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Re:



comments inserted.
JimC

----- Original Message ----- From: "Sim Koning" <simkoning@msn.com>
To: <dinosaur@usc.edu>
Sent: Wednesday, September 24, 2008 11:54 PM



These are the problems I'm having with the ground up hypothesis; assuming that protobirds already had elongated feathers on their arms, what advantage would this give a ground running predator?

Increased mobility.

If they leapt into the air, these "wings" would actually slow them down due to drag,

It is just as likely that the 'wings' would not slow them -- due to reduced form drag resulting from feathers 'reshaping' the area behind the arm (thereby allowing the arm to be moved away from the body during running without substantial penalty and with some advantages).


making them more vulnerable to predators.

Or perhaps -- making them less vulnerable to predators.

I think a more streamlined animal with powerful legs and no "wings" at all would make more sense for a specialized "jumper" (kangaroo).

Why presume that the early wings make them less streamlined when jumping?

Also, why would they be leaping up into the air in the first place? To catch insects? No animal that does this exists.

Cats do it.

If they evolved simple wings to provide lift while running,

Why would they do that? More likely that the early wings would act to increase mobility rather than lift. The increasing ability to produce lift would be initially secondary.


wouldn't this actually reduce traction

Yes, which is possibly why the wings could be oriented to modulate or increase traction rather than reduce it.


and in turn slow them down and make them less stable?

Wouldn't necessarily slow them down, and the ability to selectively modulate stability is an advantage rather than a disadvantage.


To me, an at least somewhat arboreal animal that used it's elongated feathers or even protofeathers to parachute from an elevated position makes more sense;

Since many improvements in parachuting and/or gliding ability don't lead toward the direction of powered flight, doesn't that create problems for your scenario?


Primitive bats had greatly elongated fingers with claws on all five digits which they used for climbing as well.

What is the evidence for this presumption?

Fossil bats make a good case for at least one group of flying animals evolving from arboreal gliders.

In what way do they make that case?

There is a strong selective advantage for an arboreal animal to evolve some sort of parachute or gliding system to prevent injury from falling or to conserve energy by leaping from tree to tree instead climbing all the way down and back up a tree trunk, or cliff face: this is made obvious by the large number of arboreal creatures that have evolved in this way.

This is quite true. How many of them seem to be developing the ability for powered flight?


On the other hand, you do not see any primarily ground running, leaping animals evolving some sort of simple gliding or flight system; the only animals that do anything like this are birds,

And flying fish, and hatchetfish (which also flap while aloft).

yet they already have a fully developed and highly sophisticated flight mechanism. There are plenty of jumping, kangaroo like animals, yet none of them have evolved wing membranes or any organ other than stronger legs (and smaller arms) to increase airtime.

Which demonstrates that there is more than one way to skin a cat. A good place to mention that the ground up vs. trees down scenario is a perfect example of a potentially false dichotomy.
JimC