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



----- Original Message -----
From: "James R. Cunningham" <jrccea@bellsouth.net>
To: <cbennett@bridgeport.edu>; <dinosaur@usc.edu>
Sent: Wednesday, November 12, 2003 1:05 PM
Subject: Re: Pterosaur wing membranes (a couple of short questions)


> .....
>
> 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.


Jim, here is yet another example of why it is so interesting to have your
input into pterosaurology.  You have just said that something
that I always assumed was undesireable and impossible (i.e., adduction of
the interphalangeal joints), was not only desireable, but necessary as well.
Note that despite the weird folding of the pterosaurian metacarpus, the
movement of the interphalangeal joint that would deflect the distal phalanx
ventrally when the wing was extended in a flight position, would technically
be adduction (i.e., movement toward the midline of the manus).  I am happy
to stand corrected that adduction was desireable, but I am not convinced
that it was possible.  So, I must ask, what would happen to pterosaurs if
they could not adduct the IP joints?  But first:

In support of my assumption that adduction of th IP joints was impossible I
can offer the following:

1.  I am not aware of any animal that has muscles in a position to actively
adduct or abduct its IP joints.  IP joints seem to allow only active flexion
and extension, and any adduction, abduction, and rotation that may occur at
the IP joints seems to be involuntary and passive.  Note that I have sent an
accompanying e-mail message to the dinosaur and vertpaleo listservers to see
if anyone knows of any animal that can do it.

2.  In typical non-pterosaurian tetrapods the metacarpophalangeal joints are
provided with interossei muscles, whose action is to adduct and abduct the
digits.  The fourth metacarpophalangeal joint of pterosaurs, the
wing-knuckle, seems to be highly modified so as to essentially lock the
joint in an extended position spreading the patagium.  Adduction, abduction,
and rotation of the extended metacarpophalangeal joint seem to be strictly
prohibited, though as I dicussed in the functional part of the Pteranodon
monograph, the joint seems to have been set up so as to allow the metacarpal
to rotate about its long axis when the wing-knuckle was strongly flexed so
as to fold the wing more compactly, though this rotation may have been a
result of the arrangement of the ligaments rather than direct .  Given that
pterosaurs seem to have lost the ability to adduct and abduct the
wing-knuckle even though they presumably inherited muscles to do just that
from their non-flying ancestors, it seems unlikely that they would have
evolved other muscles to adduct their IP joints when typical non-flying
vertebrates have no such muscles.

3.  It sounds to me that you are suggesting that the third IP joint is fixed
because its joint surfaces are subcircular whereas the first and second IP
joints are mobile because the joint surfaces are oval.  My interpretation of
the difference is that the proximal joints are oval so as to strongly limit
rotation of the distal phalanx relative to the proximal phalanx, whereas the
third IP joint is subcircular because it is not as important to limit
rotation and perhaps a little more rotation is desireable.  Despite the
spoon-shaped articular surfaces, their large radius would mean that even a
little flexion, extension, adduction, and abduction of the distal phalanx on
the proximal would lead to a lot of displacement of joint surfaces, which
would be strongly limited by the interphalangeal ligaments.

I think that ventral deviation of the bowed wingfinger would have to have
been produced by rotation of the wing metacarpal about its axis.  Supination
of the wing metacarpal would increase the apparent downward curvature of the
wingfinger in a flight posture when viewed from the front and would turn the
wingtip downward.  This may not be what you would like, but it might
suffice.  This then brings us back the question, what would happen to
pterosaurs if they could not adduct the IP joints?


> 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.

My inability to read music has not prevented me from playing guitar, and my
inability to comprehend Sneyd's math should not prevent me from
understanding pterosaur wings.


> ....
>
> 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.
>

I illustrated what I interpret as ligament attachment scars about the
interphalangeal joints in my Pteranodon monograph, and noted that the ones
on the ventral surfaces of joints were largest, presumably because upward
bending stresses on the joint were larger than other stresses.  Indeed the
ventral ligament attachment sites were largest on all the wing joints.  Is
there any reason to think that the features that you are interpreting at
muscle attachments are not ligament attachment scars?


> ....
>
>
> 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?
>

Huh?  what?  my eyes just glazed over and my mind was beginning to wander.


Chris

S. Christopher Bennett, Ph.D.
Assistant Professor of Basic Sciences
College of Chiropractic
University of Bridgeport
Bridgeport, CT  06601