Low oxygen levels could have triggered two giant extinctions hundreds of millions of years ago, allowing the dinosaurs to reign supreme over the ancestors of mammals, U.S. researchers said. Dinosaurs first appeared during a long period of low oxygen and therefore developed highly efficient breathing mechanisms that allowed them to thrive while many other species became extinct. The researchers arrived at the theory by tying in what is known about the physiology of dinosaurs with recent geological evidence suggesting that from 275 million to 175 million years ago, oxygen levels stayed very low -- comparable to levels found now at altitudes of 14,000 feet (4,200 metres). Peter Ward, a University of Washington paleontologist, said Friday he thinks low oxygen and hot greenhouse conditions caused by intense volcanic activity may have caused widespread extinctions 250 million years ago, at the boundary between the Permian and Triassic periods, and about 200 million years ago, at the boundary between the Triassic and Jurassic periods. The Permian-Triassic extinction is believed to have eradicated 90 percent of all species on Earth, including most protomammals, the immediate ancestors of true mammals. The Triassic-Jurassic extinction killed more than half the species, including many mammals and mammal-like reptiles. But dinosaurs flourished. Ward said he put together three pieces of the puzzle -- the extremely efficient breathing systems of birds, the finding that many dinosaurs had similar physiology, and a report that came out earlier this year showing that oxygen levels were low during the two extinctions.
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