How reptiles survived the big one

The reptile lived about 250 million years ago in what is now South Africa

By BBC News Online's Helen Briggs

Fossils of reptiles that survived the greatest extinction in the Earth's history suggest that the catastrophe had a far greater impact on ocean life than on land-dwellers.

The theory that an asteroid or comet slammed into the planet, wiping out most living things, may have to be revised following the discovery.

Scientists have found that two-thirds of a group of ancient land reptiles managed to escape the devastation, while about 90% of marine life died out.

The causes for the extinction were not extra-terrestrial

Sean Modesto, Royal Ontario Museum, Canada

They say an extraterrestrial impact would have had far-reaching effects on Earth, and propose that the extinction was caused by something else.

Fluctuations in sea level, global cooling or volcanic activity are just some of many scenarios proposed to explain an event so severe it has been dubbed "the mother of mass extinctions".

Earlier this year, scientists in the United States said they had found evidence that the extinction was triggered by an asteroid or comet hitting the planet.

And Japanese researchers said last week that sulphur trapped in rocks in southern China provides further evidence of a massive impact.

Super-continent

The disaster happened about 251 million years ago, at the end of the Permian period. It was far more devastating than the extinction that ended the reign of the dinosaurs, nearly 200 million years later.

At the time, life was flourishing on land, then largely a giant continent covered by desert. Amphibians and reptiles continued their invasion while crinoids, ammonites, corals and fish colonised the seas.

A giant impactor's effects are unpredictable
Artist's impression

What happened next is still a mystery. But it had devastating effects. Evidence from the fossil record suggests that as many as 96% of all marine species were lost, while on land more than three quarters of all vertebrate families became extinct.

Now, scientists in Canada and South Africa have pieced together the family tree of an ancient group of reptiles, which they say could shed light on the nature of the event.

They have analysed existing fossils of a relatively small group of lizard-like reptiles, called procolophonoids, which arose in the Permian.

The research shows that four out of six lines of these four-legged burrowing reptiles escaped extinction and made it through to the Triassic period, the beginning of the age of the dinosaurs.

"Our work with this group of reptiles together with other recent work suggests that the mass extinction at the Permo-Triassic boundary was not as devastating for terrestrial animals as for marine animals," team leader Sean Modesto told BBC News Online.

"It also suggests that the cause of the extinction was not extra-terrestrial."

City-sized asteroid

Dr Modesto, a palaeobiologist at the Royal Ontario Museum in Toronto, Canada, believes an asteroid impact would have wiped out most land animals as well as marine ones.

Scientists behind the asteroid hypothesis estimate that a body up to 12 km (7.4 miles) wide hit the Earth.

They say the impact would have released an amount of energy equivalent to about one million times the largest earthquake recorded in the 20th Century.

The devastation left by the space object that hit Siberia in 1908

Mark Bailey, of the Armagh Observatory in Northern Ireland, said it was very difficult to predict what might happen to life on the land or in the sea following an asteroid collision on this scale.

"When comets or asteroids hit the Earth, you get very complex environmental degradation," Dr Bailey told BBC News Online.

A collision could trigger anything from acid rain and global temperature changes to tsunamis, he said.

He pointed out that many animals - such as small mammals, crocodiles, lizards and indeed reptiles - survived the impact of 65 million years ago while dinosaurs died out.

"We see from the dinosaurs that some species can survive," he said. "And there is very little doubt that this extinction was caused by an asteroid."

But Dr Modesto says the new work is just one piece in a big jigsaw puzzle. He says there could well be alternative explanations for the reptiles' survival.

Perhaps the creatures, which were burrowers, were able to escape the environmental catastrophe by hiding underground.

The reptile research is published in the journal Royal Society Proceedings B.