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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 traveling in.

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.



Cheers,

--Mike H.