An early burst of skeletal evolution at the origin of dinosaurs

Over 230 million years of Earth 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 and geographic origins of 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 seven morphological datasets to account for
uncertainty, we suggest 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 appearances and
diversification of all major lineages, coinciding with a burst of
morphological evolution that peaked in the early Late Triassic. The
patterns 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, including birds,
has been sculpted by multiple adaptive radiations following successive
mass extinctions in deep time.