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CHEEKS AGAIN



WHY ORNITHISCHIANS HAD CHEEKS
Jaime is correct in pointing out that what ornithischians (and cerapodans
specifically) did was not exactly chewing
( http://www.cmnh.org/fun/dinosaur-archive/2001Feb/msg00346.html ), but it
was not your basic herbivorous reptile "bite off what I want to eat" action
either.

Basally in the Ornithischia, the teeth occluded in a normal herbivorous
reptile manner (Lesothosaurus etc.). In the two main branches of the
Cerapoda however, Ornithopoda and Marginocephalia, the plain ol' leaf like
cheek teeth evolved into closely packed batteries of many teeth with an
ever-increasing occlusal surface.

In marginocephalians the occlusion seemed to have a more vertical
orientation. In pachycephalosaurids however, this seems to have been lost in
favor of the more basal tooth pattern of simple slicing.

In ornithopods occlusal battery this is taken a step further. The occlusal
surface is not just enlarged, but also set at an angle, where the maxillary
teeth's occlusal surfaces face inward and downward, and the dentary teeth's
occlusal surfaces face outward and upward.

Additionally, Norman has concluded that iguanodontian ornithopods seemed to
have a unique kind of cranial kenesis where the maxillae and jugals bulge
outward when the mouth is being closed, causing even more contact between the
dentary and maxillary teeth.

All of these factors together seem to indicate that ornithischians were doing
more with their food than simply slicing it up. Additionally, it seems to
indicate that the processed food invariably ended up OUTSIDE the oral cavity,
and would have fallen on the ground if it weren't for some sort of barrier,
like cheeks

WHICH ONES HAD CHEEKS?
As I have argued above, cheeks (or some other analogous barrier) seem to be
indicated by the manner in which cerapodan ornithischians processed their
food while it was still in the mouth, as well as the fact that it needed to
stay in the mouth.  So at the very least, cerapodans probably had some sort
of cheek-analogs.

At SVP in Salt Lake, presentations were given concerning dental occlusion in
ankylosaurs, specifically Euoplocephalus tutus, indicating that ankylosaur
occlussion may have been more complicated than previously thought.  So
perhaps cheeks were in thyreohyrans as well as cerapodans.

Did Lesothosaurus and other basal ornithischians have cheeks? Maybe, but
there's nothing for sure indicative of them being there, save for the fact
they look a million times cooler with them.

BUT I THOUGHT CHEEKS WERE BIG AND PUFFY
No, in fact, the cheek-analogs in ornithischians were probably not very big,
and definitely not puffy.  Additionally the cheek-analogs probably lacked
much in the way of muscle, as reptiles generally lack massively developed
facial muscles

In some basal ornithopods, as well as basal marginocephalians, there's a
moderate degree of dentary and maxillary heterodonty. The first few teeth in
such forms as Hysilophodon and (so I've heard) the new Thescelosaurus are
lonely, almost conical projections that did not participate in the chewing
going on in the back of the mouth. These teeth may have been useful for some
sort of specialized grasping or some other task, and may have been free of
the cheeks entirely.

In heterodontosaurids, basal ceratopians and pachycephalosaurs, the cheeks
probably originated caudal to the wide variety of fangs present in these
dinosaurs.

In advanced iguanodontians like _Iguanodon_ and _Lambeosaurus_ the pre-dental
beak enlarged so much that the dental battery appears much reduced, and the
cheeks probably would not have extended in front of the teeth. The
pre-dental beak (that's the whole front of the face) becomes very narrow left
to right in these animals and cheeks rostral to the teeth would have
restricted the amount of food taken into the mouth. This is taken to the
extreme by such lovely hadrosaurs as Edmontosaurus copei whoâs pre-dental
beak is about two to three times as long as its dental battery.

So.... in conclusion:

1: Most ornithischians chewed strangely compared to other reptiles, and would
have required a barrier lateral to the teeth to keep processed food in their
mouth and off the ground

2: Cheeks were extensive throughout the Ornithischia and were possibly absent
in only the most basal forms such as Lesothosaurus and Pisanosaurus.

3: Cheeks were not as extensive on the face as commonly illustrated, nor were
they puffy, nor were they very muscular.

and finally:
4: Ornithopoda means bird foot, NOT boring :-)

Peter Buchholz
Tetanurae@aol.com