WASHINGTON -- The reign of the dinosaurs may have ended 65 million years ago with the massive impact of a mountain-sized rock from outer space. Now a new study suggests that a similar impact millions of years earlier may have helped dinosaurs dominate the animal world.
In a study appearing Friday in the journal Science, researchers said that a combination of fossil footprints, the presence of a rare mineral and evidence for the sudden bloom of ferns some 200 million years ago suggests that a massive rock from outer space hit the Earth and helped kill off large reptiles that then ruled the world.
Once the reptiles were gone, dinosaurs were able to dominate the Earth for more than 135 million years, said Paul E. Olsen of Columbia University, the first author of the study. They grew ever larger, until they, too, were wiped out by an asteroid impact, he said.
Olsen said a layer of iridium, a chemical rare on Earth but common in rocks from outer space, exists in sediments in the U.S. Northeast that have been dated at about 200 million years, the boundary between the Triassic and the Jurassic geologic ages.
During the Triassic, the dominant animals on Earth were massive reptiles, mostly four-footed ancestors of the modern crocodile. They roamed a single land mass, a huge body that would later split to form the Earth's continents. Dinosaurs then were small, runty animals, poor competition for the more imposing reptiles. And mammals, the ancestors of man, were even smaller animals, probably skittering in fear among the rocks.
That changed at the end of the Triassic and the beginning of the Jurassic age.
Olsen said that footprints preserved as fossils in lakebed deposits in what is now New Jersey, New York and Pennsylvania show that the massive reptiles abruptly disappeared and were replaced within 30,000 years by large, lumbering dinosaurs.
Fossils from the same geologic age also show that there was a sudden bloom of ferns, the first plants that would appear after a massive asteroid impact killed off most other vegetation. A similar spike of fern growth exists in the geologic record at the time dinosaurs were killed off 65 million years ago.
But the major new finding is the presence at the Triassic-Jurassic geologic boundary of what is called an iridium anomaly. The concentration of iridium quadrupled at the boundary, suggesting Earth had been hit by a huge asteroid. An iridium anomaly, although much richer, has also been found at the geologic boundary marking the extinction of dinosaurs.
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