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What really happened to the dinosaurs?

Scientist says asteroid-extinction theory too simple

By Marsha Walton

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(CNN) -- The disappearance of the dinosaurs may not be as neat and tidy as the animals being wiped out by a huge asteroid 65 million years ago.

New evidence from geoscientists, published Monday in the Proceedings of the National Academy of Sciences, suggests a much more complex hypothesis of hostile conditions spread over many years.

"Dinosaurs are very popular, and the asteroid theory is sexy, it's a perfect story, and in the past few years it's all you've read in the popular press," said Princeton University professor Gerta Keller, the paleontologist who wrote the study.

Keller said that while her theory may not be as riveting as a massive space object hitting Earth, it answers a lot of questions that she said don't seem to fit in the "wipeout" theory that's been publicized since the early 1980s.

The single asteroid theory involves the Chicxulub crater in Mexico's Yucatan Peninsula, in the time between the Cretaceous and Tertiary periods, commonly known as the K-T boundary.

This K-T boundary in the rocks at Chicxulub is evident because of the presence of iridium, an element common to asteroids.

Much of Keller and her colleagues' research involves the study of rock formations above and below the Chicxulub crater.

Evidence she's examined shows that the asteroid hit Mexico about 300,000 years before the dinosaurs and many other organisms disappeared --and that the crater was smaller than originally believed.

Over the past 10 years, Keller has studied microfossils at the Mexican site and sites in many other countries.

These tiny organisms, called foraminifera (about 10,000 could fit under a fingernail), are good barometers of what the conditions were like when they lived.

"These single-celled organisms are extremely sensitive to environmental changes," Keller said.

By studying their calcium carbonate shells, it is possible to determine temperature, salinity and other barometers of the time, she said.

If it wasn't one big asteroid, what did wipe out this planet's most popular former residents?

Keller said this and other mass extinctions can be tied to an intensive period of volcanic activity and resulting greenhouse effects, and probably a series of many asteroid hits.

She said by the time of the Chicxulub impact, there were already many signs of stress in organisms: Species were already endangered, their populations having declined and becoming dwarfed.

"We're quite confident we're on the right track," Keller said.

But she said challenging such a "perfect story" as an asteroid strike has made her unpopular, and at times vilified by those who support the massive wipeout theory.

Paleontologist Mark Norell said the feud among scientists about exactly what led to the demise of dinosaurs can get testy at times.

"Very few theories in science explain everything," said Norell, chairman of the division of paleontology at the American Museum of Natural History in New York.

He said Keller does a pretty good job of showing the Chicxulub crater is not the "smoking gun" that provides all the answers.

"Keller rightfully points out that the ecosystem was unraveling long before the impact," he said.

The asteroid may have been the straw that broke the camel's back -- or in the case of the dinosaurs, the backs of tyrannosaurus rex and triceratops.

Norell said when the impact theory was introduced more than 20 years ago, it was a good one.

"It had a lot of things going for it, namely, there was an impact, everybody agrees on that," he said.

"But to a lot of us, it was still kind of unsatisfactory," he said.

Unearthing exactly what happened may never be possible. Norell said there are few places in the world that scientists of any specialty can look at to show exactly what life was like tens of millions of years ago.

Keller said geologic studies of the Yucatan crater may also benefit the planet's current inhabitants, by providing insight on global warming and asteroid and comet research.

"Asteroids may be less dangerous than they've been made out to be," Keller said.

An asteroid that could cause a mass extinction occurs only every 200-250 million years, she said.

Smaller ones, creating craters 60-90 miles across, occur about every 500,000 or so years.

And speaking as a dinosaur paleontologist, Norell said not all the dinosaurs became extinct 65 million years ago.

"We just call them birds now," he said.

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