The early dinosaurs succeeded in conquering the land because of their ability to deal with low oxygen levels.

Credit: Ballista

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WASHINGTON: The first groups of animals to crawl from the sea may have been driven extinct by low oxygen levels, according to a new study.

Vertebrate creatures are thought to have first begun moving from the world's oceans to land about 415 million years ago. Then they all but disappeared by 360 million years ago. The fossil record contains few examples of animals with backbones for the next 15 million years, and then suddenly vertebrates show up again, this time for good.

The mysterious lull in vertebrate colonisation of land is known as Romer's Gap, named for the Yale University palaeontologist, Alfred Romer, who first recognised it. But the term has typically been applied only to the amphibians that came before the dinosaurs, and there has been little understanding of why the gap occurred.

Now a team of scientists led by University of Washington palaeontologist Peter Ward has found a similar gap during the same period among land-dwelling arthropods - insects and spiders, mostly - and they believe a precipitous drop in the oxygen content of Earth's atmosphere is responsible.

"These two groups acted exactly the same way. They proliferated, then they went away, and then they reappeared and multiplied like crazy," said Ward.

He notes that atmospheric oxygen rose sharply at the end of the Silurian period about 415 million years ago, to reach a level of about 22 per cent of the atmosphere, similar to today's oxygen content. But 55 million years later, atmospheric oxygen levels sank to 10 per cent. The level remained low for 30 million years - during which Romer's Gap occurred - then shot up again, and vertebrates and arthropods again began moving from the sea onto land.

"It matches two waves of colonisation of the land," Ward said. "In the first wave the animals' lungs couldn't have been very good and when the oxygen level dropped it had to be hard for the vertebrates coming out of the water."

Ward thinks that dinosaurs - the most successful of the second wave of colonisers - did so well because they could best deal with low oxygen levels.

Dinosaurs first appeared in the last part of the Triassic period, about 230 million years ago. That was during one of the lowest ebbs of atmospheric oxygen content of the last 500 million years.

"Dinosaurs thrived and nothing else did. There's an explanation for that, and it is that the air sac breathing system in dinosaurs and their descendants, modern birds, is more efficient than systems used by other organisms," Ward said.

He and his colleagues tested that hypothesis by examining the breathing system used by birds. They found that at sea level birds breathe 30 per cent more efficiently than mammals and at 5,000 feet in elevation birds are 200 per cent more efficient.

"I think of dinosaurs as the high-altitude ... athletes of their day. They ran rings around their prey," said Ward.