As reported by Olsen, et. Al. in the May 17, 2002 issue of Science, levels of the metal iridium that rarely occurs naturally on the Earth's surface and mostly arrives on extraterrestrial objects, has a prominant peak in rocks from the time when many species died out at the Triassic-Jurassic boundary. Simultaneously, spores of ferns, the first plants to colonize devastated areas, also rise dramatically.

Tetrapod footprints and skeletal material from more than 70 localities in eastern North America indicate that large theropod dinosaurs appeared less than 10,000 years after the Triassic-Jurassic boundary and less than 30,000 years after the last Triassic taxa, in synch with a terrestrial mass extinction. The strata from this time shows an iridium anomaly (up to 285 parts per trillion) and a fern spore spike, suggesting that a bolide impact was the cause. Dinosaur diversity reached a stable peak less than 100,000 years after the boundary, and these fierce creatures ruled terrestrial environs for the subsequent 135 million years.

Around 200 million years ago, large plant-eating dinosaurs grazed the Earth alongside primitive meat-eaters as large as ostriches. After the mysterious event that marked the Triassic-Jurassic boundary, herbivores dwindled and large carnivores flourished. The precursors to Tyrannosaurus rex were born. Any explanation for the boundary has to explain why some animals lived on while many others perished. Olsen thinks that only the hardiest creatures would have survived the extreme conditions following an asteroid impact. Dust clouds masking the Sun would have plunged the Earth into dark cold, followed by intense warming as clouds of greenhouse gases accumulated. Olsen hypothesizes that warm-blooded dinosaurs that could withstand the cold or those that scavenged varying types of food would have had a survival advantage, and these were the dinosaurs.

P. E. Olsen,1 D. V. Kent, H.-D. Sues, C. Koeberl, H. Huber,4 A. Montanari, E. C. Rainforth, S. J. Fowell, M. J. Szajna, B. W. Hartline. Ascent of dinosaurs linked to iridium anomaly at the Triassic-Jurassic boundary. Science, 296, 1305 - 1307, (2002). PMID: 12016313 [PubMed]