[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Battle of titans: ilium questions (short)



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
_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com