New Proof of How Dinosaurs Died

Go digging with dinosaur hunters and they will show you that the last of the Cretaceous beasts died out suddenly, 66 million years ago. For 30 years the prevailing theory has been that T. rex and its brethren were wiped out by a comet or asteroid crashing in Central America, kicking up so much dust and ash that the Earth cooled for years afterward and made survival impossible.

Or was it really that simple? Some scientists, digging into well-preserved layers of the earth, said the timing was off. Yes, there is a 110-mile-wide crater, called Chicxulub, in the Caribbean off the coast of southern Mexico -- but radioactive dating suggested it was made 180,000 years after the last dinosaur fossils. Something was wrong.

So Paul Renne, an earth scientist at the University of California, Berkeley, decided to test the theory. Today he and his team report in the journal Science that the death-from-the-sky theory holds up after all. The great impact happened 66,038,000 years ago -- within 30,000 years of the dinosaurs' extinction. When you're talking about things that happened tens of millions of years ago, that's pretty good.

"I wouldn't say the theory was in trouble, but there have been skeptics and the absolute timing has never quite lined up," said Renne in an email to ABC News.

Related: Dinosaur Bones Come Back to Life

He and his team took earth samples from a formation in Montana called Hell Creek -- a remote place that is heaven to paleontologists because fossils and soil, left over the eons, are well preserved there. When the Chicxulub impact occurred, it deposited a very thin layer of ash whose age can be measured by the decay of a radioactive isotope of potassium. They combined it with other measurements to confirm the date of the so-called KT boundary -- when the period of the Cretaceous dinosaurs ended.

Not all dinosaurs were killed off, you'll recall. Some of their descendants survive today as birds. And there were small mammals that thrived once they were out of the way. A separate paper in the same edition of Science describes a small insect-eating animal, a bit like a modern shrew, that appeared about 65 million years ago -- not long after the dinosaurs disappeared.

Why, beyond fascination with the past, is the Chicxulub catastrophe worth studying? Because it was not the first time much of the life on Earth was wiped out, and it may not be the last. Astronomers have been keeping an eye out for asteroids that could, in future years, be on a crash course with us. There's a small asteroid called 2012 DA14 that will come within 17,200 miles of Earth this month.

Related: Asteroid Makes Close Call

Renne and other scientists say the dinosaurs were probably already struggling by the time of the great impact, but today's paper shows that something catastrophic happened right around the time scientists find the last fossils of great dinosaurs. It's been estimated that 70 percent of all species disappeared in fairly short order.

"It's possible that the impact was enough, but there is ample evidence that other things such as rapid climate swings were going on just beforehand, so it seems likely that the impact tipped the balance of an already-stressed biosphere," Renne said. "I've always felt that we should avoid simply saying, 'OK, Eureka, it was an impact and now we're done' -- simple answers are often incomplete."

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