Re: Terramegathermy in the Time of the Titans (not so long...)

["Matthew Bonnan" <mbonnan@hotmail.com>]

David Marjanovic said:

"Some dinosaurs are supposed to have migrated
from Alberta to Alaska and back every year, and sauropods are supposed to
have walked away after having emptied a region of edible plant parts, like
gnus in today's Serengeti..."

Okay.  Now think about the sentence above.  What is the evidence for mass 
migration of hadrosaurs, sauropods, etc.?  The evidence is very indirect.  
We assume that these large, supposedly herding dinosaurs had feeding 
patterns and movements similar to large mammals.  We assume that sauropods 
were clearing out regions of edible plants in vast numbers, like gnus you 
say.  We have footprints of sauropods all heading in the same direction.  
But we don't really have any direct evidence for mass migration, or even for 
the impact of dinosaurs on the Mesozoic environment.  Yes, they were big 
animals, and yes we know that they must have had some impact on terrestrial 
plants, etc., but no one has ever seen a feeding sauropod herd, or tracked 
hadrosaurs on a migration across the Western United States.  We cannot 
simply superimpose mammalian ecology on dinosaurs and say, "well, there you 
have it."  It's much more complicated than that, because dinosaurs are not 
mammals and they are very much extinct.  Our evidence for many of their 
possible lifestyles and activities is very indirect.

You said:
"If [the heart] is big, really big, not the relative size of a human heart, 
then,
according to P&L, it does necessarily follow that you are tachymetabolic."

And I am saying this is a bit too simple and black and white.  It does not 
necessarily follow that you are tachymetabolic simply because you have a 
very big heart.  All that indicates is that your blood pressure has to be 
subdivided and your heart must be strong enough to pump blood to your head 
and hindquarters.  Don't fall into the either-or trap.  If a green 
chalkboard is not white, it does not follow that it is therefore black.  
Similarly, even if dinosaurs were not typical ectotherms, they could be many 
shades of metabolism.  They do not necessarily "have to be" tachymetabolic 
endotherms.  Consider the low metabolic rates of sloths or of montreme 
mammals.  These are both endotherms, and yet they have relatively low 
resting metabolic rates compared to placental or marsupial mammals.  Maybe 
dinosaurs had a metabolism like that.  Or maybe dinosaurs had something that 
no longer exists in the present day.  We still don't know.

You said:
"All I remember of reading that article (once, long ago) is that I found it
unconvincing :-] . Can someone tell me what Reid meant and why?"

Reid meant, in a nutshell, that dinosaurs appear to have fast, 
endotherm-like growth rates, but also have osteological features that 
suggest their metabolic rates as adults were much lower than those of 
mammalian tetrapods of the same size.  Reid also questioned the use of 
"gigantotherms" like the leatherback turtle for dinosaur models.

Again, hypotheses about dinosaur physiology are based on a large amount of 
indirect evidence.  Although it is tempting to look at the superficial 
similarities in terrestrial mammal and dinosaur skeletons and jump to the 
conclusion that similar build = similar metabolisms, it is not this 
straightforward.

Matt Bonnan
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