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Re: Senter 2006, Confuciusornis, and humeral mobility

Thanks Jason for a very comprehensive and thought-provoking response.

If I'm understanding you correctly, you're saying that the short
sternum, puny keel, prominent deltopectoral crest, and minimal dorsal
excursion by the humerus were all part of a unique kind of flight
stroke?  The kinematics of this flight stroke were found in basal
birds like _Confuciusornis_, which (according to your interpretation)
were limited to low-amplitude flapping.



On Mon, Mar 28, 2011 at 12:53 PM, Jason Brougham <jaseb@amnh.org> wrote:
> Hi Tim,
> The short answer is that it is an alternate way to get dorsal elevation of
> the humerus, and it is possibly a way that is used by extant albatrosses
> and petrels.
> In the figures of fossils that I've seen Dr. Senter's observation - that
> the scapulae slightly overhang the glenoids of Confuciusornis - is true.
> But in none of these specimens is the thorax preserved in three
> dimensions. Senter went to some length to distinguish the life
> orientations, but I don't think the life orientations can be determined
> decisively with currently published specimens. Moreover, some of the other
> metrics, like furcula angle, that he uses to determine whether the
> scapulae lay close to the dorsal vertebral column, might be better
> measured with absolute distance between furcular rami, relative to width
> of dorsal vertebrae, than by angle. Thus it is hard to say with certainty
> that dorsal excursion was limited in Confuciusornis.
> If the scapula overhung the glenoid then, perhaps, the humerus would
> strike the lateral edge of the scapula when it was elevated, and this
> would restrict the dorsal excursion. But I realized there was one other
> obvious way to get the elbows above the dorsum.
> Looking at the shape of Confuciusornis' humerus one is struck that the
> breadth of the bone from the humeral head to the lateral tip of the
> deltopectoral crest is nearly equal to the length of the shaft of the
> humerus, and that the two chords are set at a 45 degree angle to one
> another. It occurred to me that if we rotate the humeral head in the
> glenoid by 45 degrees, we would elevate the humeral trochlea (elbow) above
> the dorsum, without bringing the humeri any closer together.
> Then I learned that an extant bird, the Black Kite, with the same wingspan
> as a large Confuciusornis specimen measured by Dr. Habib, elevates its
> humeri only 25 0r 30 degrees during flight. This suggests that large,
> dorsal, humeral excursion may not be crucial for powered flight. And the
> Kite actually ways 4 times more that the estimated mass of Confuciusornis!
> So this should give the latter taxon an even larger safety margin in low -
> amplitude flapping.
> In the flight stroke of extant birds the humerus remains swept backward,
> and the elbows remain posterior to the glenoids when viewed from above.
> Moreover, the humeri do rotate or torque in their sockets during the
> normal flight stroke.
> In addition, Pennycuick (1982, the Flight of Petrels and Albatrosses, page
> 82) documents an unusual, low - amplitude, high - frequency, rotation
> flapping used by petrels and albatrosses.
> These points, in my mind, undermine the argument that restricted humeral
> elevation would make Confuciusornis more likely to be a glider than a
> powered flier.
> Jason Brougham
> Senior Principal Preparator
> Department of Exhibition
> American Museum of Natural History
> 81st Street at Central Park West
> 212 496 3544
> jaseb@amnh.org