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Re: Cheekless and lipless dinosaurs...



Dinogeorge Dinogeorge@aol.com wrote:
> The tooth rows of all ornithopods, and indeed,
> most ornithischians, not to mention segnosaurs and a few prosauropods,
are
> inset from the outer edges of the jaws, and that space must have been
taken up
> by something.

Aye.  And let's not forget that the ornithopods and ceratopids, with their
formidable batteries of teeth, were perfectly adapted to process their food
orally.  (How I loathe the characterization of herbivores as having "flat
teeth that can only chew soft plants")!  With such systems in place, it
would be very wasteful to NOT wrap cheeks around those inset teeth, thereby
facilitating the material being chewed to continuously drop out of the
mouth.  Sauropods are a different matter entirely, using their teeth for
cropping off or raking in the plant material.  Their teeth are not akin to
the ceratopid "food processors" or the ornithopod "graters" and the teeth
are not inset; hence, no cheeks.

The one ornithischian exception I have seen proposed recently was Stephen
Czerkas' cheekless _Stegosaurus_ hypothesis, presented at last year's
S.V.P., which was based largely on similarities between the mandibles of
_Stegosaurus_ and extant turtles, and the fact that the lower teeth of
_Stegosaurus_ seemed to reside within and below the sharp margin of the
mandible.  Such a jaw could conceivably function as a "gutter" to prevent
food from falling out of the mouth.  (Not to be referred to as "the
gutter-mouth hypothesis"; "gutter" was my word for it).  Thomas R. Holtz,
Jr. felt that the analysis was flawed because the _Stegosaurus_ jaw
cross-sections were based on a highly restored fossil jaw.

Regarding the lips of theropod dinosaurs, the advocacy for lips seems to be
based on the foramina which parallel the teeth, just as such small openings
do on the skulls of extant lizards.  These openings provide a path through
the skull for blood vessels and nerves.  Representing the opposing view,
Tracy Ford thinks that tyrannosaurs had no lips at all (just as we see in
crocs today).  His point was that if tyrannosaurids had a lizard-style set
of lips, the upper set of teeth would inevitably bite into the lower lips. 
(If my characterization of Tracy Ford's hypothesis here is incorrect, I
trust that I will be corrected and admonished in due time).  Gregory S.
Paul's _PDW_ states that the theropods would have had the lips, but the
tips of some of the longer upper teeth would protrude past the lips, as in
some housecats.  One problem with the lipless model is that the
tyrannosaurid would lack the means to seal out bacteria, and disease and
infection could find easy entry into the animal.  On the other hand,
monitor lizards make good use of oral bacteria when they bite into their
prey, infecting the bitten animal with bacteria which will kill the prey if
the wound itself does not.  So a tyrannosaur might have developed similar
immunities (to the monitor's).  Could we at least be permitted to restore
tyrannosaurids as having upper lips, facilitating the all-important snarl?

-- Ralph Miller III     gbabcock@best.com