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Re: The Eternal Question: Lips or Skin?



The idea that crocs don't have lips because they are constantly by water
doesn't hold water. Crocs gape frequently out of water, deliberately
dessicating their tongue. More terrestrial species of crocs, like
Paleosuchus, do not have lips and do not have problems with dessication
despite spending many daylight hours out of water. Crocs in burrows spend
many weeks out of water. Lack of lips does not tie them to water. An
important factor being overlooked here is the palatal valve at the front of
the throat (an extension of the tongue) creates a watertight seal between
the mouth and the throat and respiratory canal, limiting water loss through
the mouth.

Although I don't believe it's particularly important for crocs, it should be
pointed out that a crocodile with its mouth closed actually has a pretty
good (if not watertight) seal between the maxilla and the mandible. The
interior of the mouth remains moist for some time if the mouth is kept
closed (eg. with tape, when sedated), although I seriously doubt anyone has
measured it objectively.

Adam

----- Original Message -----
From: "Michael Hanson" <mhanson54@home.com>
To: <dinosaur@usc.edu>
Sent: Thursday, August 09, 2001 2:55 AM
Subject: Re: The Eternal Question: Lips or Skin?


Andy Farke wrote:

>Quoting Mark Hallett:
>
>"Crocs (excluding
>extinct terrestrial forms) don't have to deal with the potential
dessication
>of lip and mouth mucosa that other tetrapods do, since they're either in
the
>water or close to it. A "baretoothed" croc-like mouth in dinosaurs would be
>subject to this, as well as a having a continuing problem in keeping flies
>and other flying or crawling ectoparisitic forms from reaching these soft
>tissues."
>
>So what about the extinct terrestrial crocodylomorphs? How do their mouths
>compare to those of early dinosaurs? How about other big terrestrial
>archosaurs, such as Postosuchus and their kin? Would this imply that
>"lip-like coverings" had to evolve early in the history of archosaurs and
>were then secondarily lost in crocodiles? I'm just throwing a lot of
>questions (not necessarily well-grounded) out here. . .
>
>Andy

Lip-like coverings were very likely to have been lost in aquatic crocodiles
yet present in early terrestrial forms. Other terrestrial archosaur forms
(other than pterosaurs which had beaks) also, were very likely to have them.