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Re: Waiting for a giant bird and dino physiology revisted

Another two old answers...

----- Original Message -----
From: "Matthew Bonnan" <mbonnan@hotmail.com>
To: <dinosaur@usc.edu>
Sent: Tuesday, May 08, 2001 12:02 AM
Subject: Waiting for a giant bird and dino physiology revisted

> David Marjanovic writes:
> "> and they [birds] have IMHO not yet had a chance to grow
>  > to gigantic sizes."
> I don't see the mystery here about the absence of multi-ton birds.
> [...]
> 2) In the case of ground birds, their hindlimbs were already modified so
> that they walk or run with bent knees all the time.  This means that their
> femur is constantly being loaded in tension, not in compression.  As bone
> stronger in compression than tension, maybe there is a size limit to how
> you can get with bent legs.  After all, the biggest animals we know of on
> land have columnar limbs, not bent legs: check out elephants or sauropods.
> In contrast, even moas have bent knees.  Further, all of these big ground
> birds had no easy option for returning to all fours.  Whereas sauropods
> probably quadrupedal secondarily and able to modify the forelimbs of their
> bipedal ancestors back to support structures, the wings of these giant
> ground birds are pretty reduced.

This should still be enough for *T. rex* sizes. _All_ theropods, not just
birds, have bent knees and are bipedal. Of course, there was no selective
pressure for any neornithine to get that big.

> 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.

Turtles are graviportal. 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)

> Thus, we can wait for a giant bird to tern up (pun intended),

Won't come soon, I agree. (If tyrannosaurs are not birds.)

> Maybe dinosaur size has to do with
> MANY factors, not just physiology, and maybe tachyaerobic endothermy alone
> can't explain dinosaur success at large size.

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

> cold-blooded (boy, I miss that hyphenated word) [...]
> warm-blooded (boy, I miss that hyphenated word, too).

Warm-bloodedness is IMHO the combination of endothermy, homeothermy,
tachymetaboly, and tachyaerobics (no pun intended, if this word is correctly
formed). Cold-bloodedness is therefore the opposite, the combination of
ectothermy, poikilothermy, bradymetaboly, and bradyaerobics. Well, these
combinations aren't at all as frequent as people used to think, and lots of
people think that dinosaurs had some other combination.

> The minute we settle on a
> single factor as the reason for dinosaur success at large size, we blind
> ourselves to so much else.  Sure, maybe dinosaurs were warm-blooded and
> was a part of their success.  But that's the point: a PART.  Life history,
> reproduction, functional morphology, historical contingencies, the
> archosaurian body plan, climate, etc., must have all played a part.
> Otherwise, we should have gigantic land mammals and birds, right?  Why
> we?  That's what's interesting to me and why I remain cautious about
> so much stake in physiological models: they are a good start, but we
> shouldn't stop there.