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Re: On the Issue of Sprawling Dromaeosaurs


Ralph Miller wrote:

I don't know who to credit for the idea, but it has been pointed out that it
makes sense for a feathered gliding or flying animal to have asymmetrical
flight feathers, as this would prevent the feather vanes from twisting about
the rachides during forward motion, and thus this characteristic would serve
to preserve the integrity of the flight surface and maintain a smooth,
streamlined contour.

True. But the above criteria would also be true for a parachuting animal that is relying on aerodynamic drag to retard and guide its descent. Forelimb motions could be useful for changing body orientation in the air, as proposed previously (e.g., Caple et al., 1983; Garner et al., 1999) and serve as the forerunner to the flight stroke. Preserving the integrity of the feathers (including resistance to torsional forces) would be as beneficial to a parachuter as to a glider.

And this is one reason why I would expect the lengthy hindlimb wing feathers
of little basal dromaeosaurs to serve some function in aerial activity, even
if we don't understand precisely how the hindlimbs would come into play.

I agree. "Aerial activity" can include anything from leaping to parachuting to gliding to flapping flight. For little basal dromaeosaurs I think parachuting and gliding are the most likely behaviors. However, I will readily admit that I cannot think of a compelling reason for why _Microraptor_ and its ilk could not fly as well as (or as badly as) _Archaeopteryx_. Overall, the putative flight apparatus of these two taxa is really not that different.

IMHO the assertion that _Archaeopteryx_ was a powered flier but _Microraptor_ was a passive glider is due in a large part to analogy with extant forms: birds are two-winged fliers, and there are no four-winged vertebrate fliers (although there are two-winged flying vertebrates in which both pairs of limbs are incorporated into the wing). By contrast, there are many lineages of parachuters and gliders in which all four limbs are used to support the parachuting or gliding surface(s).

Are there any
osteological features which would falsify the hypothesis that these small,
early dromaeosaurs could have moved their arms during aerial locomotion?

I don't believe so. After all, the rationale behind the predatory-stroke-to-flight-stroke hypothesis is that a specialized prey-catching motion is biomechanically homologous to the flight stroke of birds, and very little refinement is required to convert one to the other. Many studies (including the two I mentioned above) have proposed that the aerial ancestors of birds moved their arms in the air prior to the development of an airfoil surface on the forelimbs. Thus the incipient flight stroke evolved in tandem with the incipient flight surface, and the latter did not evolve before the former.



Tim



Tim

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