By Doreen Walton
Science reporter, BBC News
Alligators and birds share a breathing mechanism which may have helped their ancestors dominate Earth more than 200 million years ago, scientists say.
Research published in the journal Science found that like birds, in alligators air flows in one direction.
Birds' lung structure allows them to breathe when flying in low oxygen, or hypoxic, conditions.
This breathing may have helped a common ancestor of birds and alligators thrive in the hypoxic period of the Triassic.
"It might explain a mystery that has been around for quite some time", Dr Colleen Farmer from the University of Utah told BBC News.
The mystery in question is why the archosaurs came to dominate Earth after the planet's worst mass extinction 251 million years ago.
Archosaurs evolved into two different branches which developed into crocodilians, dinosaurs, flying pterosaurs and eventually birds.
Synapsids, which evolved to include mammals, had been dominant in the Permian period before the mass extinction.
Some survived but were toppled from their perch by the archosaurs.
Any mammal-like synapsid survivors "were teeny liittle things hiding in cracks" said Dr Farmer. "I think it's because they couldn't compete.
"It wasn't until the die-off of the large dinosaurs 65 million years ago that mammals made a comeback and started occupying body sizes larger than an opossum."
To demonstrate alligator lung mechanisms, the scientists measured airflow in anesthetised animals, showing it flows in one direction rather than in and out of chambers.
They also pumped water containing tiny fluorescent beads into the lungs of dead alligators to observe the flow.
Studies on these alligators may explain why some animals ruled the Earth.
The researchers believe the similarity in lung structure may explain why some animals were better able to adapt after the extinction, when oxygen levels dropped.
"We know that birds are really good at breathing in hypoxic conditions. They can fly at altitudes that would kill a mammal," said Dr Farmer.
"Many archosaurs, such as pterosaurs, apparently were capable of sustaining vigorous exercise. Lung design may have played a key role in this capacity.
"That's been a puzzle, why do birds have these very different lungs? But now we can date it back to the common ancestor of birds and crocodilians.
"It implies that all dinosaurs, herbivores like Triceratops and carnivores like Tyrannosaurus, had bird-like lungs," Dr Farmer added.