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Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs
The rise and diversification of the dinosaurs in the Late Triassic, from 230 to 200 million years ago, is a classic example of an evolutionary radiation with supposed competitive replacement. A comparison of evolutionary rates and morphological disparity of basal dinosaurs and their chief “competitors,” the crurotarsan archosaurs, shows that dinosaurs exhibited lower disparity and an indistinguishable rate of character evolution. The radiation of Triassic archosaurs as a whole is characterized by declining evolutionary rates and increasing disparity, suggesting a decoupling of character evolution from body plan variety. The results strongly suggest that historical contingency, rather than prolonged competition or general “superiority,” was the primary factor in the rise of dinosaurs.
Phylogenetic relationships and morphospace occupation for Triassic archosaurs. (A) Framework phylogeny for Triassic crurotarsans (13) scaled to the Triassic time scale (13). Numbers at top refer to millions of years before present; gray bars represent the observed durations of major lineages; vertical dashed lines denote two hypothesized extinction events (CNEE and TJEE); arrowheads indicate lineages that survived the TJEE. Lad, Ladinian; Crn, Carnian; Rh, Rhaetian; EJ, Early Jurassic. (B) Empirical morphospace for Triassic archosaurs, based on the first two principal coordinates (13). Large circles, dinosaurs; ovals, pterosaurs; squares, poposauroids; hexagons, phytosaurs; stars, aetosaurs; crosses, crocodylomorphs; smaller solid circles, “rauisuchids”; larger solid circles, nondinosaurian dinosauromorphs, Scleromochlus.