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

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

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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