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Re: 11th specimen of Archaeopteryx



I think perhaps there is a misunderstanding here of the role of asymmetrical 
feather vanes in lift production.  The asymmetry has very little to do with the 
ability to generate large fluid forces.  A symmetrical feather will do this 
just fine.  In fact, an non-cambered flat plate will produce a decent about of 
fluid force, with a high L:D ratio, at the proper angle of attack.  The trick 
is that the center of lift sits closer to the leading edge of an airfoil than 
the trailing edge.  For an non-cambered flate plate, the center of lift sits at 
one quarter the chord length.  For a cambered foil (such as a feather), the 
center of lift moves as angle of attack changes, but it still sits near the one 
quarter chord location.

As a result of the above dynamics, having the central rachis near the quarter 
chord location of the feather is the most structurally efficient morphology, as 
the center of lift also defines the primary bending axis.  This produces an 
asymmetrical feather.  A symmetrical feather will also produce lift and drag at 
appropriate ratios, but it will need to be heavier to provide the same 
resistance in bending and torsion as the asymmetrical feather (note that the 
primary feathers of living birds are shaped such that they promote some 
torsion, but greatly resist bending - they have a high "bendiness to twistiness 
ratio", to quote Steve Vogel).

What this means is that the asymmetrical feather structure in Archaeopteryx 
suggests the production of large fluid forces, particularly lift, by the wing 
(either by Archie itself or an ancestor).  *However, it does not tell us 
anything about how the forces were directed or utilized*  While there are few 
papers investigating turning and other control functions, the fact is that any 
evidence for high lift production on the wing is potentially evidence for those 
sorts of models.  That same evidence could be used to validate an arboreal 
hypothesis, as well, of course, and so that is why we must look for other 
indications of arboreality.  Finding few to none, the terrestrial uses for lift 
generation would seem to be more reasonable.  In other words, high L:D wing + 
arboreal adaptations suggests arboreal living.  High L:D wing + terrestrial 
adaptations suggests terrestrial living.  The high L:D wing is not actually 
that informative, because it can apply to either scenario.

As a closing thought, I am extremely skeptical of WAIR ability in 
Archaeopteryx, for reasons I've posted previously.

Cheers,

--Mike


On Oct 24, 2011, at 2:31 PM, Matthew Martyniuk wrote:

> (meant to send this to the list...)
> 
> Sure, some things are more uncertain than others. But which of these
> hypotheses concerning the possible asymmetrical remige functionality
> is currently better supported by evidence?
> 
> 1) Lift generation for arboreal assistance (WAIR, or something similar)
> 2) Aiding in turning while running
> 
> How many studies have been done on the role of asymmetry in each function?
> 
> Scott Hartman wrote:
> "And finally, asymmetrical feathers are in no way directly linked to an
> arboreal lifestyle."
> 
> I never said they were. But the proponents of the arboreal lifestyle
> are the only ones with a wealth of studies and research on their side.
> There are plenty of papers discussing WAIR. I'd be curious to read
> those for turning, especially since in modern birds, even those that
> use their wings for turning, the feathers are not aerodynamic, and
> there was apparently no selective pressure to retain asymmetry for
> this purpose. If we're doing science, when there are two competing
> hypotheses, one backed up by current studies (WAIR) and one not
> (turning), the response should not be "well, we have no idea what they
> were for." The response should be "current research suggests WAIR is a
> likely use for asymmetrical feathers while alternate hypotheses
> require more investigation."
> 
> As Mike said above, there is no evidence to suggest that asymmetry was
> the ancestral condition, and in fact there's evidence against it
> unless you consider caudipterids and Sinornithosaurus to be
> secondarily flightless.
> 
> What gets me is that people keep citing "lack of arboreal features",
> while discounting feather asymmetry as one, despite the fact that no
> alternate interpretations of that character have been seriously
> investigated. If Archie had symmetrical feathers but a reversed
> hallux, it would be just as easy to say that the hallux is not an
> arboreal feature and that it would be possible to think up alternate
> uses for it, so therefore Archie has no arboreal features. "The
> asymmetry is not an arboreal feature because Archie has no other
> arboreal features" does not strike me as a very sound
> counter-argument.
> 
> Matt
> 
> On Mon, Oct 24, 2011 at 1:54 PM, Mike Keesey <keesey@gmail.com> wrote:
>> On Mon, Oct 24, 2011 at 10:51 AM, Matthew Martyniuk <martyniuk@gmail.com> 
>> wrote:
>>> 
>>> We need to learn a lesson from the BANDits; it's not enough to shoot
>>> down a hypothesis without proposing any alternative.
>> 
>> On the other hand, there are cases where we have to admit, "We don't
>> know [yet]."
>> 
>> --
>> T. Michael Keesey
>> http://tmkeesey.net/
>> 

Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
mhabib@chatham.edu
(443) 280-0181