Now, a University of Washington paleontologist theorizes that low oxygen and repeated short but substantial temperature increases because of greenhouse warming sparked two major mass-extinction events, one of which eradicated 90 percent of all species on Earth.

In addition, Peter Ward, a UW professor of biology and Earth and space sciences, believes the conditions spurred the development of an unusual breathing system in some dinosaurs, a group called Saurischian dinosaurs that includes the gigantic brontosaurus.

Rather than having a diaphragm to force air in and out of lungs, the Saurischians had lungs attached to a series of thin-walled air sacs that appear to have functioned something like bellows to move air through the body.

Ward, working with UW biologist Raymond Huey and UW radiologist Kevin Conley, believes that breathing system, still found in today's birds, made the Saurischian dinosaurs better equipped than mammals to survive the harsh conditions in which oxygen content of air at the Earth's surface was only about half of today's 21 percent.

"The literature always said that the reason birds had sacs was so they could breathe when they fly. But I don't know of any brontosaurus that could fly," Ward said. "However, when we considered that birds fly at altitudes where oxygen is significantly lower, we finally put it all together with the fact that the oxygen level at the surface was only 10 percent to 11 percent at the time the dinosaurs evolved.

"That's the same as trying to breathe at 14,000 feet. If you've ever been at 14,000 feet, you know it's not easy to breathe," he said.

Ward believes low oxygen and greenhouse conditions caused by high levels of methane from intense volcanic activity are likely culprits in mass extinctions that occurred about 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. He will make a presentation on the topic Tuesday at the American Geological Society annual meeting in Seattle.

The Permian-Triassic extinction is believed to have eradicated 90 percent of all species, including most protomammals, a group of mammal-like reptiles that were the immediate ancestors of true mammals.

The Triassic-Jurassic extinction killed more than half the species on Earth, with mammal-like reptiles and true mammals, which evolved during the Triassic Period, hit particularly hard. But dinosaurs, which also evolved between the two extinctions, had little problem with conditions during the Triassic-Jurassic extinction.

"The seminal observation is that dinosaurs skated across the second of these mass extinctions, actually increasing in number as they went along, while everything else was dropping around them," Ward said.

Scientists know of five mass extinction events in Earth's history, but a cause has been widely agreed upon for only one - the episode at the end of the Cretaceous Period 65 million years ago, when the impact of an asteroid is believed to have brought the demise of the dinosaurs.

Such impact also has been suggested as the cause of the Permian-Triassic and Triassic-Jurassic extinctions, but geologists have yet to unearth any indisputable evidence of such an impact, and there is no conclusive evidence of what caused either of the events.

Ward said mass spectrometer readings on fossil material, as well as the extinction pattern for fossils in rock outcrops collected from the time of the two extinctions, indicates the events were drawn-out affairs and did not happen suddenly, as they would have with an asteroid impact.

In addition, he said it is known which types of creatures, and which breathing systems, best survived the extinction events. The same breathing systems are still present in birds, which are known to fare well at high altitudes, where oxygen levels are substantially lower than at the surface.

"The reason the birds developed these systems is that they arose from dinosaurs halfway through the Jurassic Period. They are how the dinosaurs survived," he said.

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