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Re: Archaeopteryx feathers and origin of flight based on 11th specimen



On Thu, July 3, 2014 1:23 pm, don ohmes wrote:
> 
> On Thu, Jul 3, 2014 10:41 AM EDT Mike Habib wrote:
> 
>> However, I would argue that too much is being made of weather asymmetry.
>> There is nothing magical that happens when a feather is slightly
>> asymmetric. In fact, flightless birds often have asymmetric primaries,
>> they just are as asymmetric as in flying >species (on average).


As a quick error fix: I meant to say they “just are less asymmetric than in 
flying species (on average)”. I apologize for the typographical error.


> 1) asymmetric aerodynamic surfaces (more rigid/heavy on the leading half,
> lighter and more flexible on the trailing half) have evolved many times --
> eg, insects, airplanes, bats and birds. AFAIK, cross sectional symmetry in
> nature is limited to insects -- if it exists there.

Yes, the structural spar tends to sit closer to the leading edge than the 
trailing edge of the wing. The mechanical importance of this for bending and 
torsional resistance is well understood, and it does not apply if the leading 
edge is only slightly shorter than the trailing surface. The importance of 
aeroelastic stability is less well studied, but also recognized (and again 
applies to feathers with more extreme asymmetry in most cases - there are 
exceptions for curved feathers and other morphologies). As Tom noted, the 
difference here is between functional asymmetry and biomechanical asymmetry. 

[Side note: Insects have a more symmetrical camber because the way in which 
their wing twists is different from vertebrate wings and because the virtual 
air mass is relatively large, meaning that the effective wing shape is 
different from the anatomical wing shape]


> 2) if there is no advantage to a symmetrical primary (in extant flightless
> birds), why would an asymmetrical primary not be retained? The implied
> default drift-back-to-the-center tendency is likely non-existent, in the
> absence of selection moving the center of variance.

Most flightless birds do have asymmetrical primaries - typically the vane ratio 
sits around 1.2:1 to 2:1. A drifting mechanism doesn’t require selection, 
that’s why it’s drift. Incidentally, when tarsal feathers are turned on in 
chickens (easy to do; simply mutation) some of those tarsal feathers are often 
slightly asymmetric.


> 3) all the extant animals known to have asymmetric primaries are either
> volant, or have volant ancestors, again AFAIK…

Naturally - all living birds are either volant or have volant ancestors. That’s 
part of why this is difficult to sort out.


> Discounting asymmetric feathers as a trait indicating at least the
> presence of volant ancestors seems overly something -- perhaps overly
> skeptical?

Since low levels of asymmetry are not indicative of flight in living taxa, I 
don’t see this as being too skeptical. However, to your point, I suggested at 
SVP last year that Archaeopteryx might have been (with its slightly 
asymmetrical vanes) secondarily flightless.

Cheers,

—Mike