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Re: Alvarezsaur spurs (was Re: dino-lice)



Jason Brougham <jaseb@amnh.org> wrote:

> 1) The claw on the first digit is retained in alvarezsaurs as it is in all
> basal avialans, including clearly volant ones. No one suspects
> Eoenantiornis or Yanornis of using their wings to crack open termite
> mounds, do they? Claws are also retained in ostrich wings where they
> presumably have no function.


When the alvarezsaur forelimb underwent extensive truncation and
fusion, it occurred in a very uneven fashion.  For example, whereas
the lateral digits became vestigial (and were lost altogether in
_Linhenykus_), the first digit remained large and robust - and was
undoubtedly functional.


What is needed is a representative from the long ghost lineage between
Late Jurassic _Haplocheirus_ and the functionally monodactyl
alvarezsaurs of the Late Cretaceous.  That should answer some key
questions, such as whether which came first: (A) vestigialization of
the lateral fingers, or (B) forelimb truncation; or (C) did the two
occur in tandem?


For (A), did the alvarezsaurs pass through a compsognathid-like
morphology, in which the forelimbs were reduced in size, but the first
digit remained large - although still part of a grasping, functionally
tridactyl manus (as in compsognathids)?  Under this scenario, the
functionally monodactyl manus and impressive pectoral musculature both
arrived much later.


For (B), did the emphasis on the first digit (and reduction of the
lateral digits) start early, as part of a long, _Haplocheirus_-style
forelimb?  This would suggest that a digging function came first, and
the shortening of the forelimb came later in order to enhance its
power via leverage.  Thus, at one transitional stage alvarezsaurs had
a long, functionally monodactyl forelimb endowed with a strong
musculature.


For (C), this would also be consistent with a digging function, as the
forelimb shrunk, and the first digit became the only functional digit.
 As with (B), beefing up of the pectoral musculature would occur
earlier, rather than later.




Cheers

Tim