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Re: Study Backs Quick End of Dinosaurs





Ralph et al.:

Yes, this should be interesting. I was a volunteer on their 1990 outting to Glendive, Montana and was one of the young punks that scurried up and down the buttes looking for dino bones. One of the interesting things about the program (called Dig A Dinosaur at that time) was that Sheehan et al. would not tell us much about what they had found the previous two years as far as sample size, number of dino genera, etc. In fact, we were told that the data would not be analyzed until that summer's work was up.

Anyways, I look forward to reading it to and to much discussion on the list.

Matt Bonnan

From: "Ralph Chapman" <Chapman.Ralph@NMNH.SI.EDU>
Reply-To: Chapman.Ralph@NMNH.SI.EDU
To: dinosaur@usc.edu, majestic_cheese@yahoo.com
Subject: Re: Study Backs Quick End of Dinosaurs
Date: Wed, 31 May 2000 17:09:40 -0400

I believe this is an important study and look forward to reading it. The
only potential problem I see is to see exactly what taxonomic level the ids
were made at and wondering what the results would be in other areas pre
study (other Cretaceous areas) if the sampling was this intense. To a
certain degree we are comparing apples and oranges with body fossils
(partially articulated skeletons) making up the predominant record in most
of the Cretaceous, at least the part generating the taxonomic diversity as
decribed species at each time, and this study, which is almost wholly
individual elements which, mostly, are clumped (I think) into clusters for
each genus or more and each counted as one species group - tough to id
different species for disarticulated elements of closely related taxa. So
you would think that this would have two effects First it would go against
their hypothesis because it would dampen apparent diversity in this
well-sampled record - which is good procedure scientifically since they are
saying this time interval was of equally high diversity to previous times.
So the first effect is good for them. However, it also might help their
hypothesis if the greater sampling of material at the scale they did
provided more elements, and potentially higher diversity for that section
relative to earlier times, even if the starting pool of species/fossils was
the same, or even greater earlier on, which I think is what Clemens was
getting at. The answer probably is to do super sampling at selected
intervals in the record for comparisons through time of equal sampling. Sort
of like looking at diversity through time by comparing diversity in
comparable lagerstatten, which is a cool approach. Actually, one of the
neatest studies of taxonomic diversity through the Phanerozoic was done by
Dick Bambach who, unlike Raup, Sepkoski etc. did not just look at diversity
through time at a specified taxonomic level. Instead he looked at apparent
diversity within ecological guilds - marine paleocommunities that were
roughly equivalent through time and was able to really see where the changes
really did occur. A great study.


Anyway, this should be fun!

Ralph Chapman

Ralph E. Chapman
Applied Morphometrics Laboratory
National Museum of Natural history
ADP, EG-15  NHB, 10th & Constitution, NW
Smithsonian Institution
Washington, DC 20560-0136
(202) 786-2293, Fax: (202) 357-4122
Chapman.Ralph@nmnh.si.edu

>>> Larry Dunn <majestic_cheese@yahoo.com> 05/31/00 03:31PM >>>
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 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.''



=====
Larry

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

http://members.tripod.com/~megalania/index.html

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