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Re: Dinosaurs and birds




On Wednesday, April 4, 2007, at 05:50 PM, dinoboygraphics@aol.com wrote:


Now imagine that wing assisted incline running (WAIR) opened up arborreal niches (because all of the biomechanical arguements against dinosaurs in trees go out the window if you can walk up a trunk) for the first time, leading to an explosion in arborreal but largely non-volant winged theropods. What would that have looked like? I'd imagine that you would have winged birdy critters that had well developed deltoid muscles, and little need for large pectoral muscles on the sternum, nor for a stabilizing role of the AHL on the downstroke (which would be much weaker), nor even a need for the glenoid to face as upward as it does in volant birds. Sound familiar?

It is a cool scenario, and I've had similar thoughts myself. Up until quite recently, I had favored essentially the same idea. There is a catch, however. WAIR actually requires rather significant power output, and a very powerful upstroke with significant amplitude. That means that WAIR actually has pretty steep prerequisites. It requires a supracoracoideus pulley, or some other character to generate a rapid upstroke. It requires strong elevation of the wing relative to the body axis, and it probably does require the AHL because both the downstroke and upstroke actually have to be rather rapid. WAIR tends to emphasize the power on the upstroke, and upstroke-enhancing adaptations are actually *more* derived in avians than downstroke enhancing characters.


To put it another way, WAIR tends to require ring vortex gait related characteristics. Ring vortex gait usage is probably fairly derived, however. If basal avians and near-avians were able to get into the air at all, it would probably be via a continuous vortex gait (which is "easier" in many ways). Archaeopteryx, for example, might have been able to manage a continuous vortex gait, but has few characteristics to indicate usage of a ring vortex gait (see Rayner, 1991, 2001)

The one thing that WAIR doesn't require is a fully formed airfoil. That is, it can be used by birds with wing loadings so high that they cannot launch. Thus, juvenile birds without the ability to launch or sustain flight can still use WAIR, if they have a relatively well-formed flight apparatus otherwise. As you have probably already noted, the birds in which WAIR is recorded are generally precocial species in which the flight apparatus is well developed early in ontogeny, excepting airfoil size. Altricial species can use WAIR as well, but only later in ontogeny.

It is also notable that species like Microraptor have high aspect ratio wings, and probably lack slotting, which is not expected in an animal adapted to WAIR behavior (at least not as an adult; perhaps juvenile Microraptor individuals had more appropriate planforms). The planform in Archaeopteryx is closer to expectations (though its wing loading seems a bit low for what we might expect in a WAIR-adapted animal).

All that said, I do think that the general idea of derived maniraptorans moving between the ground and arboreal habitats has a lot of merit, and I think it nicely connects the previous dichotomies of arboreal and cursorial launching systems. I also think that it fits well with the growing picture of early flight evolution in birds. However, actual WAIR mechanics are unlikely to be the critical link in question, unless basal birds actually had more flight ability than we give them credit for, rather than less.

Did Sapeornis and Jeholornis fly? Quite possibly, but shouldn't we perhaps test between the above hypothesis and the prevaling assumption of volancy? As other non-volant, winged maniraptorans show, there are a wide range of things you can do with wings that don't require actual flight, and we need to be critically examining them rather that grandfathering in Archaeopteyrx, and therefore all animals closer to birds than Archaeopteryx, on little more than 150 years of tradition.

This I definitely agree with. I'm working on a different angle for testing that long-running assumption for Archaeopteryx as well, and that may also make an appearance at SVP. I'm happy that I'm not the only one!


Cheers,

--Mike H.