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Re: illustrations



Gavin Rymill (gavin@gavinrymill.com) wrote:

<In which dinosaurs is this crossover precluded? Is it limited
to therapods?>

  The realtionship of the ulna and radius has been examined in
sauropods (Bonnan, 1999, _JVP_ and on list), in theropods
(Sereno, 1998, _JVP_; also see Conrad's comments on list), and
ceratopsids (at least *Torosaurus,* Erikson et al., 1997), as
well as on the list, and there was no way the the radius crossed
the ulna, but that they laid side by side. Some dinosaurs have
locked positions for the ulna and radius (birds _sensu stricto_,
alvarezsaurids, ornithomimosaurs), others can rotate them only
slightly. The position in basal theropods may be more mobile
than in higher, shorter-forearmed forms, but this immobility
carries in the increased arms of maniraptoriforms.

<The limb proportions of Baryonxy very strongly suggest
quadrupedalism to me. I've even seen it reconstructed in recent
books on all fours (though not necessarily correctly). So either
it walked on the edges of its hands (unable to put its palms
flat on the ground) or it *could* cross its radius and ulna.>

  I don't see how *Baryonyx* could walk quadrupedally; relative
to the hindlimbs, the forelimbs are much shorter, and the
statement (Charig and Milner, 1986, 1990) was made regarding the
long and robust humerus. Spinosaurs most certainly do not
possess the ability to locomote using the forelimbs, find one
with arms long enough and the manus built for it (the hand was
not capable of extending the wrist, by the way) and this can be
discussed.

<What about Plateosauridae or Iguanadontidae? They have to have
their palms facing the ground to walk on all fours as is
commonly accepted. However, an inability to rotate their
forearms would preclude their use in pretty much anything else,
e.g. gripping braches of trees (which usually grow vertically).
Imagine trying to use your forearms for a task while keeping
them in a playing-the-piano position!!>

  Humans perform mechanics like this when lifting or climbing,
for the most part. However, ornithischians, unlike theropods
excluding Herrerasauridae and *Eoraptor,* but like
sauropodomorphs, are mesaxonal: the weight of the manus was
carried on the third digit (entaxonal is the theropod condition
where the digit is on the second, internal to the mesaxonic
condition (ent- "inter-")) and as such the manus' inclination
requires flexure of the elbow to bring direction of motion of
the manus under the shoulder and thus not a sprawl.


=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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