By Paul Rincon
BBC News Online science staff
The idea that an asteroid impact caused Earth's worst ever mass extinction has been boosted by the discovery of a huge crater that seems to date to the event.
The 250-million-year-old Permo-Triassic extinction killed off 95% of all marine life and 70% of all land species.
The cause is not known, but candidates include volcanism and space impacts.
The discovery of a possible crater off the coast of Australia may lend weight to the impact theory, US researchers report in the latest Science magazine.
However, the claim has already met with controversy: some scientists doubt the site is even an impact crater.
The evidence comes from seismic imaging, gravity data and the identification of melt rocks and impact breccias in cores drilled in and around a seabed feature called the Bedout High, off the coast of north-western Australia.
Range of dates
At the end of the Permian, Australia was part of the supercontinent scientists now refer to as Pangea.
"We have put forward a very strong, consistent story," Dr Luann Becker, of the University of California, Santa Barbara, US, told BBC News Online.
"We were able to date the thing as best as we could; we have a melt sheet with Triassic sediments piled on top. The stratigraphy is right, the timing's right."
Trilobites disappeared in the Great Dying
In core sections, drilled from more than 3,000m (10,000ft), the researchers found minerals that had been completely or partially converted to glass. They interpret these as rocks that were melted by an impact from outer space.
Attempts to date these using the technique of argon-argon dating yielded mixed results.
Six cores drilled from the Bedout area returned dates much younger than the extinction event. But one core returned a date of about 250.1 million years old - on the button for the mass extinction recorded in fossil beds around the world.
The team also found new evidence of shocked quartz - a mineral marker associated with impacts - at sites dating to the Permo-Triassic boundary in Australia and Antarctica.
And seismic data shows a central "uplift", a mountainous mass in many impact craters formed by the inward and upward movement of material from below the crater floor.
The so-called "great dying" that marks the end of the Permian period and the beginning of the Triassic period has baffled scientists for many years.
Spherulitic glass may have formed when an impact melted rocks in the crust
Many possible triggers have been proposed, including impacts, volcanism, climate change and glaciation, but the evidence remains equivocal.
The Bedout announcement recalls the discovery of the 65-million-year-old Chicxulub crater in Mexico, which bolstered the theory that an impact wiped out the dinosaurs. But so far, the Bedout data has failed to convince many experts.
One source told BBC News Online: "The evidence is pretty slim. An impact of that size should be flat, and the Bedout structure isn't flat. It's got the [central] uplift but it's lost all its edges - its crater rim.
"I don't understand how it could be so large, get rid of all the crater rim but leave the uplift standing."
'Not hugely impressive'
Professor Jay Melosh, of the University of Arizona in Tucson, US, commented: "This thing just rings off alarm bells in my mind. If it is an impact it's the most darn peculiar one I've ever seen."
Professor Melosh explained that some features in the sediments showed signs of sagging: "That's indicative of a basin that's been slowly subsiding constantly with time.
"If you simply blasted out an impact crater, all the sediments that came in later would be flat-lying, or at least parallel even if they'd been tilted later."
The super-volcanism that built the Siberian Traps could have caused the extinctions
Professor Michael Benton, of the University of Bristol, UK, told BBC News Online: "No iridium anomaly has been reported and confirmed anywhere. And the supposed spherules and shocked quartz are immensely rare and not hugely impressive."
And a source added: "At Chicxulub, there are things in the ejecta that tell you it has come from a Pan-American crust.
"There's absolutely nothing in the end-Permian deposits that link it back to the Bedout structure."