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RE: The Eternal Question: Lips or Skin?



-----Original Message-----
From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of Michael Hanson
Sent: Thursday, August 16, 2001 6:39 PM
To: dinosaur@usc.edu
Subject: Re: The Eternal Question: Lips or Skin?


Adam Britton wrote:


>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 (e.g.. with tape, when sedated), although I seriously doubt anyone has
>measured it objectively.


I think that this is very important for all crocodilians. (Theropod) Dinosaurs did not have this feature, instead, they had spaces between their maxillae (which were "V" shaped) and mandibles (which were "U" shaped). Theropods would have a large gap between the lower jaw and the upper. Therefore, lips would be required in dinosaurs to prevent desiccation.<<


What? Please explain the “V”shape of the maxillae?


The maxillae don’t touch each other, do you mean the premaxilae? No, not all theropods had the “U” shaped dentaries (or in other words, where the dentaries join each other). Tyrannosaurus has a “U” shaped prermaxillae and a “V” Shaped dentary. There IS a HUGE gap between the lower and upper jaws when the mouth was closed. When you add the width of the upper teeth (about 5 cm) to the width of the gap, you’d have to have a gap between the outside of the dentary to the inside of the upper lip of about 10 cm!!! How would they be able to sustain that gap? In order to have lips,  you need both upper and lower lips, which from, guess what, my research, yep I did research on it, theropods didn’t have lips. There is no reason why theropods couldn’t have been able to keep a ‘seal’ like a crocodile with the mouth closed. The ‘lipless’ croc’s can and do, seal off their mouths. When they are lying out in the sun they can and do (at times) keep their mouths closed and keep the moisture in the mouth.


Tracy L. Ford

P. O. Box 1171

Poway Ca  92074