RE: Dinosaurs, Size, and Land Area

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Richard W Travsky
>
>  Burness, G.P., J. Diamond, and T. Flannery. 2001. Dinosaurs, dragons,
>  and dwarfs: The evolution of maximal body size. Proceedings of the
>  National Academy of Sciences 98(Dec. 4):14518-14523. Abstract
>  available at
>       http://www.pnas.org/cgi/content/abstract/251548698v1
>
>
> Now this is something I've been curious about for a long time, the size
> dinosaurs reached and the land area required to support them.
>
> The abstract says
>
>  These patterns explain the size of the largest-ever extinct mammal, but
>  the size of the largest dinosaurs exceeds that predicted from land areas
>  and remains unexplained.

This is an interesting paper.  They measure the body size of various
terrestrial animals (endotherms and ectotherms) and plot them against the
land area in which they lived.  They find, not surprisingly, that larger
land areas support a) larger herbivores than carnivores and b) larger
ectotherms than endotherms (given the fact that ectotherms have
proportionately smaller food requirements than endotherms).

Incidentally, they give the following masses for various bigass dinos:
        _Sauroposeidon_         55 tonnes
        _Argentinosaurus_               73 tonnes
        _Paralititan_           59 tonnes
        _Tyrannosaurus_         6.25 tonnes
        _Giganotosaurus_                9 tonnes
        _Carcharodontosaurus_   7.5 tonnes

The authors point out that these dinosaurs were much, much larger than the
predicted size for endotherms for those particular land areas, and even 12x
(theropods) or 1.5-3x (sauropods) larger than ectotherms for their land
areas.  The situation is even worse than they suggest, because they use the
*total modern* land area for North America, South America, and Africa for
the North American, South American, and African dinosaurs, respectively: in
fact, given higher Cretaceous sea levels and other tectonic changes, these
dinos were probably limited to just a fraction of the area now expressed in
these regions.

On the other hand, the authors do not take into account the study of Carrano
& Janis, who suggest that mammals may have a size limit that has nothing to
do with land area productivity.  Specifically, those authors observed that
gestation period scales with body size in placental mammals, so that the
largest mammals have multi-year gestation periods: a major stress on the
individual mothers, and a very low rate of replacement.  In contrast,
dinosaurs (as egg-layers) had egg clutch sizes and rates of replacement
which were essentially mass-independant, and thus would not face the same
constraints to evolving and sustaining immense body size.

> This also makes me wonder about the KT impact event. Changes in land
> area?

Land area would actually have increased in the latest Maastrichtian due to
the Maastrichtian Regression.

>Atmosphere composition? The animals that survived, how can they be
> viewed in this context?

The authors of the PNAS article do bring up the possible effects of higher
CO2 levels in the Cretaceous (although the values for the Cenomanian would
be very different from the Maastrichtian).

The authors also point out that the largest living animals (various species
of whales) have global distributions, and wonder if their might be body
mass/area relations for the oceans as well.  (I might wonder if there might
not even be a body mass/volume relations for these critters, since they live
in a more 3-dimensional world in terms of where food occurs).

Cool stuff.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796