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Re: Fw: Dinosaurs and birds
On Wednesday, April 4, 2007, at 10:08 PM, don ohmes wrote:
Well, I disagree.
An animal that increases it's maximum speed by generating thrust w/
it's forelimbs as it runs gains an advantage. If it is predatory, it
gains advantage in both resource acquisition and escaping predation.
That is a highly intuitive counter-argument, and well stated. The
problem is that the model you describe is actually largely unfeasible.
It has been proposed in multiple forms. The original model was the
simple "adding thrust with the wings" model, and was proposed by
Burgers and Chiappe in 1999. Another model followed that attempted to
explain the advantage in terms of ground effect. Both have significant
problems, as shown by Jeremy Rayner, Ulla Norberg, and others.
This model also suffers from the same problems I mentioned for WAIR, to
an extent. Even if the velocity gain via aerodynamic thrust model was
workable, it would be far more helpful if the animal could use a ring
vortex gait. A continuous vortex gait actually retards the animal
slightly on the upstroke. That's not a big deal at high speeds. It is
a problem in the speed regime that a running Archaeopteryx would be
An animal that is adapted to assist running w/ forelimb-generated
aerodynamic thrust, receives an advantage should it produce lift, or
even alter the direction of thrust w/ respect to the ground
(advantageous exploits would include perching in trees, 'leaping' over
obstacles, catching/escaping similarly talented animals, etc).
Again, adding aerodynamic thrust to gain running speed is not
particularly effective. Note that running birds do not deploy their
wings while sprinting, unless it is during launch. Wings *can* be used
for certain agility actions on the ground, such as clearing obstacles
or certain turning maneuvers. However, those are a separate
consideration from what was being discussed before. In a WAIR type
model, the incline is required. That does not mean that an incline is
required for the evolution of a thrust-producing flight stroke.
The logical end point of your argument is the conclusion that a
ground-up evolution of flight could not evolve on a smooth planet...
no doubt inclines could be helpful, just that they are not essential
to a selective scenario that produces flapping flight.
They might not be essential, but I didn't actually say they were. I
only stated that inclines were essentially to a WAIR type model. I
don't know if a cursorial flight origin would work on a "smooth" planet
or not. There isn't really any way to tell. We could certainly
imagine scenarios that would work; that does not indicate whether those
scenarios would be likely.