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Re: alvarezsaur fingers

Jason Brougham <jaseb@amnh.org> wrote:

> but I wonder if anyone discussed the transition of the major finger in
> alvarezsauroids from finger II to finger I. Haplocheirus has the standard
> maniraptoran plan, with a large, robust, second finger. By the time we get
> to Mononykus digit I is broad and robust, while II is just a vestige.
> Anyone care to speculate on functional hypotheses that explain that?
> The only thing that comes to mind is the hand of avialans,

For me, the group that immediately comes to mind is the
Compsognathidae.  The compsognathid _Sinosauropteryx_ has relatively
short forelimbs (around one-third the length of the hind limb); the
ulna bears a huge and robust olecranon process; manual digit I is
longer than the other two (although the short metacarpal means digit I
doesn't extend quite as far as digit II); and manual phalanx I-1 is
massive, as is the ungual that it supports.

Alvarezsaurs seem to have greatly exaggerated all these features,
presumably independently of _Sinosauropteryx_.  Nevertheless,
alvarezsaurs may have passed through a superficially
compsognathid-like bauplan (especially in the morphology of the
forelimb) on their way to the highly derived morphology seen in the
Late Cretaceous forms.