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Re: flight stroke



On Sun, May 05, 2002 at 05:29:29PM -0700, Waylon Rowley sent:
> Ok, this raises several interesting questions. Is it
> easier for a beginning glider to evolve enlarged
> retrices, or flapping behavior? How suitable is the
> tail for producing lift? And are the arm feathers
> providing more lift to the center of the body than are
> distal tail feathers? Yes, I think they are. So, you
> have 2 options. Bring the cg posterior so that the
> tail can help support it better, or bring the tail
> closer to the cg. In the meantime, though, flapping
> can help with the problem.  

You're assuming that gliding was the initial phase.

This isn't parsimonious in a couple of ways.

One is that in living birds, the good gliders are all highly derived
forms; the basic bird wing, as per the bracket between tiamous and
chickens, is a high output system for energetic flight.  This is not
something one would expect a glider to evolve into.

Another is that _every other_ known arboreal vertebrate glider use a
sprawling posture, even the parachuting snakes.  The lack of any
possibility of this posture with the theropod body plan ought to call
into question the proposed viability of the tree-leaping hypotheses.

Finally, unless one wants to suppose initially scansorial modes of life
('strong BCF'), the forelimbs were initially specialized for something;
it's simpler if the flight stoke comes from this type of use, rather
than from tree-snagging, itself a fairly extensive specialization.

-- 
graydon@dsl.ca   |  Hige sceal þe heardra, heorte þe cenre,
                 |  mod sceal þe mare þe ure maegen lytlað.
                 |   -- Beorhtwold, "The Battle of Maldon"