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In most birds the keratin contribution to the beak is pretty extensive and
doubtless Oviraptor was the same. In some, e.g. hornbills, bony
contribution to a horn structure (the casque) may be fairly minimal.
As to what it was doing- I don't think parrots or seedcracking
birds are probably the analogue. Believe it or not, I think that Cracraft
was right on the money when he compared the lower jaw of Chirostenotes
(Caenagnathus back then) to the dicynodonts. Big, anteriorly fused anterior
dentary beak with anteroposteriorly elongate jaw joint for a sliding motion
of the jaws.
Turns out the rest of the skull fits in pretty well with this. Many
dicynodonts also have the extremely short skulls seen in oviraptorids, and
also if I recall have fused premaxes. Particularly damning is the
*downwardly vaulted palate*. As I understand things, the dicynodonts are
believed to have a shearing motion of the jaws. So In this sheme,
Oviraptorids could have chopped through very tough vegetation like a pair
of those scissors you see on late-nite TV cuts through a penny.
The superficially most obvious comparison- parrots- (I like to call
them "parrot-heads") is actually very misleading, i've found. Parrots have
very sharp upper beaks, and extremely well-developed kinesis. Oviraptorids
have gone the opposite direction: the skull bones are all fused up in
adults (a synapomorphy linking Avimimus skull material to the
oviraptoridae, incidentally), and the pterygoids are closely appressed
(fused?) to the basipterygoid processes. I'd guess that few theropods had
worse kinesis (I recall one of the black hills guys arguing at Dinofest
that even Rex has pretty decent kinesis in a lot of joints).
anyways, my thoughts.