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Re: Best _Guanlong wucaii_ headline



David Marjanovic (david.marjanovic@gmx.at) wrote:

<It might be the other way around. A semilunate carpal may be a prerequisite
for the evolution of wing feathers because a wing that could not be folded out
of harm's way would have little or no adaptive advantage (especially in sexual
selection). I have not done any experiments or math on this; I'm just
mentioning the existence of an alternative just-so story.>

  The semi-lunate structure of a single, or coalesced pair of distal carpals,
is known in several noncoelurosaurian taxa for which the arm appears
fundamentally incapable of having sufficient angulation to make use of the
arm-folding precipience of wing feathers. These animals include *Allosaurus*,
*Acrocanthosaurus* and the likely carcharodontosaur *Megaraptor*. This seems to
have assisted the arm in increasing movement in one plane while minimizing it
in another while at the same time also decreasing the pronatory qualities of
the forearm. This, as in birds, tends to be about increasing the resistance
with an arm motion in a single action, while retaining flexibility. It's not a
wonder why some would tend to put birds in the spotlight when looking at these
structures, but it evolved long before there were birds, and no animal evolves
(in the classic sense) to be "like birds", so any such argument would be to
imply a prescience, dare I say "designed", evolving system. That was, in fact,
the whole point of Lamarckism.

  However, the real brain behind the wing-folding for feathers mechanism seems
drawn from Hopp and Orsen, not Paul, who theorized that the elaborate folding
mechanism was coincident, rather than drawn from or precipient to, with long
feathers for the purpose of covering a nest or brood. The lateralization of the
shoulder, increased mono-planar flexibility of the wrist, and the decrease of
pronation with increase in wing-pully collapse of the arm, tends to work for
this but they may also be exapted, rather than pre-adapted with feathers. I
doubt even long armed, unfeathered animals would have been able to do this
arm-brooding quality, and indeed, this seems to be the case with
ornithomimosaurs, which reduced the arm-mechanism in favor, it appears, for
other purposes (both Osborn and Russell have written on the potential
function).

  Cheers,

Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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