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Re: Avian Monophyly (Was Re: Sheesh)... :-)
Well, I've recently been thinking that the microraptorians seemed like the
most likely deinonychosaurian lineage to have given rise to birds, with
Archaeopteryx branching off just after Microraptor but before all other
birds. The shape of the skull appears at least somewhat similar in Archie
and "Dave" the Sinornithosaurus, and Microraptor makes a good precursor to
Archie in terms of flight structure-- Microraptor is a four-winged glider,
while Archie mostly uses its arms but still has some feathers on its legs
that could be useful. Of course, Microraptor itself obviously wasn't the
actual precursor to Archie because it appears too late in the fossil record,
but it's a good example of what Archie's forerunner probably looked like.
As for Rahonavis, the idea that it was more closesly related to Buitreraptor
and Unenlagia than to birds appears to make sense in light of its
"primitive" structure in a time when things like Ichthyornis and Vegavis
already existed. Convergent evolution might have led to a body plan
resembling Archie, while being a member of a different dromaeosaur lineage.
Of course, the fact that it's geographically close to the unenlagiines is
another good argument for it being of that group.