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Re: Making Lip of It

 I think they were referring to a greater degree of overbiting, where
 the upper tooth spans most of the height of the lower jaw (as in the
 canines of cats or the teeth of Dilophosaurus).

Even so, the canines of cats are fully covered by lips.

> This leads me to my next point: I suspect that most, if not all
> ornithodiran archosaurs were 'lipless' and this may have been why
> so many lineages tended to evolve a beak of one form or another.
> Maybe a lipless mouth is a necessary precursor to a beak. I suspect
> that a beak may have often started out has a hard covering on the
> snout that gradually worked its way back along the jaws, replacing
> the teeth in the process. But as you point out, a beak of this
> nature may not have been able to coexist with lizard like lips.
> This seems to present an argument against dinosaurs having lips
> since we know that many archosaur lineages evolved beaks
> independently.

 Nice argument; it may also explain the condition in turtles if they
 turn out to be archosauromorphs as a number of molecular
 phylogenetic analyses suggested (and if non-archosaurian
 archosauromorphs also lacked lips). If the lip is an impedement to
 beaks, or at least to the beaks expanded along the outer surface of
 the mouth present in birds and turtles, the loss of lips would
 certainly permit future development of this type of beaks. I would
 not very much say that any kind of beak cannot exist if lips are
 present, because some similarly functioning horny surfaces exist in
 lipped animals as ruminants.

 Lönnberg (1904) compared separate parts of some bird beaks with
 different scales in squamates, proposing they were homologous.

I have yet to see evidence that animals such as *Sinornis* or *Yanornis* or *Eoalulavis* had either a beak or scales lining their mouths. In the published specimen of the latter, feathers reach almost all the way to the tip of the snout, and the tip itself is prepared out of the rock, so that any feathers or scales have been destroyed.

Embryologically, upper beaks are an outgrowth of the caruncle (the false egg tooth). This is the best explanation yet for why there are no beaked therians.

I wonder if beaks include keratinized lips -- are the edges of beaks homologous to lips or gums? Is it even possible to decide?