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Re: Rhamphorhynchus wrist articulation




Allan Smith wrote:

>  a 1:1 model of Rhamphorhynchus sitting perched on
> a branch and have run into a bit of a problem with the hand/wrist
> articulation and could really use some help if anyone can come up with a
> good answer...
>
> When perching (assuming Rhamphorhynchus did) would digits I-III foace fore
> or aft?

They are unlikely to have been able to perch, but if they did -- digit I would
be directed outboard.  Digit II would  be directed outboard and aft a bit.
Digit III would be directed aft and outboard.  The reason being that
attachments to Digit IV will hyperextend them outward and aft as Digit IV
itself hyperextends into the folded position.  The wingfinger is fully flexed
when in soaring flight position and fully extended when folded in the
terrestrial position.  The elbow will also be directed fairly far outboard and
somewhat aft.

> In other words, could pterosaurs rotate their wrists or were they
> fairly fixed in position.

As the elbow is extended, the entire wrist swings aft and down relative to the
r/u. As the wrist itself is flexed, it swings forward and down (sort of like a
cocked-pin joint).  The wrist itself does not rotate about the long axis of the
r/u.  However, metacarpal IV can rotate about 20 degrees about its long axis,
on a mortise and tenon articulation between it and the distal carpal.

> I would like to present the animal with the digits facing aft

None of the digits will face aft.  III will be more aft than II, and II will be
more aft than I.

> but will fold  them along digit IV if that is not possible.

That's not possible either, when the wing is folded.

All the best,
JimC