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Re: Senter 2006, Confuciusornis, and humeral mobility



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