Alligators² and birds share a breathing mechanism³ which may have helped their ancestors dominate Earth more than 200 million years ago, scientists say.

科学家称，短吻鳄与鸟类有着相同的呼吸机制，这可能有助于它们的祖先在2亿年前统治地球。

How do alligators breathe?

Research published in the journal Science found that like birds, in alligators短吻鳄 air flows in one direction.

Birds' lung structure allows them to breathe when flying in low oxygen, or hypoxic低氧的, conditions.

This breathing may have helped a common ancestor of birds and alligators thrive繁荣，兴旺 in the hypoxic period of the Triassic三叠纪的.

Mammals 'hiding'

"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祖龙，古蜥 came to dominate Earth after the planet's worst mass extinction⁴ 251 million years ago.

Archosaurs evolved into two different branches which developed into crocodilians鳄目动物, dinosaurs⁵, flying pterosaurs飞龙目 and eventually birds.

Synapsids, which evolved to include mammals, had been dominant⁶ in the Permian period before the mass extinction.

Some survived but were toppled倾覆，推翻 from their perch栖息，栖木 by the archosaurs.

Any mammal-like synapsid合弓纲 survivors⁸ "were teeny极小的，微小的 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负鼠."

To demonstrate alligator¹ lung mechanisms⁹, the scientists measured airflow in anesthetised animals局部麻醉动物, showing it flows in one direction rather than in and out of chambers¹⁰.

They also pumped water containing tiny fluorescent荧光的 beads¹²玻璃粉 into the lungs of dead alligators to observe the flow.

Puzzle solved

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高度 that would kill a mammal," said Dr Farmer.

"Many archosaurs, such as pterosaurs, apparently¹³ 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食草动物 like Triceratops and carnivores食肉动物 like Tyrannosaurus, had bird-like lungs," Dr Farmer added.