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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. 

>  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.

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).

> 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.

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?


> 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.

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.
> 
> 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.

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 strictly
> 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 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.

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'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 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.

But those joint articulations are now at right angles to one another. 
Why would the muscles not have made the same adjustment that the
articulations did?  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.
> 
> 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,

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.

> 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.

>  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.

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 have
> 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 the
> wingfinger in a flight posture when viewed from the front and would turn the
> 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.
 
>> 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 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?

They could very easily be ligament attachment scars.  And to be quite
candid, they could be miscellaneous post-mortum crud.  Interpretation of
such markings is not in my field of expertise unless the markings are so
plain that they are hitting me about the head and ears.  I still wish
you'd take a look at those joints the next time you have the
opportunity.


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


And that's happening to me now.  I started this in the dark and dawn has
now struck.  I think I'm going to close and go to bed -- or at least
drink a cup of cofffee.  And I'm too pooped to run my spell checker (but
you folks already know I can't spell or type).

Cheers,
Jim