Re: Ornithomimid beaks

[Graydon <graydon@dsl.ca>]

On Thu, Aug 30, 2001 at 08:21:51PM -0700, Jaime A. Headden scripsit:
> Graydon "do we have another name here? :)" (graydon@dsl.ca) wrote:

Why, no, there isn't another name there.

My family name is Saunders and my personal name is Graydon and I am
sometimes afflicted with terseness.

>> 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...

Granted.

>> 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.

Loss of teeth directly, no, but if the teeth are still there there's a
certain minimum width to the top margin of the tooth-bearing bones; if
the teeth aren't there, the jaw is free to develop a shape that supports
the stresses on it without having to support teeth, too.

> 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";

That's from watching ducks eat.

We're not talking about something that filter-fed *all the time*, I
don't think; the beak isn't as highly specialized as a flamingo, after
all, but more like the beaks of ducks with generalized feeding habits.

Ducks use the very tip of their beaks to grasp things they want to eat;
when it's grain, it's a very delicate operation involving small amounts
being picked up very fast.  When it's a frog they can't swallow whole,
it's not delicate at all.

(Grazing geese use the sides of their beaks, so it's not like
tip-grasping is universal.)

Anyone know of biting stress studies for ornithomimids?  The one skull
I've seen (in the Royal Ontario Museum) looks very delicate.  I don't
think they would be able to generate much biting force, even with a
beak; that's what I'm taking as most of the argument for what-it-can-
swallow-whole omnivory, I don't think tearing things up was much of an
option.  (although tearing *apart* with the hand claws might have been.)

Rotten hard to test any of that, though.

> 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.

No argument with that.

> 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.

How many times did beaks evolve?

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