Mother Of All Extinction Clues?

Scientists have discovered signs of a large impact crater buried off the coast of Australia that may be linked to the biggest extinction event in Earth's history, the "Great Dying" 250 million years ago.

Many scientists have long blamed a massive meteor near Mexico's Yucatan Peninsula for wiping out the dinosaurs 65 million years ago.

What caused the far earlier and larger Permian-Triassic extinction — when about 90 percent of all species disappeared — is subject to sharper debate.

The leading theory is that the extinction actually stretched over thousands of years, triggered by volcanic eruptions. A massive flow of molten rock over what is now Siberia injected tons of toxic gases into the atmosphere, gradually changing the planet's climate.

The new study, published Thursday by the journal Science, backs another theory that a massive asteroid strike played at least some role, too.

The researchers cite clues that an impact crater the right age and perhaps 120 miles wide is buried off Australia's northwest corner. They're calling it the Bedout crater (pronounced Beh-doo.)

"We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," lead researcher Luann Becker of the University of California, Santa Barbara, said in a statement.

But other scientists are highly skeptical.

"It's not yet persuasive that it's even a crater," said Peter D. Ward, a professor of geological sciences at the University of Washington in Seattle who has long studied impact craters and mass extinctions.

Intensive study is required to join the list of the world's proven impact craters. Most have been eroded by rain, wind and earthquakes over millions of years. This possible new site is poorly preserved and deeply buried.

Even if it is an impact crater, size must be proven, Ward added. "It's got to be a big hit" to cause global repercussions, he said. "There's going to have to be a tremendous amount of more work" done on the site.

Becker's team, funded by the National Aeronautics and Space Administration and the National Science Foundation, had been hunting a crater in the Southern Hemisphere after finding what appeared to be impact debris in Antarctica.

She learned that oil companies had drilled cores from a ridge at the Bedout site decades earlier. In those never-before-studied samples, she found melted rock layers and crystal structures displaying the "shocked" pattern distinctive of meteor impact. Sediments from an oil exploration well provided the right date.

By Lauran Neergaard