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