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

["David Marjanovic" <david.marjanovic@gmx.at>]

> 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.

OK, erm, their lower limbs are graviportal :-] They can't move their wrists
and ankles, that's what I wanted to say.

> "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
> ...

and surely isn't necessary for just 1 t, is it? I'm not talking about a
sauropod-sized bird.

> 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.

Hmmm... Some dinosaur lineages became gigantic because there were selective
pressures towards it _and_ their body plan and their physiology allowed it.
P&L say this physiology must have been tachymetabolic and -aerobic, or it
wouldn't have allowed it. Endothermy (or anything else) alone doesn't mean
one must become gigantic (obviously). So it has some curvature, but it's not
even spiral... :-)

> Your statement provides no way of
> testing endothermy/ectothermy/something else.

yep
except if someone does all the maths and finds out that even a damned good
reptile physiology does not allow gigantic sizes (or if we accept the P&L
paper: a big animal needs a big heart needs much energy needs high resting
metabolism...)

> 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.

yep :.-(