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Re: Making Lip of It



2011/9/22 David Marjanovic <david.marjanovic@gmx.at>:
>
> Even so, the canines of cats are fully covered by lips.
>
True, I was thinking about them not being covered by the lower lips,
in the way shown by some of Jaime's drawings ("Paulian 'raptor")


> Embryologically, upper beaks are an outgrowth of the caruncle (the false egg
> tooth).

Would that preclude these keratin hard patches being homologous to scales?

> This is the best explanation yet for why there are no beaked
> therians.

Can we assume horny caruncles are the only way to produce beaks? May
not all that facial nerve musculature, associated with lips or not,
also oppose to the spread of bills on the outer surface of the face?
This possibility, would agree with Sim's view on liplessness paving
the way to beak appearance.

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

Interesting...  I also think on whether we can see a beak as something
more special than simple toothless snout with greater keratinization.
Later, we can simply compare morphology without thinking in functional
theorethical baggage about beaks. Lip definition is to me a problem,
and it can be seen as defined not as a yes or not thing. For example,
although I do not see scales near crocodile alveolar borders, if they
are present, as in the drawings by Jaime, they may indicate a very
small, close to the alveolar border, lip. The same would be the case
in birds out of similarity, we may not need to give some yes or no
answer (although lack of distinction better fits with "no").

Finally, I do not find much convincing the evidence that lips are in
some way necessary in dinosaurs to avoid drying of the teeth. First,
do we have evidence of dinosaur teeth drying or not drying? Second,
even if they did not dry, they have shorter life than those of
mammals, because of being replaceable, so their surface may have much
less time to dry up than in an elephant. The more tachymetabolic
dinosaurs are inferred, the more they ate and likely, the faster their
teeth worn out and they had to be replaced. And Gregory Paul (2002)
made a great work in convincing that they were tachymetabolic.