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Re The biggest dinosaurs

John Bois wrote:
> On 20 Aug 1997, Norton, Patrick wrote:
> > ...does anyone have any thoughts about why the upper absolute
> > size of dinosaurs was so much greater than the upper absolute size of
> > terrestrial animals today?
> I believe it is because they guarded eggs.  A premium is put on defence
> rather than running.  In mammals it's the other way ariound and,
> apparently, pregnacy incurs not much of an increase in predation.
> Bolstering this claim is a finding in M de Fraipont et al, _The evolution
> of oviparity and viviparity in lizards and snakes: a phylogenetic
> analysis_, in Evolution 50(1) pp. 391-400."Our results show that
> egg-guarding is associated with an increase in size...(and)...viviparity
> is associated with a reduction in size..."  This is for snakes and lizards
> but one would expect similar forces to be in operation, i.e., it is easier
> to defend a nest when you are big.

Then again, mammals in the not-too-distant past reached quite large
proportions (Indricotherium comes to mind). I think maximum sizes
have more to do with a complex interplay of environment, climate,
and ecology (ie. other creatures in the ecosystem). Herbivores may
be able to grow to large sizes in an attempt to out compete each
other if the environment provides enough food, carnivores may
increase in size in competition with each other given the large
herbivores to sustain them, and herbivores and carnivores may then
begin an "arms race" to maintain an equilibrium. This could
conceivably happen in stages or concurrently. If the carnivores
win the arms race, they may drive large herbivores to extinction.
If the herbivores win, they may drive themselves into extinction by
eating all of the available food. The extinction of the megafauna
(in several parts of the world) has been attributed variously to
human predation (where the carnivores become too efficient), climate
change (food becomes scarce, herbivores die off, large carnivores
follow), or even a combination of the two (humans in Australia
regularly burned the vegetation, environmental balances shift, hunting
that was formerly sustainable now leads to extinction). I doubt that
any single factor contributed to the decreased size of modern mammals,
and similarly that no single factor was responsible for the
large size (or extinction for that matter) of dinosaurs.

        Dann Pigdon
        Melbourne, Australia