BOSTON -- Scientists shot down a widely held theory Tuesday that dinosaurs died out from a catastrophic collision between Earth and an asteroid, saying a drop in oxygen levels actually caused their extinction.
The reptiles disappeared not with a bang, but a whimper, over 10 million years as the Earth's oxygen supply declined sharply and they were unable to adapt, researchers told a meeting of the Geological Society of America.
Even if scientists were able to clone dinasours, as depicted in the recent movie 'Jurassic Park,' dinosaurs would be unable to survive at current oxygen levels, said Gary Landis of the U.S. Geological Survey in Denver.
'They've had their day. They'll never be back, and they shouldn't,' Landis said.
The asteroid theory was first put forward by the late Nobel Prize winner Luis Alvarez, who thought the impact of such a collision could send enough debris into the atmosphere to alter the Earth's climate and affect the dinosaurs' food sources.
Landis and colleagues said it was entirely possible that an asteroid did slam into the Earth 1 million or 2 million years before the last dinosaurs disappeared about 65 million years ago.
But by then, two-thirds of the reptiles were already extinct, and 'the rest were on their way out,' said Keith Rigby Jr., a paleontologist at the University of Notre Dame.
The dinosaur extinction 'does not appear to be controlled in any way, shape or form by an extraterrestrial impact,' he said.
Rigby said it appears geological processes deep within the Earth initially created a hospitable atmosphere for dinosaurs during the Jurassic and Cretaceous periods, when carbon dioxide and oxygen levels rose.
But during the late Cretaceous and early Tertiary periods, 100 million to 65 million years ago, the amount of oxygen in the atmosphere fell, leaving the dinosaurs literally short of breath.
Before oxygen levels declined, there were 35 species of dinosaurs in North America alone, said Robert Sloan of the University of Minnesota. But 'from there, it was all downhill,' he said.
The reptiles' physiology limited their ability to take oxygen into their lungs, said Richard Hengst of Purdue University. That problem, plus a climatic cooling trend, 'would have put enormous respiratory stress on them,' he said.
By analyzing tiny bubbles of ancient air found in fossilized amber, Landis estimated that during a comparatively short period of 3 million years or so, atmospheric oxygen levels dropped from a high of 35 percent to 28 percent, rose briefly, and then fell nearly to the present level of 21 percent.
Small mammals that coexisted with the dinosaurs, including rat-like creatures that may have evolved into human ancestors, were able to survive the oxygen fluctuations because of comparatively greater lung capacity and adaptability, he said.
The scientists said the change in oxygen levels happened as much as 20 times faster than previously estimated. It was caused by 'linked' geological processes that included a decline in volcanic eruptions and a drop in sea levels that exposed oxygen-consuming organic sediment, Landis said.
He and colleagues called their explanation for the dinosaur extinctions the 'Pele Hypothesis,' after a Polynesian volcano goddess.
Rigby described it as a 'single unifying theory that affects virtually all phases of the Earth's history,' including climate and atmospheric changes and the evolution and extinction of species.
