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Re: 11th specimen of Archaeopteryx



Thomas R. Holtz, Jr. <tholtz@umd.edu> wrote:

> I wonder to what degree "controlled flapping descent" may have a role here?


As far as I can ascertain, CFD (as expressed by the modern chukar
partridge) use a complete wingstroke, as in WAIR.  If the first birds
and their ancestors were incapable of executing a full wingstoke in
the style of modern birds - on account of the inability to raise the
humerus above the dorsum -  then it would tend to rule out CFD and
WAIR (at least as practiced by modern birds).  Both these behaviors
(CFD and WAIR) can occur with "half a wing" and a full wingstroke.
Contrast that with basal birds, which had a full wing, but only half a
wingstroke (as inferred by biomechanical studies).


Then again, CFD has dual purposes in chukars during the descent:
orientational control (by using the wings to control pitch, and
position the feet for landing), and slowing down the descent (so the
little chick doesn't go ker-splat).  So it's all about achieving a
safe landing.  I wonder if limited flapping motions by the wings in
the ancestors of birds were adequate to guide and slow descents?
(This was the essence of the "Pouncing Proavis" scenario of Garner et
al., 1999).


Note also that modern gliding mammals extend the glide in order to
facilitate a safer landing: there is a critical length of the glide
path up to which landing forces drop significantly, then level off
(Byrnes et al., 2008).  So gliding is thought to be not so much about
maximizing the distance travelled, but remaining airborne long enough
to better position the body and shed velocity prior to landing.  This
is the advantage of gliding over pure ballistic leaping.






Cheers

Tim