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Re: Cheekless and lipless dinosaurs...

-- Ralph Miller III  wrote:   >...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. <
Why developing muscular mammalian cheeks? Skin (smaller mouth opening) would do the job equally efficiently -preventing the food to fall out while chewing.

>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. <
Monitor lizards, oras in particular have very long teeth, upper ones reaching the lower line of  jaws, yet, their teeth are completely covered with lips. Tyrannosaurids had lips ( no reason at all to develop lipless feature like in some fish-eating animals). Observing the skull of say, Nanotyrannus would reveal there was no space for upper teeth to go over the lower teeth, gums and thick scaly lower lip. Their teeth were probably completely covered with both lips when mouth closed.
Another case are some specialized theropods, like spinosaurs - in which tips of some of the longer upper teeth ( or all )  "would protrude past the lips".
Unfortunately, only  better fossil specimens could prove who is right on the issue

Berislav Krzic
Beri's Dinosaur World
Dinosaur Books