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Re: Pterosaur extensors/flexors: a solution!
Some good thoughts! I don't entirely agree, but simple agreement is
boring anyway. Comments inserted:
Applying the same configuration (flexors in front) to the wing
metacarpal means flexors must open the wing and extensors fold the
I think that's weird.
Why is that weird? I understand that is seems odd to us, given that we
usually see a greater range of palmar motion than dorsal extension in
the hands of vertebrates, but ranges of motion are highly variable for
most joints if we look across different groups of highly derived
vertebrates. I find it counter-intuitive, but I'm not sure it's
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 possible, but only for MCI-III. MCIV (and PhIV) do not work in
that orientation. Thus, if the other fingers point ventrally, then
they are 90 degrees offset from the wing finger. Maybe that's the
case, but I don't see much evidence for it, and it leaves the wing
flexing into flight position.
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.
Is this really more parsimonious? Instead of having a wider arc of
posterior motion (compared to the anterior arc) in the hand, we have
spiraling tendons. That model also, if I understand correctly,
requires that the anatomical position of PhIV be rotated 90 degrees
relative to MCIV (hence the spiraling tendons). Both proposed
anatomical reconstructions are highly derived, and neither is
objectively more parsimonious. There is the added problem that a) the
spiral configuration has mechanical problems and b) the presence of the
channels is actually pretty shaky. I certainly can't find one in the
azhdarchid wing anatomy (and I've looked very hard). In fact, with the
flattened MCIV of azhdarchids, I don't think the spiral tendon
configuration could work.
And this is a great example of showing diagrams to one another instead
of trying to talk it out.
That I do agree with. Figures are worth a lot.
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.
Well, they fit above the midline rather better than below in
azhdarchids, at least. If you put them below midline, they basically
have to fit on the ventral-pointing surface of MCIV, which doesn't work
too well. But, they're derived in a lot of ways, so perhaps not the
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181