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Re: body size yet again

At 06:55 AM 5/6/98, you wrote:
>Once again, critical, knowledgeable, thoughtful responses.  I'm getting
>With regard to body size trends, it did not occur to me to compare
>outliers.  If we look at outliers I would probably have to eat my words
>that regardless of what we measure, we will see the same big picture.  But
>if we look at means, medians, modes, or any other measure of central
>tendency, I believe we will see the trends I have described.

Again, based on what data?  If we look at South American dinosaurs, I don't
think you can justify a decreasing average size into and throughout the
Cretaceous: larger sauropods remain taxonomically and specimen-wise the most
abundant dinosaurs up to the K-T boundary.  In India as well large
titanosaurs remain the most common known dinosaurs up into the intertrappean

In the Early K of North America, sauropod material seems to be relatively
common and taxonomically diverse.

>  I don't
>really know what to say about outliers, other than the fact that very few
>scientific studies use them to make comparisons.  The tails of most
>distributions are relatively flat, so the values of outliers can vary a
>great deal.

The increase in ornithischian (or subgroup) size into the Cretaceous does
not seem to be a matter of outliers.  The small forms remain around the same
size, but the big forms become more and more common.

>Herds of Triceratops horridus did not feed on tree leaves.  It requires a
>great deal of vegetation near ground level to sustain numbers of such
>creatures.  The very presence of large herds of ceratopsians and hadrosaurs
>in forests would maintain a lot of canopy gaps.

On this I would agree.

>My hypothesis that closed-canopied forests decreased in area after the
>Jurassic is based on the literature I have read, but I have to admit I am
>not up on paleobotany terribly well.

I would be interested in learning the sources concerning the decrease in
closed-canopy forests.  Most workers for the last twenty years have regarded
the Morrison as a relatively open, dry biome.

>But the remark about small birds being diverse has sparked my interest.  In
>ALL taxa that I'm aware of, including dinosaurs, species richness increases
>exponentially with decreasing size.  Why?  The answer is not at all clear
>to me.  It may be that small species tend to have shorter generation times,
>leading to more rapid rates of evolution and speciation.  Or perhaps large
>species have a higher extinction rate.  Or is it a combination of factors?
>I would be interested in hearing opinions.

Actually, I suspect it may have to do with shorter generation times,
extinction rates, and ability to keep stable population numbers over a
geographically smaller region (although, of course, some small bodied forms
have vast ranges).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661