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Re: Making Lip of It



2011/9/23 Jaime Headden <qi_leong@hotmail.com>:
>
> Lip flexibility is not really relevant.

It seems to me flexibility of lips, at least at the base, has to be
relevant, for an inflexible lip would not be able to move, or to bend.
As far as I can imagine, unflexible lips would only make sense if they
form a cutting edge or if they direct in such a sense they permit the
tooth to contact the food without problem. If sections of the bird
ramphotheca are related to scales, that would raise the possibility
that dinosaur oral scales were attached to the sides of the snout,
making them less flexible than those of lizards.

> It should behave much like in lizards _at least_, where the band of tissues 
> along the lip's margin pull at the entire margin from any one point,

I do not know too much about lizard lip function, but if the band
along the lip margin pulls in the sense of their lenght, I would
rather see the margin shortening (as much as possible) instead of
raising. This would make lip borders to get closer to the teeth, at
least near the tips of the jaws. Shortening the lip margin would raise
the lips if the lip margin contact teeth which direct laterally on the
sides (so that the circunference of the mouth is lesser near to the
base of the teeth), but would lower lips if teeth direct medially, or
if front teeth direct caudally instead of rostrally.

> much as an elastic band does on one's waist or sock.

If it functioned as the elastic bands of socks, you would have lips
more closely apressed to the teeth, which does not help, and seems to
oppose, to move upwards relative to the teeth. It seems to me the
reason of the elastic bands in socks is to get a firmer grip, not to
facilitate sliding downwards. Unless you have the lips raised above
horizontal, but to reach this you need a flexible lip in first
instance.

> In snakes, flexibility of the lip tissues is so great that the presence of 
> scales could only matter during _compression_ of the lip tissues together,

Snakes have more dorsoventrally compressed skulls than most dinosaurs,
which may make the position of their scales probably less vertical.
So, a bite woud tend to compress more a lip scale along its plane in
the case of animals with laterally compressed snouts than in the case
of less laterally compressed squamate snouts, assuming the
integumentary surface more or less agrees with the subjacent bony
surface (which to a greater degree is the case in extant archosaurs).
In any case, lip flexibility (and head integument flexibility in
general) in snakes and other lizards may relate to their greater skull
kineticism, which is much lesser in non-avian dinosaurs.

> while the same should generally be true in lizards where pushing _up_ on a 
> lip can cause you to push _out_ on the lip to involve the scales on the face 
> and jaw, moving much like a chain-link fence does.

It is easier for a scale partially directed laterally than for one
more vertical to rotate outwards instead of being compressed.

The shorter the tooth supposed to penetrate food, the lesser the
deformation to which the lip scale is subjected to. So, inflexible lip
scales would be less stressed in the case of squamates with
dorsoventrally short teethed than in the case of animals with longer
teeth. However, I agree this latter argument would not mean much if
lips did not entirely cover the teeth in dinosaurs, as you said.

Finally, Sim's argument of interlocking teeth in certain basal
crocodyliforms and pterosaurs looks good. As far as I can remember,
fishes which have a similar teeth interlocking are also lipless, as
Hydrocynus.