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Re: Waiting for a giant bird and dino physiology revisted
On Mon, 7 May 2001, Matthew Bonnan wrote:
> 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 is
> stronger in compression than tension, maybe there is a size limit to how big
> 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.
OK. What size limit would you put on them? Is an ostrich way below or
close to the limit?
> In contrast, even moas have bent knees. Further, all of these big ground
> birds had no easy option for returning to all fours.
Are there easy options for achieving columnar limbs? To me _T. rex_ size
would be impressive--no need, really, to look to sauropods--How come no
_T. rex_ sized birds? is a valid evolutionary question by itself.
> Evolutionary contingency is probably also at
> work on limiting bird size.
I agree. But I think an estimated upper limit would make this statement
more meaningful. And if the upper limit is some decent size then we still
have our rather large unanswered question.
> I am very interested in WHY and HOW dinosaurs got so big on the average, and
> why some became incredibly gigantic ON LAND. 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 that
> 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 don't
> we? That's what's interesting to me and why I remain cautious about putting
> so much stake in physiological models: they are a good start, but we
> shouldn't stop there.