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Re: Ornithomimid beaks



Graydon "do we have another name here? :)" (graydon@dsl.ca) wrote:

<If the theory of theropod beak formation that has the horn butting pads, 
lacrimal horns, and so
on extending down the snout until close enough to the mouth to be exapted as a 
cutting surface is
correct, you'd expect beaks to be initially right at the tip of the mouth. 
(Although I am not at
all sure how that theory handles *lower* beaks.)>

  What theory? At first I'm wondering what this might be alluding to, and 
recall Graydon bringing
this up before, but then super-Ornithischian Dude Pete Buchholz brings me to 
PDW, where Greg Paul
had illustrated the beaks of ornithomimids in this fasion, where nearly the 
whole of the
preorbital skull was covered in a cornified beak, including around the nares. 
First off, there is
no evidence for such a structure (not to put down Paul's skill and insight, 
just the data is
lacking) in ANY dinosaur, so nothing like this can be called even theoretical. 
You need evidence
to make a theory. It's an hypothesis, but that's it...

<The other way to look at it is that the beak is the gripping surface of a pair 
of pincers; teeth
are a high metabolic cost for a what-it-can-swallow omnivore, and that 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.> 

  This is unlikely. Loss of teeth does not result in a sharp tomial edge, as 
the tomial edge is a
derived condition beyond the reversal of tooth loss... the reduction of the 
teeth on troodontid
jaws does not result in a sharp caudal edge, and the same can be said in 
primitive birds, which
have rounded toothless margins of bone behind the dentigerous portion of the 
jaws.

  David Marjanovic writes about the extreme reduction of the placement of the 
keratinous portion
of the beak as preserved. This has led (I assume) to the position in both of 
these young men the
concept of "tongs" or "pincers"; however, aside from the taper of the preserved 
portion of the
beak, it has been overlooked that the caudal margins and the ventral margins 
appear to be broken.
In fact, the caudal portion of the keratin in the *Gallimimus* specimen GI 
(SPS) 100/1133, has a
rough, irregular, and broken margin, leading me to suspect the keratin was much 
more extensive
caudally and ventrally.

  As for selection against teeth, this would only work if the beak started at 
the front. However,
*Pelecanimimus* appears to suggest that the beak was caudal and progressed 
rostrally.


=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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