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Re: Ah ha! That's where therizinosaurs came from



Jason Brougham <jaseb@amnh.org> wrote:

> I wonder what would happen if we applied phylogenetic bracketing to Xu's
> new Xiaotingia cladogram. You have Epidexipteryx, quite certainly
> flightless, at the base of the Avialae, and Archaeopteryx, quite possibly
> flighted, at the base of Deinonychosauria, so what can we infer about
> their last common ancestor? The next successive outgroup are the
> flightless Oviraptorids, right, so if we take that little tree there we
> may argue that flight arose twice: once after scansoriopterygids and once
> in Archaeoopteryx. That's nutty, but then Biology is nutty.


This is only "nutty" if you assume that _Archaeopteryx_ was flighted.
Otherwise, flight arose in derived Avialae.


BTW, powered flight arising twice within the Avialae is not as "nutty"
as it first sounds.  There appears to be a plexus of basal avialans
(and probably non-avialan paravians) that were equipped with large
wings and rudimentary lift- and thrust-generating capabilities.  More
than one lineage might have crossed the threshold from undulating
gliding to outright powered flight.  Perhaps Enantiornithes and
Euornithes made this transition independently.  It would certainly
account for the differences in the scapula-coracoid contact in
(Enantiornithes - peg on the coracoid, socket in the scapula;
Euornithes - peg on the scapula, socket in the coracoid).  However,
most likely the common ancestor had a flat scapula-coracoid
articulation, as in the basal enantiornithean _Protopteryx_.





Cheers

Tim