> 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.