In with fire...

...and out too, although of a different nature. Everybody knows that the dinosaurs (most of them, anyway) died out suddenly about 65.5 million years ago, and the consensus, for now, is that it was an asteroid that did them in. Perhaps just as interesting as their sudden disappearance is their relatively sudden rise to prominence near the boundary of the Triassic and Jurassic Periods around 200 millon years ago.

New research led by Brown University's Jessica Whiteside suggests that it was volcanism that paved the way. During the Triassic, the early dinosaurs found were not the dominant form of carnivores in terrestrial habitats. That honor belonged to group known to paleontologists as the crurotarsans (that's one at right). These crocodilian relatives appeared early in the Triassic, rose to dominance, and were quite diverse by the latter part of the Period. Then, suddenly, all the large crurotarsans disappeared. That decline opened the door for the dinosaur dominance that would follow during the Jurassic Period. Whiteside's group have examined a number of lines of evidence to pinpont the cause of the crurotarsan fall.

The evidence points to massive, widespread volcanic eruptions, probably related to the beginning of the breakup of the supercontinent we call Pangea. Analysis of fossil pollen indicates that the changes associated with the volcanism led to the disappearance of perhaps 50% of the plant species present at the end of the Triassic. We also see that the crurotarsans, common and abundant before the period of volcanism, are gone afterward. This newly emptied niche, the large, terrestrial carnivore, was rapidly filled by the theropods, the group that would ultimately give rise to all of the carnivorous dinosaurs. And the rest is history.