Re: Pterosaur extensors/flexors: a solution!

[Michael Habib <mhabib5@jhmi.edu>]

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 
> wing.
>
> 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 
actually weird.

> 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 
> wing.
>
> 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 
best example.

Cheers,

--Mike H.


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu