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My opinion on dinosaur lips
Some points have not been addressed so far in this discussion:
The phylogenetic bracket is ambiguous: Today mammals and lepidosaurs have
lips, crocs don't -- and turtles and birds have _beaks_ which may or may not
have evolved on lipless ancestors. That crocs lost lips for whatever reason
is possible -- whales are lipless too (unlike seals and hippos).
Fig. 4-7 in PDW (p. 98) shows a cross-section through the snout of
*Dilophosaurus*; there the jaws fit quite closely and lips are shown which,
if a little longer, could have completely covered the teeth. Teeth that
extend beyond the dentary are not incompatible with lips as shown by
saber-toothed cats, elephants etc.. Keeping the teeth from desiccating is,
however, probably worth some effort: in the 19th century ivory traders
preferred forest elephants because their tusks don't have fissures.