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Re: Pterosaur wing - Part I



Chris Bennett wrote:

> I do not agree with your understanding of the pterosaur wing skeleton or
> adduction,

OK, keep in mind that I'm likely to use non-standard terminology, as my
training is more in fluid mechanics. Please try to read between the
lines in what I'm saying and help me out with jargon terms that are
outside my field (as I hope I do when responding to posters who use
non-standard terminology when talking about aerodynamics and structural
mechanics).  I'm not saying that each bone in the pterosaur arm skeleton
is in the same basic orientation as a human arm with the hand held palm
down.  Only that in life, the finger joints bend much the same direction
as in a human arm held in that position (with the exception of the mc-ph
joints that we mentioned earlier).

> and think you have ignored the weird folding of the manus that I
> referred to in my post.

I didn't think I had, but am open to enlightenment :-)  Personally, I
think the manus folding is a wonder to behold.  I do doubt that normal
flight position matches most of the illustrations we see (though
high-lift, high-drag positions might well do so), and I am taken with
the way the manus usually sits on the ground, seemingly with two fingers
out and one aft.  I do not have the mechanics of that folding fully
sorted out in my mind.

>  As you suggest the human forelimb can be used as a
> model for the pterosaur wing, but only up to a point.

I agree.

>  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.

Again, I agree.

> 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.

That I don't remember, but don't argue either.  I looked, but can't find
your monograph.  I filed it away all too well.  It's safe from me
anyway.

> ............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.

Quite true.  And does not mean that there cannot be such muscles.  I
don't have a pteroid bone or the muscles to operate it either. Some of
the articulations of joints in the small fingers are rather ball-like,
and I'm not sure just what their limits of driven motion might be.

> 
> 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.

No argument.

> The interphalangeal joints also flex
> posteriorly and extend anteriorly.

Not in the 4th digit they don't, though ancestrally they likely did. 
They are effectively fixed in that direction, with substantial aftward
projections to help them resist the aftward bending moment caused by the
membrane spanwise tensile force.  Note that the distal end of phIV-1
doesn't have that aftward projection because it is broad enough fore and
aft to match the proximal end of phIV-2, which does have the
projection.  They may well have originally flexed in that direction, but
they don't now (by now, I mean when they were alive).  And, as you know,
the 'stems' of the 'Tees' in 2&3 actually are cocked slightly aftward
(forming more of a sideways 'Y' to also help resist that aftward bending
(but the purpose of the 'Tee' shape in general is to serve that purpose
while accomodating the need for increased unit spanwise twist in the
relatively shortened outboard azhdarchid wing. 

> Movement downward in the vertical plane,
> which is what I understand you have suggtested in desireable is what I have
> been calling adduction.

I'll buy into that.

> 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.

Using your terminology (which I trust more than mine), I'd buy into that
as well.  But with that terminology, I do think they have gained the
ability to manage limited downward adduction.  I've got a quetz
wingfinger lying in my lap as I type this, and I still feel that way. 
They could manage the same gross wing manipulation (for roll control) by
flexing 2&3 downward along their length using inboard driven tendons in
the grooves along each side of the 'Tee', but I don't see any evidence
that such long structures existed, and it bothers me that if they did,
they would substantially increase the compressive forces in two bones
that were already subject to overloading in compression. I don't see
anything in the skeleton that would preclude deflection in a direction
that would alleviate that compression.  However, what I'm really asking
is that next time you have an opportunity, would you look at those
joints with that thought in mind and form a fresh opinion (which might
well be the same opinion as the one you presently hold -- I'm not asking
you to change your mind, only to provide fresh insight).

> 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?

Yes, we do. With the exception that I think at least some pterosaurs may
be able to.  As you say, the bones and folding of the pterosaur hand are
quite weird.
> 
> Yes but Digits I-III flex forward, and Digit IV flexes backward.

Yes. I would add (from memory, and you know how dependable memory is)
that some (not all) of the joints of the small fingers in the UT Austin
specimens were rather ball&socketish and may have had a quite
respectable range of motion.

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

For descriptive purposes, I'll go for the pinky finger being whichever
one is aftmost.  I never cared much which digit it was that was actually
lost.  I'm not saying that the choice of lost digit number isn't
important, just that there are others better able to make that
interpretation than me.  I'm interested more in the effects of the ones
that remain, whichever ones they are.

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

Again, I'm just asking that you look specifically at that issue the next
time you have an opportunity to look at the original material (in lieu
of these casts). BTW, I looked at the original of the pteroid tip that
we were talking about some months ago, and you were right.
> 
> 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 do agree that those are the greatest external forces. There is also a
substantial compressive force along the long axis of the bone due to
spanwise membrane tension, but I know that you are as well aware of that
as I am.
> 
> 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.

Sounds like a plan to me.  I've got to go run some errands, so may not
say much else till this evening.

Thanks,
Jim