Re: pterosaur aerodynamics

[jrc <jrccea@bellsouth.net>]

Comments inserted below.
Jim

----- Original Message ----- 
From: "Jaime A. Headden" <qilongia@yahoo.com>
To: <dinosaur@usc.edu>
Cc: <davidrpeters@earthlink.net>
Sent: Thursday, April 13, 2006 3:14 AM
Subject: Re: pterosaur aerodynamics



>  All studies dealing with the wing aerodynamics and flight performance of
> pterosaurs have used a deep chord or medium chord model (it doesn't really
> matte that the membrane touches the ankle when dealing with measures that 
> only
> regard the mid-wing chord, and such aspect studies DO).

This is not true, unless one makes the assumption that the inboard portion 
of the membrane that swings back and touches the ankle is left slack.  And 
if that is the case, it creates a different set of implications, regarding 
initiation and control of flutter.  In other words, it should be a testable 
hypothesis.

> The studies make the
> assumption of the general chord from more than one specimen, although some 
> have
> argued (here on the list and in at least ONE letter, Peter's response to 
> Unwin
> and Bakhurina on *Sordes*) that all pterosaurs were narrow-chorded.

In my opinion, there is currently no positive proof of that (or the inverse) 
in the fossil record of which I am presently aware.  However, flight is 
possible with all three wing attachment scenarios.  And there were enough 
species over the eons, that there may have been a fair possibility of 
differing attachments, in at least some species.  My personal take on this 
is that, so far, an ankle attachment and broad wing have not been 
unequivocally demonstrated.  An example of this, is the Vienna specimen of 
Pterodactylus kochi (illustrated on page 150 of Wellnhofer's Illustrated 
Encyclopedia), where the material is taken by some to be a clear indication 
of a thigh attachment and by others, to be a clear indication of an ankle 
attachment.  To me, the only thing this specimen demonstrates clearly, is 
that the trailing edge of the wing membrane passes very close to the elbow. 
As an aside, I've not seen any specimen in which the location of the 
trailing edge of the wing membrane at the elbow, is located more than 45% of 
the length of the humerus behind the elbow.  This implies that if there was 
an ankle attachment of the wing membrane, it was a relatively narrow fillet 
from inboard of the elbow back to the ankle.  Personally, I see no selection 
process that would encourage retention of such a feature over a number of 
generations (which doesn't imply that it didn't exist in at least some 
species).

> There is no
> positive evidence that all pterosaurs had ONE chord style, nor is there 
> any
> positive evidence that pterosaurs WEREN'T ankle-anchored save for 
> unpublished
> studies that argue the membrane might attach to the hip in some pterosaurs
> (note: not all).

I agree, though I note that the wing membrane must also have been attached 
to the hip in any of the three configurations  :-)
Nor, does it seem, that there is presently any positive and definitive 
evidence that any pterosaurs were ankle-anchored.
I would also rephrase the statement to give a connotaton that the membrane 
might also attach to the thigh or ankle in some pterosaurs,  but not 
necessarily all, or any.
In other words, it wouldn't surprise me if fossil materials were to show up 
that did show definitive attachments at differing locations.  But I'd bet 
that the preponderence would be for hip attachments.  We need more fossils. 
For you guys that have a talent for finding them, get out there and get to 
work !!

> Meanwhile there is evidence in some specimens that ankles WERE  membrane 
> anchoring.

Interesting.  Which ones?  If we're referring to wing membranes, I haven't 
seen them.  If we're referring to uropatagium attachments, of course they 
were.

>  One important piece of evidence in the nature of pterosaur wing membranes 
> is
> the nature of the fifth toe. Peters has reconstructed this toe as a free 
> digit
> in virtually all reconstructions I have seem. However, in at least two
> different pterosaurs (*Sordes* and *Jeholopterus*) the digit is associated 
> to
> the margin of a membrane. Unlike bats, the leg is not twisted distally and 
> I
> think it unlikely the digit was oriented medially to support a 
> uropatagium,

It was on the outboard side of the ankle when in terrestrial position. 
However, in any of the three wing attachment scenarios, when the hindlimb is 
rotatated into flight position without being twisted, MT5 winds up being on 
the top-aft-inboard side of the ankle, where it is perfectly located for 
accepting tensile loads out of the uropatagium  when the uropatagium is 
uploaded -- but not perfectly located for accepting tensile loads out of the 
wing membrane.  This is also true of those specimens that haven't lost the 
5th digit. MT5 places uropatagium tensile loads back into the tibia as 
compression, while keeping the forces free of the foot, so that the foot has 
independent mobility.

> though possible if the digit itself twists toward the tail.

This is true, but not required.  When the leg is in flight position, MT5 and 
therefore the digit (in those specimens which retain the digit) will be 
directed toward the tail without having to twist it.

> However, it seems
> more likely the digit, as articulated in countless specimens, is laterally
> oriented, and may even fold laterally, collapsing into the foot during
> taphonomy (BIG speculation!), which would nonetheless allow the 
> integumental
> association be tarsal, not hip-based.

MT5 and/or the 5th digit appear to me to be appropriately located for 
association with the uropatagium, NOT the wing membrane.  Both can attach, 
but the load paths coming out of a wing membrane would be awkward, while the 
loads coming out of the uropatagium would not be.

> Yet this does not argue for a particular
> chord design, only distal articulation of the wing membrane.

I see it only as arguing for a proximal articulation of the wing membrane, 
and distal articulation of the uropatagium.

> These are two distinct pieces of anatomy.

They are indeed two distinct pieces of anatomy.

All the best,
Jim