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
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.
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]