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Re: Pterosaur wing membranes (a couple of short questions)

Chris Bennett, et al, wrote:

> > Do you have any idea how flexible the outer wing bones are, is it possible
> they could be bent or moved independently?
> It is generally accepted that almost all pterosaurs had lost the capability
> of voluntary motion of the interphalangeal joints.......The wingfinger thus 
> would be somewhat flexible as
> a result of the design of the interphalangeal joints and minor flexion of
> the phalanges themselves, but could only be flexed and extended voluntarily
> at the metacarpophalangeal joint.  There was paper that looked at wing
> flexibility some years ago (Sneyd, A. D., M. S. Bundock, and D. Reid. 1982.
> Possible effects of wing flexibility on the aerodynamics of Pteranodon.  Am.
> Nat. 120 p. 455-477), but it has so much math in it that my eyes just glaze
> over and my mind wanders.

While it is indeed generally accepted that 'almost all pterosaurs had
lost the capability of voluntary motion of the interphalangeal joints',
there is a distinct need for active, powered ventral bending at the
joints between phIV-1&2, and phIV-2&3, and in Quetz, those joints are
spoon-shaped and allow exactly that motion.  It appears to have been
critical to roll control, and to gust load alleviation. As I'm sure you
all know, passive upward bending of the phalanges in response to gust
loading can lead to increased tensile stress in the membrane sufficient
to collapse the outer phalanges in compression.  There are several
mechanisms that can inhibit the effect, and active ventral bending is
one of the most effective.  Perhaps other extant pterosaur phalanges
should be reexamined with that thought in mind.  That said, in Quetz
joint phIV-3&4 is indeed fixed and is very robust both vertically and
horizontally, possibly to resist loading from both water slaps and the
generally increased tensile stress near the wingtip.  Alfred (A.D.
Sneyd) has given me some of his unpublished work on pterosaur wings and
it is well worth reading too. I wish he would actively participate in
this field again. I think math is mandatory in the analysis and
understanding of pterosaur wings, if one keeps in mind that the math
must either be compatible with the preserved physical evidence or
alternatively, be used to help predict potentially valuable avenues for
further research.
> watch out, there
> is so much dull anatomy in there that there is a good chance that your eyes
> will glaze over and your mind will wander!

Chris, nothing you do re pterosaurs is boring!!
> There is evidence of muscle attachments to move most everything out to and
> including the metacarpophalangeal joint.  There is no evidence of muscles to
> move interphalangeal joitns of the wing............Sordes pilosus and the
> nature of the pterosaur flight apparatus.  Nature 371:62-64) described small
> fibers in the wing of Sordes that in my opinion are not actinofibrils and
> might be muscle fibers, though they also might be elastic or collagen
> fibres.

I think that the distal end of phIV-1 in Qsp shows some potential
evidence of muscle attachment markings appropriate to modulate the
cupping of the outboard phalanges, but it is not distinct.  Chris,
sometime when you have the opportunity, I wish you'd look at that, as it
is more down your line than mine. I think it deserves some additional
attention as it is one of the things that potentially converts the
pterosaur wing from crappy to viable, even extraordinary.

> > It seems clear there is a possibility of wing fibres providing enough
> strength to allow the pterosaur to form a somewhat independent shape to
> parts of the wing and also allowing different areas to have different levels
> of strength. Is this correct?

Sort of.  One should keep in mind that the membrane fibres must act
in-plane parallel to the surface of the membrane.  Therefore, they must
work in conjunction with the pressure jump in order to affect the shape
of the wing perpendicular to the chord (which they do seem capable of

> > Also could it possible for such structures to be used to produce tension
> only in some directions, possibly allowing the wing to change shape slightly
> if muscles put pressure in at one angle as opposed to another?
> See my actinofibril paper cited above.

They appear to me to allow the membrane to operate in differential
shear.  Chris seems to be of the impression that the actinofibrils
operate in compression (Chris, if I'm putting words in your mouth please
set me straight, as that's not my intent). I'm of the impression that
they operate mostly in differential tension, with only very limited,
localized compressive capability in order to locally manipulate the
aeroelastic number and inhibit the initiation of flutter.
> > Would the wing fibres be springy?
> Elastic and collagenous fibers would be springy to varying extents, but
> according to my interpretation of actinofibrils they would have functioned
> in compression, would have been essentially inextensible, and would not have
> been stiff enough to have significantly effected wing camber by themselves.

Keeping in mind that they are intercalated rather than continuous, and
that I think the column slenderness ratio is such that they could not
have carried substantial compression, I see them as extensible BETWEEN
fibers, and relatively inextensible within fibers (also, actually
visco-elastic rather than elastic).  That said, my scenario has many
consequences similar to those that Chris describes and I agree with him
that they would not have been stiff enough to substantially affect wing
camber by themselves.  They must work in conjunction with the pressure
jump to do that, and I expect that Chris would probably agree in that
regard.  Chris?

> > And most imporantly would the wing taste good?
> Absolutely, though you've got to get the sauce just right.

I s'pect that's right.  Some of the Quetz bones show toothmarks, so
something found them tasty, though they may have required some aging