Re: Bird flight once more (was Re: What is a Dinosaur? and semilunate carpal)

["Jaime A. Headden" <qilongia@yahoo.com>]

I wrote:

<<[A]s well as the biplanar movement afforded by a lack of a large distal 
carpal block with a
trochleate surface>>

Chris Fisher (killerraptor20@hotmail.com) wrote:
 
<How does the large carpal block restrict biplanar movement. The semilunate 
carpal helps movement
in one plane (mediolateral) but does it necessarily restrict movement in the 
other
(dorsoventral)?>

  First, it would be good to clarify that dorsoventral movement of the wrist 
only occurs if the
arm is held outward, level to the horizon. Otherwise, "dorsoventral" in the arm 
held down is the
natural folding-wrist movement.

  Second, it is not the neccessarily the carpal block itself, but rather has 
more to do with the
trochleate surface (esentially, a mediolateral groove with larger dorsal ridge 
than palmar ridge)
that allows the proximal carpal bones to slide along it, rather than forming a 
long-axis hinge
(transverse hinge). Along with the large block, the transverse hinge is 
irregularly formed, with
an arc instead of a strait line between proximal and distal carpals. This 
prevents any transverse,
long-axis hinge, and thus the carpal block with a trochleate _proximal_ surface 
prevents biplanar
movement. It should be noted that the shallower the block and reduced the 
ridges, the more
possible the hinge. However, the twisting of the trochlea, depsite size of the 
block, will also
restrict the hinge.

I wrote:

<<The stiffening wrists of theropods occur after coelophysids and kin, not 
before. This appears to
correlate with larger hands and more robust claws and arms, indicating a 
greater predatory
capability in the arms.>>

Christ writes:

<I am not certain why the carpal block (semilunate carpal in Maniraptroriforms) 
makes the wrist
LESS flexible or mobile. Can you please explain.>

  See above.

<The correlation is certainly their, but how does stiff wrists help predation?>

  Not helping predation [generally], but as I wrote: "a greater predatory 
capability in the arms."
This is previously explained as the abscence of a "wobble" in the hands when 
the hand strikes the
prey target. If flexible along the transverse axis, this strike would cause the 
hand to rebound
from the strike, and reduce penetrating depth of the claws, thus reducing 
capability of affirming
a grip. In a pulling motion, this also prevents the hands from pulling outwards 
by keeping the
manus in line with the forearms.


=====
Jaime A. Headden

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

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