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Re: Some (basal) intramandibular joint questions



Heinz Peter Bredow (HPB1956@aol.com) wrote:

<Thomas R. Holtz mentioned earlier on this list that the geometry of this
joint is different in herrerasaurids and neotheropods. He cited Sereno
1993 (3). Is this report anywhere online available or can anybody please
explain?>

  The geometry is that the dentary has a long dorsal posterior process
that interdigitates the surangular rostrally, and that would prevent much
widening of the jaws, but would allow the dentary to hinge downward from
the postdentary. The shape of the dentary articulation indicates that the
jaw would have to hinge outward before it could swing upward, but that
that is undemonstrated and unlikely in any event as the lower jaw would
need to hinge out at nearly 45 degrees to do this.

<Contrary to earlier remarks on this list Sereno states in "The Evolution
of Dinosaurs" (4) that Eoraptor also has this joint. Is it like in 
herrerasaurids?>

  No, the dentary is rather flush without processes, but it also has not
been well-described, and Sereno is working on a monograph for the animal
so we will need to be patient. :)

<Up the (phylogenetic) tree. Gregory  Paul speculated in PDW (5) after a
look at the type of Ceratosaurus that it may have evolved it's own kind of
intramandibular joint. With all the new material of Ceratosuarus what's
the current view regarding this?>

  No one has done any sort of work on this except Bakker. In
neoceratosaurs, the lower jaw is very different in that the external
mandibular fenestra is very large and the contacts for the dentary to
postdentary are very slender and sit as small pegs in notches. Such
articulations permit a great deal of flexibility in different directions.

<Down the (phylogenetic) tree. Can we deduce anything regarding the kind
of intramandibular joint in (real) basal dinosaurs? If the first dinosaurs
were small bipedal carnivors then they already should have had a basal
kind of joint. If this is correct then it was lost first independently in
Ornithischia and later in Sauropodomorpha and some theropods. Seems to be
no problem to loose it due to changes in diet.>

  Not all carnivores have such a joint. Only neotheropods appear have
this, plus *Herrerasaurus* distinctly. Prosauropods and sauropods have
flush dentary/postdentary margins or a semi-fixed herrerasaur pattern, and
all ornithischians have reduced fenestra in the jaws that correlate with
peg-in-notch and scarf joints between the two mandibular halves. The most
dynamic predators today, such as falconiforms and cats, have very fixed,
immobile jaws. Innovations in the joint were to increase gape and volume
of the bite withing increasing skull size.

  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.

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