Even more on giant birds and dino phys (joy!)

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

David M. says:

"This should still be enough for *T. rex* sizes. _All_ theropods, not just
birds, have bent knees and are bipedal."

Yes but not in the same way.  The femora of most theropods, including T. 
rex, are held much more vertically than bird femora.  Knee flexion plays a 
bigger role in theropod locomotion than in many mammals, but birds have a 
very horizontal femur which is unlike what we see in theropods.  They walk 
differently, and load their femora differently.

I has previously said:
> The contingencies of history can sometimes
> limit the development of certain body plans.  A good example of this is
> turtles: they develop the shell early, are successful and radiate into
> numerous forms, but in over 200 million years there have been no flying
> turtles or running turtles.

David responds:

"Turtles are graviportal."

No they are not.  Being graviportal does not just mean slow moving or heavy: 
it involves having columnar limbs for supporting weight.  An elephant is 
graviportal.  Some turtles get big, yes, but not graviportal.  They are slow 
because their limb movements are restricted by a large shell which prevents 
the limbs from ever becoming vertical or columnar.

"Large changes in body plan, along with improbable
selective pressures, would have been necessary to produce running or flying
turtles (and tachymetaboly for the latter)"

Right -- this is what I just said in my previous post.  The historical 
constraints on body plans can limit certain animals from filling certain 
econiches.  So ... birds may also be restricted by their bent-knee (groucho) 
locomotion and history of flight adaptations to getting really big -- 
gigantic.  It would be awfully tricky for big ground birds to revert to 
being quadrupedal for instance.  This would a large change in body plan that 
might require improbable selective pressures, as you say above for turtles 
...

David continues:

"I (rather, P&L) don't say that dinosaurs became so big because they were
warm-blooded. They say that this is a prerequisite that ALLOWS one to become
gigantic."

What's the difference between "dinosaurs became so big because they were 
warm-blooded" and "endothermy is a prerequisite that ALLOWS one to become 
gigantic"?  Those are both the same thing.  I understand in the second 
instance we are talking about potential (i.e., that endothermy does not 
necessarily lead to gigantic size).  However, dinosaurs are big.  If you 
argue a priori that endothermy is a prerequisite of large size, then large 
size = endothermy.

You see what I'm saying?  Its circular.  Your statement provides no way of 
testing endothermy/ectothermy/something else.  Ostrom, Bakker, Paul, Ruben, 
and many many others have looked valiantly for osteological characters that 
unambiguously correlate with physiology, but all the evidence has ended up 
being equivocal because so many other factors were at work.  What about 
reproduction?  What about climate?  What about indeterminate growth?  What 
about diet?  What about a gynmnospermous dominated flora?  What if dinosaur 
physiology is a form of endothermy/ectothermy that no longer exists in 
extant animal?  Again, it would be nice if we had unambiguous osteological 
characters that showed us an organism had a certain type of physiological 
mechanism, but we don't.

Matt Bonnan
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