We usually think of asteroid impacts as closing certain chapters of life. But new findings indicate that in the case of dinosaurs, just such a collision may have also permitted their rise to power. Researchers writing today in the journal Science report that geological and paleontological evidence points to a bolide impact occurring around 200 million years ago and clearing the world stage for the dinosaurs' ascent.

Geochemical analyses of sediment layers from sites in eastern North America, say Dennis V. Kent of Rutgers University and his colleagues, revealed a telltale spike in the element iridium at the boundary between the Triassic and Jurassic periods. (Iridium, it turns out, is rare on Earth but fairly common in space objects.) Importantly, that elevated iridium level immediately precedes the first known footprints of large, carnivorous dinosaurs. Likewise, further clues from the fossil record indicate that while other beasts were dying off at a prodigious rate during that time, the dinosaurs were flourishing, increasing in both size and diversity.

"The evidence suggests that the dramatic decrease in non-dinosaurian diversity was caused by an extrinsic environmental catastrophe," the authors conclude. "The resulting drop in competitive pressure was the trigger for the global spread of large theropods, and these, along with surviving prosauropods and ornithischians, established the familiar global dinosaurian-dominated ecological pattern seen across the terrestrial world for the [next 135 million years]."