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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.