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Re: Lots of teeth, then beaks



Larry Dunn wrote:
>So, we have an early, (presumably basal?)ornithomimosaurid
>(_Pelecanimimus polyodon_) that had lots and lots of teeth, and then
>there are all sorts of ornithomimoids running around with beaks.
>
>Is there any likely correlation between the extreme toothiness of
>basal ornithomimosaurids and the beakosity of ornithomimoids?  Do the
>theories about beak development in birds have any relevance here?

        This was, I believe, the theory under which the original
_Pelecanomimus_ authors were working. They attributed the change from a row
of closely spaced teeth to a cutting beak to the "process" of exaptation.
For those of you who are not up-to-date on current jargon, "exaptation" is
what we used to call "preadaptation", the use of existing anatomical
(perhaps behavioural as well?) features for new functions. Great examples
include the lobe fins of lobe-finned fish becoming arms, which later were
exapted to wings in three separate groups.
        With all due respect to those authors (hi Nino!), their usage of
exaptation seems a bit unconventional. What they are describing is more the
retention of a particular function, but with a change in the anatomical
structure tasked to perform that function. I do not know of a name for this
phenomenon, but it does occur.
        A very simple example of this may be taken from the vertebrate
skull, where bones can be seen to almost "flow" through evolutionary time
around structural features such as fenestrae. The fenestra, as a structural
feature (presumably associated with elements of the soft-anatomy) remains
more-or-less constant, while the bones making up the fenestra may change
through time (making homologizing structures especially difficult in widely
disparate forms).
        While I have no personal comment on the relationship between closely
spaced teeth and a beak, I think it is a *fascinating* concept. It is
especially interesting given what we know of the beak in _Caudipteryx_.
        :)
        Wagner
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     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
 "Only those whose life is short can truly believe that love is forever"-Lorien