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Re: How did dromaeosaurs use their arms?



Henri Rönkkö wrote:

<Like birds, dromaeosaurs had wrists that could bend
sideways to quite a great extent. In birds the
sideways movement is critical for gaining the thrust
that moves the animal forward in the surrounding air.
Dromaeosaurs, in turn, are believed to have utilized
the structure of their wrist when killing their food.>

  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. Such ways, the wrist was constantly
held in line with the proximal ends of the
ulnae/radii. Such a structure would allow the wrist to
fold easily against the forearm, pulling it out of the
way, and allowing sufficient leverage for a powerful
forward "swap" of the hand, to be simplistic.

<But how, actually? I myself find the movement pretty
hard to visualize. As the dromaeosaur would have swung
its hand forward in horizontal plane, the claws on the
ends of the fingers wouldn't have pointed towards the
pray animal, but towards the ground, right?>

  No. For one thing, the distal end of metacarpal one
is turned dorsomedially, and the first phalanx was
"twisted" slightly along the axis with the distal
articular turned medially, so that the digit would
have faced partially inward with the arm extended, or
down when flexed. The other two digits would have
faced down in the former, and inward inward in the
latter, though the third finger may have been much
more mobile. *Archaeopteryx* shows the ability to
rotate the third digit under the second, and
*Deinonychus* and *Velociraptor* have similar manal
structures. This would have offered some complexity in
the digital orientations capable; mark, not probable,
or true, since soft-tissue must be taken into account
as well when dealing with bony articulations (Stevens
and Parrish, 1998).

=====
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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