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Re: Size limiting factors, (was:the biggest predators)

-----Original Message-----
From: William.BONNET@tcc.thomson-csf.com
To: dinosaur@usc.edu <dinosaur@usc.edu>
Date: Tuesday, January 16, 2001 3:11 AM
Subject: RE: the biggest predators

>The maximum size limit sounds interesting to me.
>Is there any paper about the maximum size a predator can reach ?
>About what would be his constraint in getting larger. What would prevent
>him from growing ?

>William Bonnet

I`m not sure what studies have been done on this topic, but I have mentioned
before that respiration might be a factor in size some theropods achieved. I
know that the avian respiratory system not only supplies sufficient oxygen
for a high metabolic rate, but also allows for the removal of excess heat
during flight. Large animals also have the problem of dumping excess heat.
Elephants use their large ears, large predatory theropods had to chase down
prey, and were likely more active than elephants in doing so. This activity,
plus the large size some of them attained (over 1 ton), must have required
very efficient (bird-like) respiratory systems. However, not all theropods
were equally equiped in this respect. If you look at Greg Paul`s  "Predatory
dinosaurs of the World", he shows that a "birdlike ribcage" is just starting
to develop in Metriacanthosaurus, and later becomes more pronounced in
Allosaurs, and better yet in Tyrannosaurs.

Spinosaurs, however, don`t have this type of ribcage. Theirs is a more
primitive structure, and the respiratory system probably not as efficient as
in the later Avetheropods. It is interesting that the Spinosaurs have the
tall fins along their backs, (Metriacanthosaur is intermediate in this
trait).  This might seem to indicate that in such finback species the fin
might  serve a a heat dumping mechanism, which allowed them to reach such
great size despite the restrictions of their respiratory systems.