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alvarezsaur fingers

We had wide ranging discussions about alvarezsaur biology on these threads:

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

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, where finger II
is largest but supports the wing feathers, so finger I is the one that we
would expect to function in quadrupedal climbing. If alvarezsaurs had
large wings in their ancestry, but then went on to specialize in a
terrestrial habit and myrmecophagy, I can see how digit I might take over
with tearing into bark or rotten wood, and then digit II might be free to
reduce as the wings were reduced. But alvarezsaurs are usually recovered
way lower in the cladogram than where we might expect arboreality and/or
large wings.

Any thoughts?

Jason Brougham
Senior Principal Preparator
Department of Exhibition
American Museum of Natural History
81st Street at Central Park West
212 496 3544