University of Utah scientists discovered that air flows in one direction as it loops through依次通过 the lungs of alligators短吻鳄鱼, just as it does in birds. The study suggests this breathing method may have helped the dinosaurs' ancestors dominate Earth after the planet's worst mass extinction 251 million years ago. Before and until about 20 million years after the extinction – called "the Great Dying" or the Permian二叠纪的-Triassic三叠纪的 extinction – mammal-like reptiles爬行类 known as synapsids were the largest land animals on Earth.

The extinction killed 70 percent of land life and 96 percent of sea life. As the planet recovered during the next 20 million years, archosaurs祖龙，古蜥 (Greek for "ruling lizards") became Earth's dominant land animals. They evolved into two major branches on the tree of life: crocodilians鳄目动物, or ancestors of crocodiles and alligators, and a branch that produced flying pterosaurs飞龙目，翼龙目, dinosaurs and eventually birds, which technically are archosaurs.

By demonstrating one-way or "unidirectional" airflow within the lungs of alligators, the new study – published in the Friday, Jan. 15 issue of the journal Science – means that such a breathing pattern likely evolved before 246 million years ago, when crocodilians split from the branch of the archosaur family tree that led to pterosaurs, dinosaurs and birds.

That, in turn, means one-way airflow evolved in archosaurs earlier than once thought, and may explain why those animals came to dominance in the Early Triassic Period, after the extinction and when the recovering ecosystem was warm and dry, with oxygen levels perhaps as low as 12 percent of the air compared with 21 percent today.

"The real importance of this air-flow discovery in gators佛州大鳄 is it may explain the turnover in fauna动物群，动物区系 between the Permian and the Triassic, with the synapsids losing their dominance and being supplanted by these archosaurs," says C.G. Farmer, the study's principal author and an assistant professor of biology at the University of Utah. "That's the major reason this is important scientifically."

Even with much less oxygen in the atmosphere, "many archosaurs, such as pterosaurs, apparently were capable of sustaining vigorous有力的，精力充沛的 exercise," she adds. "Lung design may have played a key role in this capacity because the lung is the first step in the cascade喷流，层叠 of oxygen from the atmosphere to the animal's tissues, where it is used to burn fuel for energy."

Farmer emphasized the discovery does not explain why dinosaurs, which first arose roughly 230 million years ago, eventually outcompeted other archosaurs.

Farmer conducted the study – funded by the National Science Foundation – with Kent Sanders, an associate professor of radiology放射学 at the University of Utah School of Medicine. They performed CT scans of a 4-foot-long, 24-pound alligator.