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Study Backs Quick End of Dinosaurs

Study Backs Quick End of Dinosaurs

By 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

``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.'' 


"I've been ionized, but I'm OK now."


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