Re: vaulting pterosaur launch, questions

[Michael Habib <mhabib5@jhmi.edu>]

> 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