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Re: my working hypothesis for body size

At 06:31 AM 4/29/98, Dave Adams wrote:

>If you look at dinosaur body size trends, as I have, I believe you will
>find that the average herbivorous dinosaur in the late Triassic was about
>the size of a deer.  By the early Jurassic this had increased to the size
>of a bison, and by late Jurassic the AVERAGE herbivorous dinosaur was over
>2 tons!  In the Cretaceous this declines, although herbivorous dinosaurs
>remain quite large.  Predatory dinosaurs show exactly the same trend, only
>the numbers are different.

This decline is somewhat hard to demonstrate.  First off, the largest
theropods and the largest sauropods known are from the Cretaceous.

Additionally, dinosaurs (unlike mammals) went through a LOT more size
changes during growth than do terrestrial mammals: lion cubs and gnu calves
are a substantial fraction of their parents' size, but hadrosaurid and
sauropod hatchlings were only about as big as their parents' foot!

So, how do we calculate average size?  Average maximum size of adults?  That
would seem to be the easiest way, and could indeed be the most significant.

Average size within a population?  Our (admittedly limited) knowledge of
dinosaur growth and populations seems to indicate something very different
for dinosaurs than for mammals or birds.  In a mammal or bird flock, there
are a few young and a LOT of full grown individuals.  It looks as if
dinosaurs, as r-selectors (that is, organisms that produce a LOT of young,
most of which died before adulthood) had populations where a substantial
part were not fully grown, and only a small fraction were maximum size. 

Also, are global averages ecologically significant in the Late Cretaceous,
say, compared to the Late Triassic?  In Pangaean times, the world is
effectively one place, but due to continental drift and the rise of
epicontinental seaways, the landmasses of the early part of the Late
Cretaceous (in particular) were even more divided up than they are now.

For one discussion on dinosaur body masses (using maximum adult body size)
and comparisons with Cenozoic mammal mass distributions, see:

Farlow, J.O., P. Dodson & A. Chinsamy.  1995.  Dinosaur Biology.  Annual
Review of Ecology & Systematics 26: 445-471.

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