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Pterosaur extensors/flexors: a solution!



Greetings!

John Conway was kind enough to send me his schematics and I am now very clear 
on what our differences are regarding the pterosaur manus. I also now 
understand Chris Bennett's hypothesis and how it differs from John's. After 
some earlier correspondence with Chris, I assumed they held similar opinions 
since some of their results are the same, but I was wrong on that count. I have 
apologized to John for my misunderstanding and I appreciate his patience with 
me during the correspondence. I have also sent John my own schematics and I 
await his comments.

Using Pteranodon for a model, Chris Bennett rotated metacarpals I-III against 
the anterior face of metacarpal IV with metacarpal I dorsal, III ventral. This 
produces a situation in which the claws face anteriorly during flight. You can 
see a model of same in Wellnhofer's Encyclopedia of Pterosaurs: 170, so this is 
not a novel idea. With this configuration it is clear that flexors I-III also 
faced anteriorly while the extensors were squeezed between the set of three 
tiny metacarpals and the big wing metacarpal. Applying the same configuration 
(flexors in front) to the wing metacarpal means flexors must open the wing and 
extensors fold the wing. 

I think that's weird.

In John Conway's schematic, distal metacarpals I-III assume an arc more or less 
in the plane of the wing, so we're off to a better start with this model. The 
key is McI-III terminate _above_ the midline of the big wing metacarpal. Since 
the flexors of I-III are beneath metacarpals I-III, John also placed the flexor 
for mc IV in the same area. That is why in John's model, as in Chris's model, 
flexors open the wing and extensors fold it. 

The solution to Chris's problem is rotating the metacarpals back to the 
standard tetrapod configuration, in the plane of the wing, with claw tips 
oriented ventrally during flight. This is the typical situation in basal 
pterosaurs and a sample of this can be seen on the painted slip cover of David 
Unwin's book, The Pterosaurs From Deep Time. It is also illustrated in Peters, 
D. 2002. A New Model for the Evolution of the Pterosaur Wing â with a twist. 
- Historical Biology 15: 277â301 using Eudimorphodon. Chris will argue this 
because almost all pterosaur manual unguals face anteriorly in situ, but I 
think they were pushed flat while being crushed, most often in that direction. 
In Chris's sample specimen and any others that show the dorsoventral 
configuration, taphonomy must have pushed the tiny metacarpals I-III up against 
mc IV  This is a rare situation as a survey of fossils will show.

The solution to John's problem is lowering distal metacarpals I-III _below_ the 
midline of mc IV. With this new configuration, all the extensors are on top, 
including the big one that spirals around  metacarpal IV (there is a channel 
for this!) to wrap around the anterior rim of the distal trochlea and insert on 
the extensor process of the first wing phalanx. Another spiral channel under 
the wing metacarpal inserts on the first wing phalanx as a flexor to fold the 
wing.

This is what I mean by parsimony. Fewer changes. Less weirdness.

And this is a great example of showing diagrams to one another instead of 
trying to talk it out. Diagrams are succinct and falsifiable. 

Hope this helps. If someone has a better argument for raising metacarpals I-III 
above the midline, I'd like to hear it, because, yes, that will change 
everything.

Best,

David Peters
St. Louis