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Re: Ornithomimid beaks



On Thu, Aug 30, 2001 at 10:13:31PM +0200, David Marjanovic scripsit:
> Don't know that one, but the hypothesis that links beaks and egg teeth
> (which explains why therians never evolve beaks) predicts the same. But I'd
> expect teeth behind, not simply nothing, which the paper implies.

What are the teeth going to do?

Teeth are good for tearing, for gripping, and for chewing; the beak is
doing the gripping, and there's no application for chewing in an
eat-what-you-can-swallow omnivore.

Tearing is the tricky one; modern ducks can strip meat from chickens,
and I don't suppose an ornithomimid with no teeth would have much
trouble using its beak to tear meat off a carcass or a leg off a big
frog or similar, but can't begin to suggest would would make good direct
evidence of this.

> > [...] as soon as there *was* a beak
> > there, there was strong selection pressure against teeth.  The sharp jaw
> > margins may just be side effects of tooth loss.
> 
> Sure, but what covered the jaw margins, if not a beak?

Skin?  Cartilage 'gums'?  It's not clear that there's any need to cover
the jaw margins with anything, is it?

The other option is that the beak preservation is partial, rather than
complete, but I'd hate to try to invoke that.

> *Pelicanimimus* which has such sharp margins on the caudal parts of the
> maxillae looks even stranger now...

If there's minimal side-to-side stress and lots of stress in the plane
of the bite, the vertical depth would be retained, and the thickness of
the edge necessary to support teeth wouldn't be.  Is there something
about the sharp edge that can't be explained by that?

-- 
                           graydon@dsl.ca
               To maintain the end is to uphold the means.