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Re: BCF and BADD



In a message dated 95-11-28 15:41:26 EST, Robert.J.Meyerson@uwrf.edu (Rob
Meyerson) writes:

>All this centers around the assumption that flight evolved in the trees;=
> from what I have read, the jury is still out on that one.  Now it could be=
> argued that the enlarged claw in dromaeosarus is a holdover from a=
> cursorial ancestor, but this could just be using circumstancial evidence. =
> What would happen to BCF and BADD if flight evolved on the ground?

As you might imagine, I have considerable difficulty comprehending the
evolution of avian flight from the ground up. The physics is against it: how
do you get airborne in the first place if you're stuck on the ground? And
until you get airborne _somehow_, how can you evolve adaptations for flight?
All kinds of absolutely absurd scenarios have been suggested to get cursorial
theropods with small forelimbs into the air: they took broader and broader
jumps in pursuit of prey until they took off, they jumped off rocks and even
cliffs (natural selection: those that survived evolved into birds), they ran
faster and faster and flapped their arms until they magically sprouted
feathers and took off, they evolved feathers as flapping insect traps and
took off, and so forth. In the BADD scenarios, all the many adaptations for
flight evolved independently in theropods for reasons unrelated to flying
(e.g., large wings with flight feathers as sunshades!), and then, when the
one group of theropods emerged in which all those adaptations converged, they
just...flew! At the bottom line, none of the BADD scenarios works. You can't
fight city hall, and you can't fight _gravity_.

An arboreal animal can become airborne simply _by letting go of the tree_ (or
cliff face, if you just can't live with the idea of an arboreal archosaur).
It stands to reason that arboreal animals will evolve mechanisms to prevent
falls, to control falls that cannot be prevented, and to relieve the effects
of impacts of uncontrolled falls. Such mechanisms have a clear and obvious
selective advantage within that lifestyle. Gliding and flying emerge
_naturally_ as evolutionary elaborations of such mechanisms; powered flight
is the ultimate adaptation within an arboreal lifestyle. I assert that we
have positive evidence of such ameliorative mechanisms in all theropods:
hollow bones (to ease the effects of impact), furculae as shock absorbers for
the pectoral girdle (so they could snag branches with their forelimbs),
retroverted halluces and large, grasping manual claws (for clinging to
branches), and even tail and forelimb feathers (for trajectory control). With
the increased specialization of the forelimb toward trajectory control and,
eventually, gliding, the hind limbs became the principal means of locomotion
when the animal was grounded (and all arboreal animals become grounded on
occasion), so a bipedal stance ensued. And so forth.

The jury you mention is asleep in the box, my man.

In a related development, don't miss Luis Chiappe's review of avian evolution
in this week's NATURE (23 November 95: 349-355). It's long and meaty, and a
cladist's dream.