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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 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