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Re: Catch-22



 Vertical climbing cannot be excluded by pointing to the hands --
 juvenile Hoatzins make it obvious that wing-claws are functional for
 (small) vertical climbers. Hence, the focus on the feet

...and the very long legs with very long metatarsi and insanely long shins of *Microraptor*. *Archaeopteryx* isn't that extreme, but still has very long legs for the standards of climbing birds.

 Unfortunately, merely using a tree as refuge is not an arboreal or
 even scansorial lifestyle. E.g., bears are not "arboreal"...

Bears do, however, have plenty of "adaptations" for climbing. It just so happens that, in terms of their evolutionary history, they aren't adaptations; some are mammalian plesiomorphies (like the fairly short but not too short limbs and their wide ranges of movement), others are probably exaptations. Many of these character states do not occur in the theropods we're talking about.

 It follows that a "jumping-out-of-trees" evolution-of-flight scenario
 is not falsified by a perceived lack of "arboreal adaptations", given
 vertical climbing competence.

That competence is exactly what I doubt.

> The hands weren't very flexible either...

But, you're right, neither are those of a hoatzin chick. Point taken.

 That would in turn highlight a potential advantage accruing to
 pre-wings, feathers and tails in an animal that climbs up a tree,
 sleeps in it and then jumps/climbs/falls down -- slowing the
 descent.

Why, then, don't birds look more like *Ptychozoon*?

 Which again reminds me -- video of Hoatzins ascending is available.
 Is anything known of their manner of descent, which is actually more
 interesting?

The babies never descend, except accidentally or to flee. In both cases, they simply drop uncontrolled into the river under the nest.

The adults fly... and they don't climb much, do they?

> But more importantly, it's a _sufficient_ adaptation to keep an
> animal from bobbing up like a cork. Climbing, if you start from a
> theropod, requires more modifications.

 Human vertical claw-climbers use prosthetic devices to *reduce*
 flexibility.

Yes, because we're descended from brachiators. Our flexibility isn't on the same planet as the ballpark you can find *Microraptor* or *Archaeopteryx* in.

 BTW -- they also make it a point to keep their CG *away* from the
 trunk.

That makes sense if you're so big you can grasp the trunk _from behind_.

Brian Choo's painting of *Microraptor* hunting birds shows it high up in the crown of a tree, where the branches are thin. How did it get there in the first place?

> Importantly, this doesn't change if the only reason to climb a tree
> is to sleep in it.

 It is consensus that the physique of Arch[a]eopteryx was competent to
 climb a tree.

Then I disagree with the consensus. :-|

 Adopting as a working hypothesis that it did in reality both climb
 and sleep in trees, what evolutionary mechanism would change the pes
 beyond that seen in the fossil?

Natural selection for better trunk-climbing, of course.

> I don't think we can take wing, leg or tail feathers as evidence
> for any degree of arboreality. They may all have evolved for
> brooding and display.

 I agree.

 That said -- the arithmetic of probability shows that a structure
 that acquires multiple functions increases it's contribution to
 overall reproductive success.

 There are no functional conflicts in this arbitrarily sequential
 suite of feather functions: insulation --> sun shield --> camouflage
 --> display --> brooding --> air resistance...

 Nor is there only one valid sequence and/or combination of
 functions...

Exactly. The trick is to find out which of them has actually happened.