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Re: Catch-22



On 12/3/2011 7:25 AM, David Marjanovic wrote:

What about the forelimbs? Why is so much attention focused on the
feet, and the hands almost completely ignored?

Because you can't climb with forelimbs alone.

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, in hopes that an "arboreal lifestyle" can be excluded.

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

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.

So a conclusion that a wing-clawed vertical climber is/is not "arboreal" is interesting ecologically, but irrelevant in terms of "tree's down" vs "ground-up".

The hands weren't very flexible either...

Which might imply that the wing-clawed Archie/Archie-ancestors/Archie-siblings/ilks and sundry et als could ascend a tree stepwise more competently than the reverse process, which is descending stepwise -- as is typical in vertical climbers.

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.

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

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

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 Archeopteryx was competent to climb a tree. It follows that it was competent to sleep in most trees, given that even a beach ball can "perch" in *most* trees.

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?

To use your term, it was _sufficient_.

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...