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, US researchers said on Friday.
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 were very low -- comparable to levels now found at altitudes of 4,200m.
Peter Ward, a University of Washington paleontologist, said he believes 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.
"Someone told me they had heard of or seen geese flying above (Mount) Everest -- at 31,000 feet [10,000m]," said Ward, who presents his findings next week to a meeting of the Geological Society of America.
"If you put a human at 30,000 feet they'd be very, very, quite dead. And the birds are not only up there, they are doing major heavy exercise," Ward said
Birds and dinosaurs both have holes in their bones. And many of the largest dinosaurs, such as the brontosaurus or apatosaurus, seem to have had lungs attached to a series of thin-walled air sacs that may have acted something like bellows to move air through the body.
"The reason the birds developed these systems is that they arose from dinosaurs halfway through the Jurassic Period. They are how the dinosaurs survived," Ward said.
"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," he added.
"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."
Currently at sea level, atmospheric oxygen levels are 21 percent.
If giant dinosaurs had to breathe in a low-atmosphere environment, then such an efficient breathing system would have given them a survival advantage.
"You'd be really favored for survival in very bad, nasty, low-oxygen world," Ward said.
Dinosaurs dominated the world for hundreds of millions of years, perishing 65 million years ago. Most scientists agree the impact of an asteroid or meteor was the catalyst for their extinction.