RE: beaks and teeth

["Witmer, Larry" <Witmer@exchange.oucom.ohiou.edu>]

I'd better chime in here as the purveyor of the "beaks or cheeks"
project coming out of Ohio University.  Tom Holtz correctly pointed out
that when people talk about "beaks," they often mean "rhamphotheca": a
modification of the epidermal stratum corneum in which there are not
only multiple layers of keratin but also variable degrees of
mineralization (yes, actual hydroxyapatite crystals between the keratin
proteins).

Although extant taxa with rhamphothecae (birds and turtles) lack teeth,
there is nothing in either the ontogeny or histological structure of
rhamphotheca that denies the co-occurrence of rhamphotheca and teeth.
And of course, there are indeed extinct taxa that possessed rhamphotheca
and teeth in the same jaws.  These are the ornithischian dinosaurs, the
very group that is the focus of the beaks/cheeks project.  This is not
controversial.  Virtually all workers have accepted that the premaxillae
(and rostral bone of ceratopsians) and predentary of ornithischians were
covered with a "horny bill"---there even are a few hadrosaurs that
preserve the premaxillary rhamphotheca.  So, our research is suggesting
only that ornithischians may have had a more extensive rhamphotheca,
with a less mineralized portion sheathing the buccal areas of the
maxillae and dentaries.  This is the same pattern we see in many modern
birds: a heavily mineralized portion up front and along the edentulous
portion of the tomial (triturating) surfaces and a less mineralized
portion supporting and anchoring the adjacent more mineralized portion.
In birds and turtles, the entire triturating surface is edentulous, so
the more mineralized portion is relatively more extensive, but in
ornithischians the caudal part of the triturating surface is formed by
the teeth and hence the adjacent rhamphotheca would be less mineralized.

Patrick Norton is correct that both my talk and Mike Papp's talk at SVP
focused more on examining the hypothesis of mammalian-style muscular
cheeks in ornithischians.  Testing ideas on fleshy cheeks in
ornithischians has been our major initial effort.  The notion of "beaks"
(i.e., rhamphotheca) is being offered as an alternative hypothesis to
explain the novel buccal anatomy of ornithischians.  We're in the throes
of exploring this idea: e.g., how does rhamphotheca differ from other
specializations of the stratum corneum? what are the osteological
correlates? etc. etc.  It's very time consuming to do it right.  But
stay tuned...   

As cherished a notion as it has been, the inference of fleshy cheeks in
ornithischians may be very difficult to sustain---no matter how much we
want we them to have cheeks. We set out expecting to bolster not reject
the hypothesis. It's fair to say that the ultimate fate of the
rhamphothecal hypothesis is uncertain.  Right now we're pretty high on
it, and it explains a variety of findings that are inconsistent with the
fleshy cheeks idea.  Nevertheless, we have more tests to do, and are
doing are best to falsify it.

--Larry Witmer
~~~~~~~~~~~~~~~~
Lawrence M. Witmer, PhD, Assistant Professor of Anatomy
Department of Biomedical Sciences, Grosvenor 114
Ohio University College of Osteopathic Medicine
Athens, Ohio  45701  USA
Phone: 740 593 9489; Fax: 740 593 1730
email: witmerl@ohiou.edu  OR  witmer@exchange.oucom.ohiou.edu


>----------
>From:  Norton, Patrick[SMTP:Patrick.Norton@state.me.us]
>Reply To:      Patrick.Norton@state.me.us
>Sent:  Wednesday, February 03, 1999 10:46 AM
>To:    dinosaur list (comment line)
>Subject:       beaks and teeth
>
>A few questions about beaks and teeth:
>
>1) Except for dinosaurs, including some Tertiary birds, I am not aware of   
>any animal that had (or has) both a beak and teeth. Are there other   
>examples of this?
>
>2) Is the presence of beaks on some dinosaurs inferred or confirmed by   
>fossil evidence? I think I recall reading somewhere of an Oviraptor skull   
>that was found with bits of a fossilized beak, but I may be wrong.
>
>3) Beaks are found among several vertebrate and invertebrate taxa,   
>including dinosaurs, birds, turtles, fish and mollusks. Are the beaks of   
>these animals considered examples of convergence, homologous structures,   
>or what?
>
>4) Among the vertebrates with beaks, are there any osteological features   
>on the  bones of the mouth region that indicate the presence of a beak? I   
>recall a talk at the last SVP meeting speculating on the presence of a   
>keratinous sheath covering the buccal region in some dinosaurs instead of   
>"cheeks", but the evidence presented seemed (to me) more to refute the   
>existence of muscular cheeks than confirm the presence of a sheath.
>
>Patrick Norton
>Patrick.Norton@state.me.us
>
>