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I took yet another look at the AMNH T. rex, noting yet again
those large blood vessel openings on the upper and lower jaws. I
suggested a while back that maybe birds didn't evolve beaks
themselves, but took them from their ancestors, and it seems to me
that this might explain a few things. The large-lipped mammals don't
show anything similar, and I can't see anything very similar along the
cheeks of any of the ornithischians. I do see something somewhat
similar along the jaws of a pelican skull I have, where it supported
the horny beak.
So I'm wondering, could Deinonychus (which also shows this pretty
clearly), T. rex and kin have had beaks along with their teeth. This
would solve one problem- figuring out why beaks popped up so often- in
ornithomimes, segnosaurs, oviraptorosaurs, Confuciusornis, and
ornithurines. Assuming our present cladograms are correct, the other
option is postulating at least four separate beak evolutions (if you say
that segnosaur and oviraptors take their beaks from a common ancestor) in
coelurosauria alone. So to evolve a toothless beak, you don't have to
develop a beak and lose your teeth, you just lose your teeth. If you
wanted to take it back far enough, you could just postulate a horny lip
for the entire dinosauria which could explain why prosauropods and
ornithischians have it too. Beaked sauropods might be a little much, tho.
Sauropods seem to have pretty well vascularized snouts as far as I can
tell, whether this is a convincing argument against, I leave it to the
reader to decide. This kind of fits in with the "dinos aren't mammals"
theme (i.e. maybe they didn't have mammal-like lips), too.