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Re: vaulting pterosaur launch, questions



That's interesting, Mike. You could probably count me among that number, because I thought a running take-off was considered almost canonical for early birds (such as _Archaeopteryx_, for example). This was an issue addressed (and redressed?) by Burgers and > Chiappe....

True; it has been assumed as the "basal" launch style by many workers. I'm not sure why that is, other than perhaps tradition built on some misunderstandings.


Burgers, P. and Chiappe, L.M. (1999). The wing of _Archaeopteryx_ as a primary thrust generator. Nature 399: 60-62.

Yes, a good example of authors assuming running launch for basal avians, and providing little reason for this assumption. The topic was indeed revisited in a proceedings volume, as well, if I recall correctly. There are a few problems with the model, sadly, both in terms of the use of the wings for extra running speed (especially the amount of extra speed they proposed) and in the way that the authors expect ground effect to work. And, of course, I would suggest that forcing a running launch is not a good assumption from the start.


Sorry to switch focus from pterosaurs to birds, but how do you picture _Archaeopteryx_ getting itself airborne from terra firma? By leaping from a standstill off the ground?

No worries, birds are always a great topic as well. Yes, I would expect a leaping launch. Archaeopteryx is a long-legged form, and would tend to achieve better instantaneous speed by leaping than by running. It does not match the morphotype expected for a running launch. Running launchers usually have to take off from compliant surfaces (water), and they usually have high wing loadings, short hindlimbs, and long wing spans. All of these characteristics occur regularly in aquatic birds, of course, and are synergistic.


Nice point. Also, birds have shifted stride generation from the hip to the femur, as part of the forward migration of the center of mass (a flight adaptation). As such, the more-or-less horizontally oriented femur is exposed to a lot more stresses than a femur that is oriented vertically (as in non-avian theropods). Hence, femur of birds is short and stout, to resist these forces.

Indeed. In particular, actually, it the type of stress that changes: the horizontal femora are loaded largely in bending and torsion. Bones fail much more rapidly under bending and torsion than under axial compression (it is compression that dominates in a more vertical limb bone). Thus, to keep safety factors within limits, bird femora tend to be short and stout, just like you described. Birds also don't receive much limb excursion from the femur any longer, so shortness carries limited cost (stride length is largely unaffected).


Cheers,

--Mike


Michael Habib, M.S. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280 0181 habib@jhmi.edu