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Re: 11th specimen of Archaeopteryx



On Wed, Nov 2, 2011 at 12:54 AM, Don Ohmes <d_ohmes@yahoo.com> wrote:
> On 11/2/2011 12:40 AM, Scott Hartman wrote:
>
> I have not assigned Archie an arboreal lifestyle. A ground-foraging
> tree-roosting lifestyle (GFTR) is not in any meaningful way "arboreal", nor
> would it result in "arboreal adaptations" -- this is because the critical
> tasks (food and nesting) occur on the ground, and contra Tim, sleeping in a
> tree just ain't that hard to do.

Really?  Which birds that "only roost in trees" demonstrate the same
total lack of arboreal adaptations seen in the foot of Archaeopteryx?
Not just claw curvature, but phalangeal curvature, phalangeal
proportion, hallux position, and the utter lack of an enlarged flexor
tubercle of any kind on the claws.  And then add in "lack of flight"
and see where that puts Archaeopteryx.

> Nor is my central point Archie-specific -- assuming limited upstroke in bird
> ancestors severely weakens the plausibility of "active" scenarios
> culminating in flapping flight, no matter what animals you designate as
> bird-ancestors, and no matter what wing-creating scenario you prefer.

That's fine; we could quibble over details, but I have no real issue
with the general hypothesis, just its application to Archaeopteryx.
And if it doesn't happen in Archaeopteryx, then we need to more
closely examine where the origin of flight really happens, and also if
the behavior you are looking at could have evolved more than once.


> By exclusion, "passive" scenarios become more plausible -- and Archie-*type*
> animals were entirely capable of accessing potential energy on a daily
> basis, simply by roosting nightly.

Oops...forgive my ignorance here - I presume you mean "potential
energy" as in "not kinetic energy"?  In that case I think the problem
again is you are applying it to the wrong taxa.  There's simply no
reason to assume that Archaeopteryx flew.  As a generic view of how
flight could have evolved from animals that already possessed
wings...sure.  I'm not sure I'd favor it, but it's certainly a
reasonably hypothesis.

> As a corollary, I ask "What other scenarios are there (for accessing
> potential energy)?"

As I said, there may simply be no need of potential energy.  Almost
all extant birds (Mike, correct me if I'm wrong, but perhaps Puffins
actually need some potential energy to take wing efficiently?) simply
don't make much use of it.  Gliding animals do, but it's not clear
that gliding animals actually evolve into active fliers.  They may,
but in general the selective pressures that make you a better glider
do not favor you becoming an active flapping flier.  I don't get too
far into that issue right now (I have to fly to SVP in a few hours!)
but at the least it's not a "given" precursor.

> The special pleading occurs when people insist that the simple act of
> sleeping in a tree inevitably results in measurable skeletal changes in an
> animal already physically capable of climbing, but fully optimized to a
> cursorial lifestyle. It might, or it might not...

Hold on there.  Your speculation isn't without merit, but what modern
analog are we looking at?  It's not just lacking optimization, but
rather that Archaeopteryx appears optimized for something else
entirely.  Modern birds that roost in trees but forage on the ground
can, you know, fly.  If you want to argue that Archaeopteryx simply
climbed up each night, then we should at least expect some sort of
climbing adaptations.  Perhaps an argument could be made for the
grasping hands and hyper-extendable second toe, but the wrists and
ankles don't show any of the normal range of motion seen in
trunk-climbing animals.  There needs to be some sort of positive
evidence to support such an assertation, otherwise it's untestable,
even if plausible.

> The relevance of GTFR hinges on the physical reality of limited upstroke in
> archie + ancestors + siblings -- and that is where I hoped this discussion
> would wander to eventually.
>
> Is "limited upstroke" fact, or opinion?
>

While there is certainly a bit of debate, I think the solid money is
on there being a limited upstroke.  The issue I take is whether
there's any reason to even hypothesize theropods in trees prior to a
full upstroke.  Once you move past the ongoing (not just you) fixation
of trying to pin all of this on Archaeopteryx I think its a fine
working hypothesis.

-Scott


-- 
Scott Hartman
Scientific Advisor/Technical Illustrator
(307) 921-9750
website: www.skeletaldrawing.com
blog: http://skeletaldrawing.blogspot.com/