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fliers and diggers
I don't really agree with the comment that there isn't any positive
evidence for fliers coming from gliders. Gliders have wings and can fly
(by fly I mean generate lift sufficient to compensate for their tendency
to fall, if not necessary thrust sufficiet to compensate for drag). They
already can fly, already have wings, already have large forearms. There
aren't any cusorial animals for which this claim can be made.
Somebody stop me if the fossil evidence here contradicts this, but it
seems possible to argue that the reason monotremes sprawl is that this is
a secondary adaptation for digging. Moles if I recall have the forelimbs
out to the side (and marsupial moles too), and both echidnas and platypi
are good diggers.
On the subject of Mononykus, I think that the keel was also for
digging or something similar. It may or may not have come from a keeled
ancestor but the phylogeny can't really tell us that.
I guess if I had a point it would be something about wondering whether
we spend enough time looking at why animals had certain structures,
sometimes it seems like the easy way out is just to say "because their
ancestors had them."