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Re: CNN: how first birds flew
At 06:40 AM 5/7/99 -0400, Larry Febo wrote:
>Some of Archie`s relatives..." seem to have been able to have moved their
>forelimbs in a flapping fashion. "
>I was under the impression that Maniraptoriformes (at least) were restricted
>in the motion of their forearms to this "flapping type" of motion in a
>single plane. Is this not true? What about theropods in general, wasn`t the
>arm motions of all similarly restricted? How was it in T_Rex?
The restriction to one plane is at the wrist: add in the joints at the elbow
and shoulder, and you get additional degrees of freedom. Motion seems to be
greatest in maniraptoriforms, and more restricted outside that clade.
The observation that the exact same motions (although not to the same
degrees) used in flapping are also useful in a predatory grab goes back to
Gauthier & Padian (1984), at least.
>Theropods may have been fast enough for takeoff flight from the ground, but
>there are other factors involved. Flight feathers (for one), which to me are
>extremely specialized structures, and flapping motion, which if restricted
>to such would seem to be an unlikely "pre-adaptation" to have developed for
>the future use in flying. These two items alone seem better explained as
>holdovers from a previously specialized function of actual flight rather
>than being of significant use to a non-flight -capable organism in it`s
>forays on the ground.
Unless you are suggesting that remiges ("flight feathers" - note it is
better to use the morphological name rather than a usage name in this
context!) and flapping motion had some previous other use (gliding,
parachuting, etc.), you are playing in the game of magic. That is: this
novel structure can only be used for this purpose, therefore it has always
been used for this purpose. However, barring magic or genetic tampering,
novel structures had some morphological precursor, which may indeed have had
a different use.
The scenario Burgers & Chiappe suggest is that elongation and elaboration of
the arm feathers and the flapping motion original began to add thrust, and
was later exapted for use in true powered flight. This is clearly not the
only scenario out there, but these workers present data that
*biomechanically* this is feasible. Claims that it is unlikely, ungainly,
or that one just plain doesn't like it does not invalidate their
conclusions. Reanalysis of their model, however, could concievably show
that the numbers do not add up, but that hasn't happened yet.
So, your last sentence should say that "These two items alone seem better
explained **TO ME** as...". Opinion of a model and fitness of a model are
two separate issues, and subject to different types of validation.
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:email@example.com
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661