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Re: Dinosaur diversity
Tim Williams wrote (3-14-96):
>A lot of authors have pointed to a loss of dinosaur diversity in the
>final few million years of the Mesozoic as evidence for a gradual
>decline before their final extinction, rather than a single great
>catastrophe at the K/T boundary.
Keep in mind that there might have been a gradual decline in dinosaur
diversity (both taxonomic and _ecologic_, as Tom Holtz pointed out)
toward the end of the Cretaceous that was causally unrelated to the final
extinction event, especially if the latter were really a catastrophic
event such as a bolide collision.
Dinogeorge added (3/16/96) this perceptive point:
>Another problem is extrapolating the Lance and Hell Creek of western
>North America to the entire world at the end of the Mesozoic.
I think this is indeed a problem. Assessing the significance of observed
declines and increases in diversity is difficult, because our
observations (equivalent to experimental results in other sciences) are
_uncontrolled_. In other sciences, experiments are strictly controlled
to assure us that the results are indeed significant. The experimental
system is compared to a controlled system, where certain variables of
interest are held constant. Obviously, we can't do this in paleontology.
There is only one Judithian and one Lancian, for example. These are two
independent "experimental systems" with no controls to help us assess the
significance of the observed differences between them. Perhaps if there
were a "second Lancian" (i.e., a Late Maastrichtian deposit of similar
paleogeographic setting), it would have some other diversity value (of
course, it might also have the same diversity. Without controls, I don't
think we can determine what the Judith-to-Lance diversity decline means.
I wonder if we really know why the Judith has its observed dinosaur
diversity, and why the Lance has its observed dinosaur diversity. What
are all the reasons that observed diversity values of dinosaurs might be
different between two different formations? We would have to have a
command of those factors, and have a way to identify the factor(s)
determining diversity in the formations in question before we can assess
the _difference_ in diversity between them.
What does it mean to talk about the dinosaur diversity of the Lance?
What part of the Lance, and where? Do the bones come from the same bed?
If not, then conditions might have been different for the deposition of
different beds. Perhaps one dinosaur-bearing bed involved in the
analysis is tens of thousands of years older than another. Should we
lump the diversity data together, especially since age may not be the
only difference between them?
Just some things to think about before forming a conclusion or using any
particular diversity data to support or dispute some particular
I admit it--I'm a paleontological nihilist.
Norman R. King tel: (812) 464-1794
Department of Geosciences fax: (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712 e-mail: email@example.com