The evolution of dinosaurs

The ascendancy of dinosaurs on Land near the close of the Triassic how appe ars to have been as accidental and opportunistic as their demise and replac ement by therian mammals at the end of the Cretaceous. The dinosaurian radi ation, launched by 1-meter-long bipeds, was slower in tempo and more restri cted in adaptive scope than that of therian mammals. A notable exception wa s the evolution of birds from small-bodied predatory dinosaurs, which invol ved a dramatic decrease in body size. Recurring phylogenetic trends among d inosaurs include, to the contrary, increase in body size. There is no evide nce for co-evolution between predators and prey or between herbivores and f lowering plants. As the major Land masses drifted apart, dinosaurian biogeo graphy was molded more by regional extinction and intercontinental dispersa l than by the breakup sequence of Pangaea.