosaur fossils. The emergence of the dinosaurs was followed by the rapid appearance and diversification of all major lineages, coinciding with a burst of morphological evolution that peaked in the early Late Triassic. The patterns that we infer are consistent with the expectations under a scenario of evolutionary radiation, in which ecologically disparate lineages rapidly diversify from a single common ancestor. In turn, our results provide a biological explanation for the instability surrounding early dinosaur phylogeny and suggest that the diversity of dinosaurs has been sculpted by multiple rapid radiations following successive mass extinctions in deep time.