Re: Dinosaur weights

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 07:05 PM 8/21/97 -0400, Matt Fraser wrote:

>You are right, of course, about how to define density, but I think that
>point at issue here is whether the dinosaurs were less dense than water,
>since it is being argued that there volume was, in part, composed of
>non-living gas poskets.
>
>What about birds?  Same density as water?

Birds have a much lower density than water.  Greg Paul estimated birds at
about 0.8 the density of water in Predatory Dinosaurs of the World, but a
more recent paper (in Science? Nature? have to find it, know I xeroxed
it...) put the value even lower.

Furthermore, in Colbert's classic study of dinosaur "weights" (1962), he
indicated that the Gila monster and crocodile he examined both were less
dense than water, but chose to use the 1.0 value he found in mammals.

I think that Greg's idea of a 0.85-0.9 value for nonavian theropods is quite
likely, especially given the pleurocoelous structure of avetheropod vertebrae.

Also, the main failing in R. McNeill Alexander's model-based mass estimates
are that the models (to put it politely) suck.  (Not Alexander's fault: he
was just using the British Museum toy series...).  You cannot fit the shape
of the skeleton of the appropriate dinosaurs into its model!!  As such,
Alexander may have found the expected masses of giant versions of these
models, but that has nothing to do with the expected masses of real dinosaurs...

the main failing with Colbert's _Tyrannosaurus_ estimate (beyond the shape
of the model) is the overestimate of its size.  Colbert gave something like
a 3.8 m limb length for the model, but the then-largest known _T. rex_ had a
limb length of only 3.12 m and Sue's is probably only 3.3-3.4 m.  Since mass
increases with the cube of linear increases (for isometric systems), Colbert
may have overestimated the mass of _Tyrannosaurus rex_ by a factor of 1.8 or so!

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