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Re: Pterosaur wing - Part I



----- 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)


> Chris Bennett wrote:
> > So, I must ask, what would happen to pterosaurs if
> > they could not adduct the IP joints?
>
> I'll come back to this later in this post.  I want to be sure that we
> are using 'adduction' in the same orientation.  I'm likely to
> inadvertently use it the term in an improper orientation because that
> isn't where my training lies. In non-standard terminology, if one were
> to treat his arm as a pterosaur wing, extending it horizontally with
> palm downward, the the 4th digit would flex with fingers moving down and
> up with horizontal being a constrained limit.  If they can flex, so do
> pterosaur phalanges -- EXCEPT for the joints which connect the fingers
> to the palm.  All four of those swing fore and aft parallel to the
> palm.  In the pinky finger, that motion extends and pretensions the
> patagium.  The ventral flexing outboard of that is the motion I'm
> interested in at the moment, the one that affects camber by modulating
> spanwise membrane tension. Those outboard joints have their long axis
> oriented horizontally and do not flex substantially in the fore and aft
> direction while in flight. But in the fiberglass Qsp wing casts, in
> which those joints appear to be in good shape, they are constructed to
> allow downward movement similar to that our fingers allow. There are
> some indistinct markings that might be muscle or tendon attachments, but
> they are not clear enough for me to make a determination.  I'd like for
> someone with your expertise to look at the originals and express an
> opinion.  More later.
>

Let's clear up this first part before going on the worry about all the other
stuff.

I do not agree with your understanding of the pterosaur wing skeleton or
adduction, and think you have ignored the weird folding of the manus that I
referred to in my post.  As you suggest the human forelimb can be used as a
model for the pterosaur wing, but only up to a point.  Extend your left
upper extremity out, with elbow back a little, and the wrist extended so
that the palm faces more or less forward.  This then is a model for the
pterosaur wing with your thumb and second through fourth fingers  comparable
to the four fingers of hte pterosaur manus.  The structure of the three
small digits of the  pterosaur wing shows that the metacarpophalangeal
joints flex and extend more or less in what is now a horizontal plane, and
can be deflected up and down in a vertical plane, which is adduction and
abduction.  As I noted in teh Pteranodon monograph, the proximal phalanx of
Digit III has a large flange that probably served for the insertion of the a
muscle that would depress it and spread the three small fingers.  The
interphalangeal joints of the three fingers are mostly ginglymoid, which
would limit their motion to flexion and extension in the more or less
horizontal plane.  The unusual joints of the small phalanges of Digit III
may well have allowed abduction and adduction, but there is no evidence that
there were muscles to actively produce adduction and abduction.  As an
aside, note that if you flex the proximal interphalangeal joint of your left
index finger and then grab and manipulate the middle and distal phalanges of
the left index finger with your right hand, you can adduct and abduct the
proximal IP joint and rotate the metacarpophalangeal joint to some extent,
but you do not have muscles to actively produce those motions; thus just
because a joint motion is possible, that does not mean that there are
muscles to produce it actively.

OK, so far I have been talking about Digits I-III.  Turning now to Digit IV,
its metacarpophalangeal joint does not flex forward and extend posteriorly
in a horizontal plane as do Digits I-III, rather it flexes posteriorly and
extends anteriorly, and so it has been rotated almost 180 degrees about its
long axis relative to Digits I-III.  The interphalangeal joints also flex
posteriorly and extend anteriorly.  Movement downward in the vertical plane,
which is what I understand you have suggtested in desireable is what I have
been calling adduction.  In typical vertebrates, there are tendons from long
and short flexors and extensors that pass out onto the finger and can flex
and extend it, but it seems that most pterosaurs have lost the power to
actively flex and extend the interphalngeal joints.  And as far as I know,
no vertebrate can actively adduct or abduct its interphalangeal joints,
which in this context woudl be elevation and depression of one phalanx
relative to another in the vertical plane.  Do we agree on this?

So copying bits of your paragraph:

> All four of those swing fore and aft parallel to the
> palm.

Yes but Digits I-III flex forward, and Digit IV flexes backward.

> In the pinky finger, that motion extends and pretensions the
> patagium.

Yes, but it is Digit IV and the pinky has been lost (Err, according to most
people...).

> The ventral flexing outboard of that is the motion I'm
> interested in at the moment, the one that affects camber by modulating
> spanwise membrane tension.

Yes.

> Those outboard joints have their long axis
> oriented horizontally and do not flex substantially in the fore and aft
> direction while in flight.

Yes.

> But in the fiberglass Qsp wing casts, in
> which those joints appear to be in good shape, they are constructed to
> allow downward movement similar to that our fingers allow.

There do not seem to have been any muscles that could have actively
depressed those joints.

> There are
> some indistinct markings that might be muscle or tendon attachments, but
> they are not clear enough for me to make a determination.

I am not aware of any tetrapods that have muscles out on their fingers, and
those scars on the ventral surface are almost certainly ligament attachment
scars, which in Pteranodon are largest and most easily seen on the ventral
surface because they are resisting the greatest force, namely the upward
force on the wing resulting from lift and the downflap in flight.


I'll stop here and see if we can agree on this before going on to worrying
about your replies to my points 1-3, etc.

Chris


S. Christopher Bennett, Ph.D.
Assistant Professor of Basic Sciences
College of Chiropractic
University of Bridgeport
Bridgeport, CT 06601-2449