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Re: flapping from gliding



From: achut@troodon.isp.net (Achut Reddy)
  
 > I think this alleged "conceptual ease" exists only if you are already
 > predisposed to believe this idea.  To me, the evolutionary path from
 > running to wing-assisted running to full powered flight seem just
 > as conceptually easy, in fact more so.

Then please specify a series of ecologically viable steps involving
attested categories of evolutionary change (like exaption and adaption)
which goes from running to flight.

I have yet to see any believable adaption model that will develop
proto-wings from arms in a non-arboreal terrestrial organism. This
is, to me, the sticking point. (Once proto-wings exist, exaption
can take over - but the proto-wings are a necessary precursor to
flight; this is why the aquatic origin of insect flight is, IMHO,
a *viable* theory - it provides an observationally based model for
the evolution proto-wings with a demonstrable advantage).

 >... Consider extant flightless birds. I believe
 > all of them are good runners.  If you believe that modern flightless
 > birds are descended from previously flighted forms, then you
 > acknowledge that the reverse transition has taken place.  Why then
 > is the transition in the other direction conceptually hard?

Simple: the transition is NOT symmetrical.

Loss of flight is a *loss* across the board, even down to atrophy
of the wings - and losses are usually evolutionarily easy, especially
if they involve atrophy or simplification.

However the *origin* of flight is a very different thing - it is the
origin of a new, previously non-existant anatomical structure. This
virtually always involves exaption, and is certainly much more situation
dependent than a loss is.

 > Besides, pterosaurs must have descended from walkers/runners, not gliders.

Oh?  On what grounds?

Here I *would* cite such more or less arboreal forms as Podopteryx and
Longisquama.

 > The further back you go in the pterosaur family tree, the better adapted
 > they are for running. 

Perhaps running in trees? (like squirrels)

 > Scleromochlus, considered by many to be
 > the dinosaur/pterosaur ancestor.  It was not capable of flight,
 > but seems to have been adapted for jumping and hopping.  It could not
 > have been a glider since it lived in a non-arboreal desert environment.

True - but look at the range of forms in small mammals today.

We have kangaroo rats in deserts and gliding rats in rain forests,
both within the same subgroup of Rodentia. So a close relative of
Scleromochlus that is arboreal is quite believable.  In fact we have
a good candidate for such in Longisquama.

swf@elsegundoca.ncr.com         sarima@ix.netcom.com

The peace of God be with you.