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Re: Xenicibis the club - winged, flightless, ibis



On Thu, Jan 6, 2011 at 3:25 AM, Jason Brougham <jaseb@amnh.org> wrote:

> The bizarre wing of the Jamaican flightless ibis Xenicibis xympithecus: a 
> unique vertebrate adaptation
> Nicholas R. Longrich and Storrs L. Olson
> Proceedings of the Royal Society B. published online 5 January 2011 doi: 
> 10.1098/rspb.2010.2117
> http://rspb.royalsocietypublishing.org/content/early/2011/01/03/rspb.2010.2117.full.pdf


I'm sorry to arrive so late to the party, but I've been incommunicado
for most of 2011 (maritime voyage to Antarctica), and I'm still
catching up on old emails.

This is a nice paper, but I think it's worth pointing out the obvious
here: That the use of the avian forelimb as a weapon is a reversion to
the primitive/ancestral theropod function.  It's the specific adaptive
strategy of _Xenicibis_, with the forelimb turned into a jointed club
or flail, that is unique.  However, among theropods, I suspect that
the use of the forelimb as a purely defensive device (such as against
would-be predators) might also have arisen among non-avialan
theropods.  Although the forelimb of therizinosaurs was modified for a
herbivorous lifestyle (Zanno, 2006), the manual unguals of
_Therizinosaurus_ itself appear to be poorly adapted for bringing
branches to the mouth.  So it is possible that _Therizinosaurus_ used
its long forelimbs for purely defensive purposes, such as for lashing
out at approaching predators.  Something along these lines was
mentioned in Tom Holtz and Luis Rey's _Dinosaurs_, but I believe it
was also proposed in a recent scientific paper (which I can't recall
at the moment).  I suspect this behavior might have been prevalent
among non-predatory Mesozoic theropods, such as _Deinocheirus_ and
many oviraptorosaurs.  In this context, the following paragraph from
the _Xenicibis_ paper caught my attention:

               "Several morphological adaptations would have further
increased the wing’s effectiveness as a weapon. Reduction
                of the extensor process and elongation of the manus
would decrease the mechanical advantage of the wing
                extensors, producing a more rapid wing extension. The
retention of long wing bones allows the forelimb to be
                swung rapidly, while the ability to hyperextend at the
elbow and wrist increase the wing’s effective length, and
                therefore its angular velocity when swung."




Cheers

Tim