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Re: Pterosaur wing - Part 2

Hah!  The original subject line was "a couple of short questions" and look
what it turned into.

----- Original Message -----
From: "James R. Cunningham" <jrccea@bellsouth.net>
To: "Chris Bennett" <cbennett@bridgeport.edu>; <dinosaur@usc.edu>
Sent: Thursday, November 13, 2003 9:59 AM
Subject: Re: Pterosaur wing membranes (a couple of short questions)

> ......
> > 1.  I am not aware of any animal that has muscles in a position to
> > adduct or abduct its IP joints.
> Humans flex them in the same direction as pterosaurs would (with the
> exception of the palm/finger interface where I know of no other animal
> that flexes at right angles like pterosaurs do (that motion of the small
> fingers that you, Wann, and I discussed at Toulouse is the same as for
> the wingfinger).

The short phalanges of the small fingers do not have any bearing on the
question of whether active adduction of wing interphalangeal joints, and as
I suggested elsewhere there is no evidence that active adduction and
abduction of them was possible.

> > IP joints seem to allow only active flexion
> > and extension, and any adduction, abduction, and rotation that may occur
> > the IP joints seems to be involuntary and passive.
> Again, I think we may be using different terminology for the same
> motion.  I see the vertical bending of the IP joints as being similar to
> that in humans with their arms and hands held in the same position.  I
> would call that flexing, but recognise that my terminology is likely
> non-standard.  My take is that you are calling it adduction?

No, we are not using different terminology for the same thing.  IN Part 1 we
agreed that flexion and extension of the wing phalanges was essentially not
possible, but that that movement would have been flexion and extension.  The
active addutcion in the vertical plane that you see as desireable is a
movement that as far as I know is not possible for any tetrapod, and one
that is my opinion was severely limited by ligaments in just the same way
that flexion and extension in the horizontal plane is.

> > Note that I have sent an
> > accompanying e-mail message to the dinosaur and vertpaleo listservers to
> > if anyone knows of any animal that can do it.
> Good idea.  Humans can do it, with the exception of that proximal swing
> at the palm, parallel to the palm.  But that isn't the motion we are
> discussing.

No, humans cannot actively adduct or abduct the interphalangeal joints.  All
adduction and abduction takes place at the metacarpophalangeal joints and in
Part 1 we agreed that the pterosaurian metacarpophalangeal joint did not
permit adduction and abduction.

> >
> > 2.  In typical non-pterosaurian tetrapods the metacarpophalangeal joints
> > provided with interossei muscles, whose action is to adduct and abduct
> > 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.
> Yes.  And the equivilent joint of the other pterosaur fingers also
> rotates in that same plane, though the locking isn't as distinct (nor
> does it need to be).
> > Adduction, abduction,
> > and rotation of the extended metacarpophalangeal joint seem to be
> > prohibited,
> I agree.
> > 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
> > to rotate about its long axis when the wing-knuckle was strongly flexed
> > as to fold the wing more compactly, though this rotation may have been a
> > result of the arrangement of the ligaments rather than direct.
> In Quetz that long-axis rotation appears to be approximately 15-18
> degrees or thereabouts, facilitated by the mortise and tenon interface
> between DC4 & MC4. It has the effect you describe and also was important
> during flapping flight, helping to faciliate the appropriate pronation
> and supination of the wing.  As an aside, this might be a good place to
> mention that the upstroke supination relative to the torso so often
> described is usually a simultaneous pronation relative to the
> freestream.

I think that I have been aware of that and may even have commented on it in
my non-aeronautical-engineer kind of way.

> I'm not sure that that gets described as often since it is
> a vector effect.
> > 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
> > 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.
> But those joint articulations are now at right angles to one another.

No, the joints are not at right angles to one another.  In the typical
non-pterosaurian tetrapod flexion and extension of the metacarpophalangeal
and interphalangeal joints all takes place in the same plane.  For example,
if you place your hand flat on a horizontal surface flexion and extension of
the metacarpophalangeal and interphalangeal joints would occur in a vertical
plane, and the metacarpophalangeal joints could adduct and abduct in a
horizontal plane.  In the pterosaurian wing digit the situation is the same
except that the entire digit has been rotated roughly 180 degrees about the
long axis of the metacarpal.  That rotation does not change the fact that
the metacarpophalngeal and interphalangeal jonts all move in the same plane.

> Why would the muscles not have made the same adjustment that the
> articulations did?

I assume they did, just following along with the rotation.

