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Re: Keratinous skulls



Ralph Miller (dinoguy@sbcglobal.net) wrote:

<Of course, hair, feathers, scales, and skin are also composed of keratin,
not just claw sheaths, horn sheaths, and beaks.  Alligator skulls are
heavily textured dorsally, though perhaps not in the same manner as
ceratopian skulls.>

  Cornifaction of the dermis and epidermis results in the keratinized
beaks of modern birds. But the keratin molecules in skin have a different
structure and are underlain by fatty tissue with dermal muscles and
innervating nervous fibers which leave different traces on the bone. In
birds today, the bony beak is heavy covered in pits and channels for blood
vessels and nerves, and these lack regularity; in mammals, the facial
region is limited to one or two foramina in each jaw which ennervates and
feeds this tissue entirely for lips; in reptiles, the foramina are
numerous, composed in regular rows, and result in innervating the thick
but relatively immobile extra-oral tissue of the mouth. One sees heavy
lips in snakes and lizards, but these are affected by opening and closing
of the jaw, or by movement of the mandibular halves, or by the rostrum in,
and have no independant movement _sensu_ mammals in the absence of a set
of distinct circumoral muscules, or oral elevators/depressions on the jaw
and cheek. Thus, they lack mammalian "cheeks." The presences of
regularization of blood vessels and nervous foramina in ceratopsids is, in
fact, a logically concluding description of the keratinous face in these
animals; note that these foramina on the lower jaw are limited to the
predentary, and the maxilla and dentary have large, regular foramina in
rows which innervate and feed the lateral oral muscules and skin;
depression of these bones inward would imply the prescence of a homologous
tissue arrangement as in mammals, but on a different order, obviously.

  Take it as you will; some dinosaurs have a pattern of facial foramina
that resemble many birds today, whereas most have one that resembles the
reptilian, lizard condition; and a few, such as *Diplodocus*, have a very
few, large foramina coupled with long fossae or channels that as in
Witmer, 2001, may pertain to extensive narial soft-tissue, but may also
pertain to lips of a similar form to mammals (excluding Bakker's trunk
theory, which lacks corroboration as elephants ennervate the truck from
within the narial opening).

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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