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