The ascendancy of dinosaurs on land near the close of the Triassic now appears to have been as accidental and opportunistic as their demise and replacement by therian mammals at the end of the Cretaceous. The dinsoaurian radiation, launched by 1-meter-long bipeds, was slower in tempo and more restricted in adatpie scope than that of therian mam- mals. A notable exception was the evolution of birds from small-boided predatory dinosaurs, which invovled a dramatic decrease in body size.
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