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Re: Archaeopteryx not the first bird, is the earliest known (powered) flying dinosaur


----- Original Message ----- From: "Tim Williams" <twilliams_alpha@hotmail.com>
To: <dinosaur@usc.edu>
Sent: Tuesday, December 06, 2005 4:31 PM
Subject: Re: Archaeopteryx not the first bird, is the earliest known (powered) flying dinosaur


Why? I see the terrestrial requirements as easier, at least if you are headed toward flapping flight.

In this case, I meant 'easier' from a biomechanical standpoint. As you say, even a flat plank can glide to some degree; but it takes a lot more for an object (either animate or inanimate) to propel itself off the ground.

See below.

One of the things I object to (and I think you'd probably agree) is the idea that flight *must* have evolved in the trees because it is 'easier', given that gliders can use gravity to their advantage every step of the way.

I do agree on that, with the proviso that, as the animal develops the ability to more easily change elevation, the difference between terrestrial and arboreal will blur rapidly.

To me, this seems irrelevent: the animal does not make a conscious decision about 'easy' and 'hard', it can only work with what it's got. 

Yes, and what its got changes rapidly.

Nevertheless, I think an animal that is fighting against the force of gravity

Here's an area where we differ. I think the force of gravity is a major asset that allows the animal to play off the exchange between kinetic and potential energy.


would require more anatomical refinements (especially in the forelimb and pectoral girdle) than an animal that habitually glided with the assistance of gravity. In this sense, the flapper would accrue more pre-adaptations for powered flight than the passive glider.

I agree with this. And this may be a good place to mention that if you distinguish between soaring and gliding, evolution toward gliding leads toward an albatross-like planform. Though both vultures and albatrosses are good soarers, they do not use the same gameplan. Relatively speaking, vultures are not good gliders. Albatrosses focus on reducing induced drag and thereby flattening the glideslope while increasing the sinkrate and airspeed due to high wingloading.. Consequently dropped from a given height in no-lift conditions, they can cover more range in less time. This can be driven directly by gliding evolution (it wasn't in birds), or later, by a response to a high energy interaction between the atmosphere/surface effects. Vultures focus on reducing wingloading at the expense of induced drag, so that they reduce their minimum sink rate and dropped from a given height in no-lift conditions, they can remain aloft longer, but cover less territory in the process. This can be a response to attempting to stay aloft in minimal lift, in a less energetic atmosphere with less surface interaction. The albatross planform can be reached directly by an evolving glider. The vulture planform can more easily be reached from a flapping origin. Note that I am talking about planform origin here, not the direction that birds actually took. Since I lean toward flapping origins for bird flight, that implies that I see early birds as relatively low aspect ratio, with high aspect ratio forms coming much, much later. And this might also be a good place to mention that I see pterosaurs as evolving gliders, developing high aspect ratio quickly, with effective flapping coming later, reaching its pinnacle in the heavily loaded azhdarchidae. This doesn't have to begin arboreally either, It can start with flatland leapers (a possibility for Sharovipteryx as well, though I don't know what sort of terrain Sh inhabited). I've just been rattling along in free association in the paragraph above, so I hope it makes some sense (at 4 in the morning, I ain't gonna go back and rewrite it :-)

This is the strength of the WAIR model: characters and behaviors that assist in incline-running can be exapted toward powered flight, and even the incipient stages serve to benefit the animal.

Agreed.

As an aside, I'm neither a trees-down or ground-up guy. I think that is a false dichotomy.

Me too. Padian always hones in the fact that the most important development in the evolution of powered flight is the evolution of the flight stroke. In my experience, some gliding-to-flight models gloss over this detail.

Yeah. It's always worthwhile listening to Kevin. He oftentimes makes a lot of sense.

That implies that good gliders don't evolve toward better gliders. If they followed the scenario you describe, then we would expect the first flapping flyers to have high aspect ratios. Does the fossil record support that? 

Don Ohmes answered Jim's question by saying...

I don't think the record does support early high aspect fliers (quite the opposite, IIRC), a strong
piece of evidence against "trees down" for flappers, to go w/ the theoretical objections Jim mentions.

I actually see trees down and ground up happening simultaneously as the animals develop the ability to exchange kinetic and potential energy. But I see the utmost origin as being terrestrial.

.... but I don't think we have the evidence yet to back this up. We would need theropods that exemplify the pre-_Archaeopteryx_ stage.

Agreed. Just as there are currently no proven incipient pterosaurs. I expect these intermediate stages to turn up over time.

The microraptorans/sinornithosaurs may approximate this pre-flight stage, but this is a leap of faith at the moment. Microraptorans/sinornithosaurs may actually represent a dead-end experiment in aerial locomotion, totally separate to birds.

Not my field of expertise. I'll just sit quietly and listen.

I personally favor a flapping phase as a prelude to bird flight, but that may be just my intuition at work. To me, nothing about bird flight implies gliding as a beginning. Gliding isn't the easy way to start.

Gliding *might* (and I stress *might*) be a good place to start if the animal is already spending its time in the trees and wants to get down to the ground fast, or to the next tree.

But that's contrary to the planform developed by the early birds, unless they were flapping from the start (which I think they were).

Jim