Re: MY THOUGHTS ON THE 'DINO/BIRDS'

[Ronald Orenstein <ornstn@inforamp.net>]

>This is not some kind of untested assumption. I've yet to see a published
>scenario in which avian ornithoptering flight evolves with any facility from
>cursorial flightlessness. The physics is against it, and the change from a
>jumping-and-fluttering behavior to a taking-off-and-flying lifestyle is
simply
>too profound.

I fail to see why this should be the case.  Anyway, as you know I think
fluttering could have been involved in the evolution of bird flight from
arboreal ancestors, too - and the biggest problem with gliding as an
intermediate stage in the evolution of flight for birds is the absence of a
known fossil bird precursor that is clearly a glider (something like
Sharovipteryx, which though highly derived in itself might represent an
ancestral pterosaur stock.  That doesn't mean that no such animal existed, but
a Heilmann-type "protoavis" has yet to appear.  In an arboreal scenario, of
course, you could have a creature with both a fluttering and a gliding (or at
least parachuting) capability, so both could be involved - though for me the
big question is why, if gliding was vital in bird evolution, birds never
developed the types of gliding membranes that have evolved over and over again
in other gliders, but instead evolved a mechanism whose usefulness to a
glider,
in its early stages of development, is a bit hard (for me at least) to
picture.

The evolution of avian flight--or any vertebrate flight--with
>all its highly specific anatomical modifications requires environmental
>selection pressure that will act on a large group of animals for tens of
>millions of years. It is not a simple modification of some quirky behavioral
>pattern; if it were, there would be hundreds of different kinds of flying
>vertebrates instead of only three.

I must assume that in its initial stages flight mechanisms, like any other
adaptation major or minor, must have started as fairly insignificant-seeming
modifications of some existing structure that may, at first, merely have
allowed a minor, but selectively advantageous, shift in a pre-existing
behaviour - "quirky" or otherwise.
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net