> This is an area where you know far more than I do,
> but it appears to me that the orientation of the articulation between
> the MC & Ph1 rotated by 90 degrees, while the outer rotations remained
> unchanged.  This seems easier for me to visualise than the alternative
> that the MC/Ph1 orientation remained unchanged and all the outboard
> joints in all fingers rotated 90 degrees.

I do not see any rotation of the phalangeal relative to the metacarpal.  It
is rotation of the metacarpal rotation the entire finger.

> >
> > 3.  It sounds to me that you are suggesting that the third IP joint is
> > because its joint surfaces are subcircular whereas the first and second
> > 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
> > rotation of the distal phalanx relative to the proximal phalanx,
> I see it as strongly limiting fore and aft relative motion while
> allowing downward relative motion (and upward motion untill straight).
> Ph rotation about the long axis is prohibited at the joint (though in
> the azhdarchidae, spanwise twisting of the whole bone along its long
> axis is facilitated by the 'Tee' shape -- really more of a 'Y' shape, as
> it is designed to facilitate long axis twisting while resisting aftward
> bending.

Well even if the joint were to permit adduction (which I am not yet ready to
accept), that still doesn't mean there was a muscle that could actively
adduct the joint.

> > whereas the
> > third IP joint is subcircular because it is not as important to limit
> > rotation and perhaps a little more rotation is desireable.
> In quetz that joint looks like two golf tees crammed together head to
> head.  I don't think it was going anywhere in any direction or
> rotation.  It looks fixed to me. Phalange compressive stresses were
> substantial in this region due to the intense increase in membrane
> tensile stress at the tip (alleviated by the actinofibrils of course). I
> think that if this joint were allowed to bend or rotate substantially in
> any direction, it would have collapsed.

What 'it'?  the IP joint or the distal phalanx?  If the joint, then why
would it be more susceptible to failure than the more proximal joints?

> >  Despite the
> > spoon-shaped articular surfaces, their large radius would mean that even
> > little flexion, extension, adduction, and abduction of the distal
phalanx on
> > the proximal would lead to a lot of displacement of joint surfaces,
> > would be strongly limited by the interphalangeal ligaments.
> Here, I think an active vertical bending of perhaps 5-15 degrees would
> have been invaluable.  And it seems to me to be well within the limits
> available to the articular surfaces.
> >
> > I think that ventral deviation of the bowed wingfinger would have to
> > been produced by rotation of the wing metacarpal about its axis.
> I agree that that did contribute, in the amount of about 15-18 degrees
> of rotation about the long axis.  But that rotation isn't always
> suitable for modulating gust loading.


> >  Supination
> > of the wing metacarpal would increase the apparent downward curvature of
> > wingfinger in a flight posture when viewed from the front and would turn
> > wingtip downward.
> It would.


> > This may not be what you would like,
> Well, actually I would like that, and recognise it.
> > but it might suffice.
> It doesn't appear to, to me. And that rotation appears to have a
> somewhat different purpose during flapping flight anyway. One that might
> preclude its simultaneous use to modulate gust load alleviation.  Note
> that it could be more easily used to modulate gust loads during soaring
> flight.
> > This then brings us back the question, what would happen to
> > pterosaurs if they could not adduct the IP joints?
> In the extreme, compressive failures of the phalanges as a consequence
> of gust loading.  BTW, though you and I think that actinofibrils
> operated slighly differently, I think we both see them as minimising
> phalangeal compression.  For those of you who aren't aware of our
> difference, Chris sees actinofibrils as operating primarily in
> differential compression. I see them as operating primarily in
> differential tension with only a very localised, very minor compressive
> ability.  Both descriptions would have much the same consquence, with
> the real question being whether the actinofibrils had significant
> compressive ability, something I find rather doubtful because of the
> column slenderness ratio.  But I digress. That isn't the issue in this
> post.

Well, can pterosaurs get by without active adduction of the joints?  We have
always known that Quetzalcoatlus was living pretty close to the limit with
rather slim margins of safety.  Is this just one more place where they were
really close to the limit, which most other pterosaurs were safely further
away from?

> >> 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 didn't mean to imply that you don't understand pterosaur wings.  I
> think what I was trying to say is that an amalgum of aerodynamics,
> structural mechanics, and biology is needed to understand them fully.
> None of the three areas is capable of explaining them alone, and all of
> us need to work together to meld our individual expertises.  I wish I
> knew how to say that more articulately, but I don't.

I took probability and statistics in college rather than calculus, and it
has served me well for most things I have wanted to do.  I have once or
twice attempted to work my way through books that offered "teach yourself
calc at home the easy way for fun and profit" and I just couldn't handle it.

That's enough for the moment.


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