BOSTON, OCT. 27 -- Dinosaurs were not done in by a giant asteroid, as the prevailing theory holds, but by a change in the atmosphere that provided insufficient oxygen to support their inefficient respiratory systems, a new theory suggests.

A new analysis of air trapped in 120 million-year-old fossils, researchers said, casts doubt on the notion that dinosaurs became extinct after an object hit the planet 65 million years ago, sending up dust that blocked the sun and killed plant life.

Dinosaurs were already on their way out by the time the meteor purportedly hit, said Gary Landis, a U.S. Geological Survey geologist and one of four scientists presenting their theory at the Geological Society of America's annual meeting today.

"The dinosaurs did not keel over with their feet up in the air, but they found it increasingly difficult to compete in their environment," he said.

The theory also carries implications for humans because it presumes that worldwide atmospheric changes can occur 10 to 20 times faster than previously thought, the researchers said.

These conclusions met some skepticism.

Other animals that lived at the time survived the drop in oxygen without any major changes, said Mark Norell, assistant curator of vertebrate paleontology at the American Museum of Natural History in New York. The latest hypothesis "doesn't explain the selectivity of the extinction," he said.

Landis and his colleagues tested air found trapped in amber in east-central Minnesota. They found that the proportion of oxygen in the atmosphere fell from a high of 35 percent to a low of 28 percent within the relatively short period of 300,000 to 500,000 years.

"It would be like taking a dinosaur for a stroll from sea level up to 6,000 to 7,000 feet," Landis said.

The oxygen-rich air resulted from volcanic activity that pumped out carbon dioxide, which was converted into oxygen by plants.

Dinosaurs evolved during this time, when oxygen was plentiful, and had weak respiratory systems, according to the theory known as the Pele Hypothesis. (Pele is a Polynesian goddess of volcanoes.)

"The dinosaurs survived and thrived in that environment because they didn't have to have anything more efficient," Landis said.

An 80-foot-long brontosaurus had a set of nostrils about the same size as a horse's, for example, said Richard A. Hengst, a Purdue University physiologist.

"There were some serious problems with trying to get air into that animal," Hengst said. "Dinosaurs could not have existed without having more oxygen in the air to start with."

The scientists said that smaller cold-blooded animals around at the time, such as snakes, lizards and turtles, were able to survive because of their relatively modest oxygen needs. So did some of the smaller mammals with more efficient respiratory systems, the scientists said.

About two-thirds of known species of dinosaurs were gone by the time the asteroid is believed to have fallen in what is now Mexico's Yucatan Peninsula, the researchers said.