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RE: Making Lip of It
To reiterate my point about pterosaurs: there were insectivorous and
toothed inland species (some Istiodactylids) that probably weren't making a
regular habit of dunking their heads into water. Even the species that were
fishing would have been doing so in saltwater, something that tends to rapidly
desiccate tissues. Plus I doubt they spent their *entire* day soaking their
heads in sea water, as I'm sure they would have needed to roost at some point.
Kaprosuchus was most likely a terrestrial croc with what was apparently a
largely terrestrial ancestry, and it almost certainly didn't have lips. Then
there are the numerous mammalian examples with exposed teeth despite the fact
that they are not easily replaced. I really don't think tooth dessication was
that much of a problem at all for most animals with exposed teeth. Nor is a dry
mouth for that matter: parrots have very dry mouths as they spend most of the
time with their mouths partly open, and they are *dinosaurs*. In fact, if a
parrot has a wet tongue it is usually a sign of illness. I really see no
barrier to a dinosaur evolving a largely dry mouth, or exposed teeth, or both.
What can phylogenetic bracketing tell us? If we look at terrestrial
crocodylopmorphs such as Sebecus and Kaprosuchus, we see jaw morphology with
interlocking teeth that would not have left much room for lizard like lips.
Pterosaurs seem to have been a largely toothed but lipless group with some
non-fishing inland forms. modern birds have beaks as did many dinosaurs, many
of which seem to have evolved independently. Kaprosuchus seems to have had a
keratinous 'bumper' of some sort on the tip of the snout. Perhaps this is how
the typical archosaurian beak started out? I can see the advantage in having a
hard covering on the parts of the the mouth that were most prone to abrasion
and injury. All this suggests to me that 'liplessness' was the basal condition
for most of Archosauria. Perhaps a lack of fleshy lips allowed for the rather
large sharp teeth we see in so many predatory archosaurs. Lizard like lips may
very well have been more trouble than they were worth for a great many
archosaurs. This is something I have yet to see addressed. The assumption often
seems to be that lips would have been beneficial for predatory archosaurs, and
so no good reason can be found for why they would be lacking. If you look at
the open mouth of a lizard such as Komodo dragon, you will notice that their
teeth are not even visible despite their large size. If a dinosaur such as
Deinonychus had lips of this sort, it would have looked toothless when it
opened its mouth. The reality is that the sharp teeth of Varanids actually tend
to lacerate the tissue surrounding them. I suspect that this would have been a
problem for many theropods, not an advantage.
I suspect the typical pattern that independently lead to beaked jaws in so
many archosaur lineages may have been something like this:
1. Lizard lips with interlocking teeth > 2. Lizard lips covering jaws with an
overbite > 3. Reduced lips with an overbite > 4. No lips with exposed teeth
(most of Archosauria) > 5. Exposed teeth with a hard scaly covering on the
skull > 6. Teeth with a rostral 'proto-beak' > 7. A reduction in size and/or
number of teeth > 8. A toothless beak.
Simeon Koning