May 31, 2000- 2:00 p.m. EDT - The Associated Press WASHINGTON (AP) -- Dinosaurs died quickly, snuffed out by the impact of an asteroid that sent a wall of fire and death racing across North America, an analysis of fossils found in Montana and North Dakota concludes.

The finding casts doubt on a theory the dinosaurs died out slowly and that the asteroid impact was simply an end-the-misery trauma for an almost-vanished species, said Peter M. Sheehan of the Milwaukee Public Museum, first author of the study appearing Thursday in the journal Geology.

Researchers analyzed the number and distribution of fossils across large parts of the two states, where the animals roamed some 65 million years ago.

``What we found suggests that the dinosaurs were thriving, that they were doing extremely well during that time,'' Sheehan said. ``The asteroid impact bought a sudden and very abrupt demise to species that were healthy and doing well.''

The research adds weight on one side of a debate among experts who study the dinosaur and how the huge animals died.

One group, often called the gradualists, believes the dinosaurs were slowly dying out, that they were weak and beginning to disappear when the asteroid hit.

William A. Clemens of the University of California, Berkeley, a leader of the gradualists, said the Sheehan study fails to prove the asteroid theory of dinosaur extinction.

Sheehan and others believe it was the asteroid impact's alone that killed the dinosaurs in one, swift fiery eruption, followed by weeks of deep cold.

The gradualists base their argument on a 20-year-old study that found few dinosaur fossils in the top 9 feet of a rock deposit, called the Hell Creek Formation, that was laid down in North Dakota and Montana during the
last two million years before the asteroid impact. Based on the scarcity of fossils, the gradualists believe the 200-million-year reign of the terrible lizard was already drawing to a close when the asteroid arrived.

But Sheehan said a three-year survey of outcroppings of the Hell Creek Formation shows fossils throughout the deposit and that dinosaurs lived there in vigorous numbers and varieties until the very end.

``We looked at the community of dinosaurs in the Hell Creek formation and found they were not changing,'' Sheehan said. ``If they were going through a gradual extinction, we would have expected to see some change. We found no evidence of a decline.''

Sheehan said that through the whole 180-foot depth of the Hell Creek formation, the species mix and numbers of dinosaurs were the same, with Tyrannosaurus as the most common carnivore and the Triceratops the
most common plant eater.

This was true, he said, right up to the 2 centimeter layer that marks the impact. This layer, found virtually everywhere on Earth, is rich in iridium, a rare element brought to Earth by the asteroid. The iridium layer sits atop the Hell Creek formation.

``The abundance of dinosaur fossils in the upper three meters (9 feet, 9 inches) of sediment immediately underlying the impact layer is well within the range of many intervals lower in the Hell Creek formation,'' the study says.

After the impact layer, there are no dinosaur fossils.

To gather the data, scores of volunteers spent three summers combing more than 11 million square meters of North Dakota and Montana, walking shoulder-to-shoulder in a search for dinosaur fossils. They found the bones of almost a thousand dinosaurs sprinkled throughout the exposed levels of the Hell Creek formation.

Clemens said that the weakness of the Sheehan study is that it fails to go back far enough in history. He said that deposits five million and six million years old contain a much richer variety and number of dinosaur fossils,
suggesting the animals were declining when the Hell Creek formation was deposited.

Clemens also said the Sheehan study does not consider the effect an asteroid extinction would have on other species.

``You need to consider the whole fauna,'' Clemens said. ``Why did amphibians go through this period unaffected? There was a diversity of birds and they go through this period unaffected.''

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