Battle of titans: ilium questions (short)

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

Greg Paul writes:

"There is currently nothing in the nature of gigantic longoschian
dinosaurs that indicates that they were not tachymetabolic tachyaerobic
endotherms that had completely diverged from the reptilian pattern. Does not
mean that they were all exactly like birds and mammals in their
thermodynamics, but they were much closer to them than to reptiles."

Okay.  I would agree with you that dinosaurs diverged from the typical 
reptilian pattern.  You have attempted to tie, among other factors, ilium 
size with metabolism.  I have a few questions for you below.

Paul:
"Mesoschian proto/dinosaurs - Lagosuchians, eoraptors, staurikosaurs,
prosauropods have ilia whose length is higher than measured in reptiles even
the semi-bipedal ones, but shorter than those of birds and mammals. This
means that they were intermediate in terms of leg muscle mass and exercise
energetics."

A few questions.  First, is there a direct, causal correlation between ilium 
length and metabolism?  A broad or long ilium qualitatively suggests more 
muscle mass than a short ilium, but beyond that, what does it really tell 
you about "exercise energetics?"

Second, the ilia of mammals are different morphologically from those of 
dinosaurs.  Do you feel its fair to compare the size of mammalian ilia with 
dinosaur or bird ilia?

Third, the tail muscles caudofemoralis longus (CFL) and caudofemoralis 
brevis (CFB) were apparently the major femoral retractors of most dinosaurs. 
 Therefore, although herrerasaurids, prosauropods, etc., have shorter ilia 
than other dinosaurs, birds, or mammals, what would the effect of the CFL 
and CFB have been on compensating for this?  In other words, maybe you don't 
need as long of an ilium to move your limbs if you are being compensated in 
part by these large tail muscles.  Any thoughts?

Fourth, does relative ilium size compare with metabolic rate in mammals?  Is 
the relative ilium size of a shrew, which has an incredibly high metabolic 
rate, longer than that of an elephant which has a lower metabolic rate?  
This is admittedly ridiculous, and I'm not suggesting you think this, but 
can you see what I'm getting at here?

Here's something less ridiculous: what are the relative sizes of the ilia in 
montremes, marsupials, and placentals?  Do the placentals have larger 
relative ilia than marsupials, etc.?  Having a relatively bigger ilium may 
just be correlated with needing to move bigger or longer limbs and have a 
less direct correlation with exercise energetics.

It would seem to me that the size of the ilium and its morphology would give 
you some idea of the size and basic arrangement of the pelvic muscles in 
various terrestrial tetrapods, but not necessarily correlate with metabolism 
per se.  In other words, ilium size and shape is an indicator of functional 
morphology in locomotion but not a metabolic indicator.  What do you think?

Paul:
"The
relatively sluggish armored dinosaurs and therizinosuars may have been less
energetic than other dinosaurs."

But again, can you correlate functional morphology directly with physiology? 
 An animal might be slow because of heavy armor or joint restrictions, not 
necessarily because of a lower metabolic rate.  An elephant cannot run, not 
because it is incapable physiologically, but because its joint structure and 
weight do not allow it to achieve fast speeds.

I must emphasize that I am not suggesting or defending the hypothesis that 
dinosaurs were gigantotherms.  I am instead questioning the correlation 
between functional morphology and metabolic rate.

Matt Bonnan
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