Re: Ornithoms, Parrots, and others

[Ronald Orenstein <ornstn@home.com>]

>But it isn't compatible with the "ground up" cursorial scenario of the origin
>of flight. In the "ground up" scenario, birds evolve flight as ground-dwelling
>animals by accumulating the anatomical features associated with flight for
>non-flight reasons. They become arboreal only >after< flying makes trees
>accessible. If the standard model admits tree-climbing non-avian coelurians,
>then it is virtually indistinguishable from BCF, and I can get off my soapbox.

This assumes that "anatomical features associated with flight" equates to
"anatomical features associated with arboreality".  While the two should
overlap, I do not see them as synonymous.

I think the two models ARE pretty much indistinguishable, but that is
because at some point in the evolution of arboreality (whenever it
occurred) there must have been, presumably, partially-arboreal forms, but I
think that these would be extremely hard to identify.  Arboreality involves
a much less specialized suite of characters than flight (and has evolved
far more often); these characters are quite reversible.  Consider the
recently-discovered 'dingiso', a terrestrial tree kangaroo from New Guinea.
 This species evolved from arboreal tree kangaroos, which in turn evolved
from terrestrial kangaroos, which in turn evolved from arboreal
phalangeroids!  If those sorts of reversals (including the final shift to
the ground WITHIN the genus Dendrolagus) can occur that frequently in a
single line of marsupials, presumably the same sorts of shifts could have
happened with equal ease in non-avian coelurians - and sorting such shifts
out based on fossils could be well-nigh impossible.
--
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@home.com