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Re: Bird lips and dinosaur jaws

Matti Aumala (mjaumala@nettilinja.fi) wrote:

<I know Jaime A. Headden already covered the subject of
oviraptorosaurian jaws quite thoroughly (though I have to admit
that as at best an amateur at dinosaur anatomy I'm not exactly
sure if I understood half of how it was connected to my question
:-) but I'm most interested about the changes that took place in
dromaeosaurs and truly avian dinosauria.>

  Okay, here's why I answered the way I did:

  In looking at the jaw of a bird (say, a crow, for which Bock
has done an excellent study of the jaw mechanics) one sees that
the coronoid region is strongly retracted in the jaws, and the
position of the origin of the pterygoideus in the upper jaw is
similarly posterior in the jaw, compared to lizards. This pulls
the flesh of the jaw back so that in birds, at least, it seems
to have no flesh to the jaw joint but a little "cheek" muscle.
Not so in lizards, which have this flesh up far forwards. This
is analogous to each other. The illustration you provide suggest
only to me the concept of bare skin versus extra-integumented
skin (fur, feathers, etc.) or the extent of a lateral flesh that
would have hidden the edge of the jaw, and to that I would have
to say no, because that is a supremely mammalian thing, formed
from the flesh that covers the circumoral musculature, and so is
not pertinent, at least as far as theropods go (except for
therizinosauroids, maybe). Birds and lizards have essentially
the same pattern of skin at the joint between upper and lower
edentulous portions of the jaw ... oviraptorosaurs would have
extended the region wrapped in skin farther forward than other
theropods, unique to Dinosauria (including birds), and even
perhaps the notosuch morphology (I haven't had opportunity to
look closely at a notosuch skull, sorry: it would be helpful if
someone could supply referrences of the two known croc-like
archosaur genera with edentulous jaws?).

Jaime A. Headden

  Where the Wind Comes Sweeping Down the Pampas!!!!

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