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In the Science magazine assessment, the 41-scientist team led by geoscientist Peter Schulte of Germany's Universität Erlangen-Nürnberg, looked at hundreds of land and sea-floor clay layers to examine microscopic fossils and chemistry dating to the event. They counter recent suggestions that massive volcanoes might better explain the end of the Age of Dinosaurs.

Instead, the evidence suggests a single 6-mile-wide asteroid traveling 45,000 mph blasted Mexico's Yucatan peninsula at the time, creating "one of the biggest holes the Earth has ever seen," says study co-author Kirk Johnson of the Denver Museum of Nature & Science.

Scientists have debated the impact theory since 1980 when a team led by Nobel prize-winning physicist Luis Alvarez reported a layer of the element iridium, commonly found in asteroids, resided in rocks dating to the so-called "K-T boundary." The mass extinction event killed off the dinosaurs and many other animal and plant species. Alvarez and colleagues traced the impact to the 120-mile-wide Chicxulub crater in the Gulf of Mexico, an idea that won increasing acceptance afterward.

But over the last decade, scientists such as Princeton's Gerta Keller have contested the Chicxulub impact theory, suggesting that volcanism and multiple impacts may have combined to cause the mass extinctions seen in the fossil record. Keller calls the Science assessment inaccurate, saying by e-mail, "(t)here is no new information here whatsoever. They build their case largely on assumptions." Keller disputes the central confirmation of the assessment, matching the impact debris pattern revealed by the core records with fossil extinctions.

But Schulte and colleagues say critics of the impact hypothesis have confused continental shelf "slumping" that resulted from a magnitude 11 earthquake following the asteroid impact with evidence for later impacts. And they find evidence that large-scale volcanism, in the "Deccan traps" region of modern-day India, started at least 400,000 years before the Chicxulub impact with no effect on life. The team traces the extinctions to within plus-or-minus 10,000 years of the impact 65.5 million years ago.

"So we are back to where we started with the Alvarez hypothesis, a single, large, (6-mile-wide) impact," Johnson says. "We certainly don't want it to happen again."

http://www.usatoday.com/tech/science/discoveries/2010-03-04-dinos05_N.htm