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Re: How did dromaeosaurs use their arms?



I wrote:

<<Ostrom (1976) showed the arm supination/pronation
was limited, due largely to the wrist, whereas in less
derived theropods, such as allosaurs, it was more
capable of s/p.>>

Henri Rönkkö wrote:

<This (dromaeosaurs having less s/p ability in
ulna/radius than allosaurs) doesn't really suggest
that the origin of dromaeosaurs was arboreal, does it?
I find s/p of ulna/radius highly useful in climbing,
far more useful than a sideways flexing wrist would
be.>

  How does this work?

  Movement of force along an arm when being swung in
an arc will result in the force attempting to move the
arm in any direction it can; the more flexible the
joints, the less likely it is the arm was useful for
"snapping" or "swapping" at something and actually be
able to grab it. The loss of a great deal of
supination/pronation in the maniraptoran arm results
in an increase of force transmitted towards the tip of
the arm (or lever, since we're talking force here) and
an increase in the resistance at the joints (elbow,
wrist, axial [md1, major] digit) that would force the
arm to behave in particular ways when force acts along
it. Thus, the arm, when pulled into forward extension,
would still have nearly all the force transmitted into
the hand, rather than acting on the wrist or elbow;
human arms act differently, due to our relative
increase in flexibility owing to primate (arboreal)
ancestry and the brachiating mode of our ancestors.

  If the maniraptoran arm lost flexibility [presumably
for manipulation] to increase the snatch-n-grab
capability that is expressed in flying theropods as
the flight stroke, then nearly all osteological
features of the maniraptoran arm can be explained as
dietary acquisitive tools, rather than pre-flight or
post-arboreal conditions. Not to say these are not
possible scenarios. Lack of flexibility in the arm is
advantageous for a "swapping" function, and can be
interpreted as a climbing adaptation.

=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

Qilong---is temporarily out of service.
Check back soon.

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