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Re: Senter 2006, Confuciusornis, and humeral mobility



On Tue, Mar 22, 2011 at 1:52 PM, Jason Brougham <jaseb@amnh.org> wrote:

>>>> The paper makes two assumptions that I question:
>>>>
>>>> 1) the glenoid and scapula  of extinct taxa must have the same 
>>>> configuration as in volant neornithines in order for the humerus to be 
>>>> raised above the dorsum.
>>>>
>>>> 2) the humerus must be raised to 90 degrees above horizontal during the 
>>>> recovery stroke in order for flapping flight to be effective.
>>>>
>>>> I don't see why 1 must be true. There must have been transitional phases 
>>>> leading from the anteroventral glenoid of lower theropods to the lateral 
>>>> one of birds, and very subtle changes, maybe even  just longer ligaments, 
>>>> could effect greater mobility in the humerus.

The argument is not that the morphology has to be identical, it's that
it has to be possible to elevate the humerus without disarticulating
it from the glenoid.  Now if someone can establish that he's wrong
(i.e. the humerus does not disarticulate when raised above horizontal)
then you have a case.  Having looked at multiple specimens of the taxa
in question I'm inclined to agree with Senter.

>>>> But I'm especially skeptical of assumption 2, not qualitatively but 
>>>> quantitatively. >Consider the paper by Sokoloff et al. (The function of 
>>>> the supracoracoideus muscle >during takeoff in the European Starling 
>>>> (Sturnus vulgaris). New Perspectives on the >origin and early evolution of 
>>>> birds, Gauthier, 1999.)
>>>>
>>>>
>>>> In their experiments they surgically destroyed the supracoracoideus 
>>>> muscles of >test subject starlings, then filmed them taking off and 
>>>> flying. The starlings' normal >range of humeral motion during the recovery 
>>>> stroke was 90 degrees above horizontal. >After surgery, this range was 
>>>> reduced to just 50 degrees above dorsal. Yet the birds >could still take 
>>>> off perfectly well from the ground and fly just slightly less well than 
>>>> >normal.
>>>> Again, modern birds have superlative aerodynamic capabilities and highly 
>>>> >derived anatomies. They also have a wide safety margin built in. In many 
>>>> cases the >basic functions of powered flight may be possible with far less 
>>>> refined equipment.

The thing is that given the orientation of the glenoid and the
supracoracoideus it can't stabilize the arm in the socket of
Confuciousornis; modern birds (even with a cut supracoracoideus) still
have the modern glenoid orientation, which provides better bracing to
the humerus during flapping motion.  Your argument about modern birds
works against you here; it's the superlative flying ability of modern
birds (and their already existing morphological adaptations) that let
them squeak by and still fly more or less ok even when parts of the
system are disabled surgically.  As an imperfect analogy - take away
half of a 747s maximum thrust and you still have a capable flying
machine.  Take away half the maximum thrust from the Wright brother's
at Kitty Hawk and the Wright brothers don't get off the ground.

>>>> Any assumption that one feature or another is necessary for powered flight 
>>>> is a >hypothesis, and it should be tested if we are to say anything 
>>>> meaningful about it.

Absolutely, and I'm sure Phil would look forward to additional
research.  Certainly I know I do.  My objection is it sounds like you
are making flight your null hypothesis.  Each response feels "this
doesn't prove they couldn't fly".  There is no reason (outside of an
accident of history) to consider flight as the a priori condition in
these protobirds.  Indeed...for the umpteenth time I'll site this sort
of thing as a prime example of why the term "bird" should be
restricted to the crown group - it makes people want to extend
assumptions about true birds too far down the tree.

Certainly there are many hypotheses out there that are competing right
now - as most are aware Greg Paul has proposed that all of these guys
not only flew but that many of the flightless ones are secondarily
flightless (i.e. powered flight happened even further down the tree).
I on the other hand think powered flight happened quite a bit higher
in the tree (and what we'd think of as "modern" style bird
flight...the ability to take off and not have to worry about where you
are going to land, I think happened MUCH further up the tree).  Senter
is probably a bit closer to my side here, but probably more or less in
middle all things considered (you'd have to ask him though).

So yes, all hypotheses about what is needed for powered flight are
indeed just that, and should be further tested (although there's quite
a body of literature out there on it already).  But that's not the
same thing as assuming flight until proven incapable.  And frankly,
the way the published studies have been piling up I think "not powered
fliers" has been the clear leader the last decade, although not to the
point where anyone could claim a clear consensus.

Best,

-Scott




-- 
Scott Hartman
Scientific Advisor/Technical Illustrator
(307) 921-9750
www.skeletaldrawing.com