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Re: Lots of teeth, then beaks



At 02:11 PM 1/19/99 -0800, 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?
>
First off, taxonomy:
(Tom get's out the whacking stick...):

There IS NOT, NEVER HAS BEEN, AND NEVER WILL BE an "ornithomimosaurid".

There is Ornithomimidae, which yields the vernacular form "ornithomimid".
There is Ornithomimosauria, which yields the vernacular forms
"ornithomimosaurs" or "ornithomimosaurians".

"Ornithomimosaurids" requires a taxon "Ornithomimosauridae", which requires
the *genus* _Ornithomimosaurus_.  Such an animal hasn't been named.

Yes, Lucas uses this name in his textbook, even though he was corrected by
some reviewer in JVP (;-); yes, Dingus & Rowe have this name show up
occasionally in "The Mistaken Extinction"; but it is wrong in these cases.

Stop using that name, please.
(Oh, and since no one has officially named "Ornithomimoidea" yet, please
don't use "ornithomimoid" either).

The previous brought to you by Tom "Defender of the Ostrich Dinosaurs" Holtz.

Now, to actually do something useful and answer the question:

Perez-Moreno and his colleagues address this issue briefly in the original
paper in Nature.  They suggest that the extraordinary number and
extraordinarily small size (they are really, really tiny) of _Pel._'s teeth
might have been an adapation for a slippery diet, and that as tooth size
reduced the beak grew to take its place.  If troodontids or
therizinosauroids are the sister taxon to ornithomimosaurs, numerous small
teeth would be ancestral for ostrich dinosaurs.  The few teeth in
_Harpymimus_ and the no teeth in the others would be further reductions.

However, it could be that _Pel._'s numerous teeth are its own
specialization, and that the ancestral ornithomimosaur had a more typical
number.

These hypotheses can be tested by a) new basal ornithomimosaur skull
discoveries (oh, please!!!) and b) better corroboration on the sister taxa
to Ornithomimosauria, in order to  more firmly establish the ancestral
condition.

How does this apply to bird origins?  Hard to say.

_Archaeopteryx_, enantiornithines, and most basal birds don't have jaws full
of numerous teeth.  Instead they have dental formulae comparable to or less
than that of most coelurosaurs.  (A consipicous exception is the buttload of
teeth in the jaws of hesperornithiforms, which are known to be fish eaters.
Polydonty seems to be associated with piscivory (try to work those two words
into a sentence today!) in many, many lines of tetrapods: cetaceans,
crocodilians, parasuchians, champsosaurs, spinosaurids, etc.).

So, if the true history of tooth loss in ornithomimosaurs went: typical
coelurosaur tooth count -> increase in number, decrease in size -> decrease
in number -> loss, then this is either a different process than avian tooth
loss or we are missing that second step in bird history.

Hoping this helps,

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661