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Re: Cheekless and lipless dinosaurs...



Christopher Srnka wrote:

<According to the article, Witmer claims carnivorous dinos didn't have
lips, but rather scales covering the teeth, as in modern lizards.>

  Lips lizards and crocodiles are lacking due to several reasons, one
being that they lack musculature about the jaws. In crocodiles, there
isn't even loose skin or scales to cover the teeth as in lizards and
snakes. However, they differ from theropods in the arrangement and
number of the mental foramina that run the gamut of the premaxillae,
maxillae, and dentaries (also called "lip-holes"). Theirs are
relatively similar in size, and along a line. They accomodate few
nerves and blood vessels.

  Dinosaurs, (I'll use hadrosaurs, ceratopsians, coelurosaurus, and
sauropods for examples) possessed farm more foramina than those
lizards and crocs did, similar to the mammalian condition near the
bony cheek (you can check this out in various biology and encyclopedic
books, like _Encyclopedia of Mammals_ [McDonald (ed.), 1984]) while
the anterior of the dentary was foraminated to a very extensive
degree, retained by us in the very large mental foramina on either
side of our chin. Dinosaurs therefore has more vervous- and
blood-vessel--fed "lips" than lizards and snakes, and especially more
than crocs, who retain but a thin band of ligamentary tissue about the
jaws (Bakker, in his well-published _Heresies_).

  It is a reasonable conclusion that dino lips were more mobile and
muscular than that of lizards, snakes, and their closer relatives,
crocs.

  Greg Paul, in _Dinosaurs Past and Present_ discusses this to some
degree, though I know he has somewhere else discussed to a greater
degree; somewhere on this list, perhaps. Anyway, in Paul, 1989, as I
believe elsewhere (other authors, sorry) it was shown that no, dinos
did not have muscular cheeks like mammals, formed by the _m.
buccinatoris_ muscle, but in fact retained an _m. pinna_ muscle (named
because it resembles a feather, with a large band of ligament and
muscular fibers radiation forward and out, like the vanes of a feather).

  The function of this muscle, as differs from the _m. buccinatoris_,
would have been to merely hold food in, while the mammalian cheek
actually pulls the jaw side to side.

  I may be a bit off on some of the points, and I appologize for not
offering more authors and better research, but I'm not at the library
right now, and cannot remember the details on the jaw musculature,
though it was discussed to some extent on the list. When the Archives
are back up, I suggest we all peruse it for the data.

<He also says "We have a very mammalized view of the world. We're
mammals, and we have cheeks, and cows and sheep and horses have
cheeks. And so we see dinosaurs as being very similar to that.>

  One cannot cross mammals and dinosaurs or reptiles. I see no reason
why he would make this statement (not a flaming, Larry, if you're out
there). There are, in fact, similar (read: homologous) structures in
the jaws of mammals, ornithschian and sauropod dinosaurs, and even
theropodan dinosaurs, dealing with the foramina, the muscles (though
the structure and fuction differs, as does, I think, the origin), and
the construction of the skull bones that are related to the processes
of feeding (personal observation).

<But today we have herbivorous reptiles and birds, and none of these
guys have cheeks." The article goes on to say that Witmer feels that
herbivorous dinos had extended beaks, like eagles or turtles, and
there is an accompanying illustration of a Leptoceratops depicting a
very odd-looking beak on the dino. Because of the lack of "cheeks" in
the illustration, the dino appears to be grinning, and the beak covers
the whole of the lower jaw and portions of the upper jaw.>

  I do not see how the beak would encompass the entire lower jaw. The
lateral (outer) surface of the mandible possesses many scars and
processes and features relating to the attachment of tendons and
ligaments. A beak of that extent would serious hamper jaw movement.
Hmm. I need this ref. I hope that I'm not blowing off this conclusion
based on a misunderstanding [on my part].

<Wouldn't the lack of cheeks present some problems in drinking water,
among other things?>

  Not unless the animals in question sucked their water up. Birds use
air pressure by moving the tongue in an undulating pattern (this is
also how the phalarope bird feeds); cats "lap" their liquids up, and
dogs are very sloppy about it. Cheeks certainly help, but they sort of
limit your choice in drinking style.

  Lizards drink by sticking their entire snout into the water and suck
up that way. Crocodiles---I don't know. Drinking mechanisms in
sauropods were also discussed just recently.

> Paul, G.S. 1989. The science and art of restoring the life
appearances of dinosaurs and their relatives: a rigorous how-to guide.
p. 5-49. (in Czerkas and Olsen, eds., _Dinosaurs Past and Present_
vol. 2 [Natural History Museum of Los Angeles County and University of
Washinton Press (Seattle and London)]).

> McDonald, D.W. (ed.) 1984. _The Encyclopedia of Mammals_. [Equinox
(Oxford) Ltd.] pp. 895.

> Bakker, R.T. 1986. _The Dinosaur Heresies: New Theories Unlocking
the Mystery of the Dinosaurs and their Extinction_. [William Morrow &
Company, Inc., New York] pp. 481.

==
Jaime A. Headden

Qilong, the website, at:
http://members.tripod.com/~Qilong/qilong.html
---
All comments and criticisms are welcome!

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