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Pterosaur extensors/flexors: a solution!
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
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