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Bird lips and dinosaur jaws



The corners of theropod mouths are usually restored with a stretch of
skin that covers the jaw muscles and folds between the jaws when they
are closed, basically like in most modern reptiles. Birds however seem
to have sort of lips and skin covering the jaw muscles that remains on
top of the jaws at all times, and is often covered with feathers. But
when and how did this change from reptilian mouths with fold-away skin
to avian mouths with lips happen?

Some dinosaurs, most notably oviraptorids, actually look very strange
with the classic reptilian mouths, but seem much more "ordinary" when
restored with avian mouths. I've seen some maniraptorans reconstructed
with avian-like mouths, as well as birds with reptilian mouths (most
notably G.S.Paul's Phorusrhacids in Predatory Dinosaurs of the Word).
Can you tell for sure what the corners of a bird's/theropod's mouth
was like from the skull, or is it mostly based on speculation?

(I made sketch to illustrate this problem, but unfortunately my
scanner broke down, so I hope you can make some sense out of my
explanation :)

--
Matti Aumala
mjaumala@nettilinja.fi