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In a message dated 5/24/00 6:55:01 PM EST, firstname.lastname@example.org writes:
<< Since there are species of moss that live exclusively in trees, even the
absence of a skeleton can't be interpreted as absence of arboreality. I'm
just curious about what characters can be pointed to as evidence for or
against an arboreal lifestyle. >>
As far as I know, there is no skeletal feature or combination of skeletal
features that is unique to arboreality among vertebrates; in nature,
everything has a counterexample. If you're a vertebrate, you don't even need
>limbs< to be arboreal.
On the other hand, here is the abstract of a paper titled "The Birds Came
First" I was working on a while ago, as time permitted:
Two mutually exclusive scenarios currently compete to explain the origin of
birds and avian flight. One, the "trees-down" scenario, explains avian flight
as an arboreal adaptation, and argues that, because all known dinosaurs are
terrestrial cursorial forms, they cannot be intimately related to birds.
Anatomical similarities between birds and dinosaurs, particularly theropods,
must therefore be convergent. The other scenario, "ground-up," argues that,
because common ancestry of modern birds and non-arboreal, cursorial theropod
dinosaurs is strongly supported by cladistic analyses, avian flight must have
evolved in these dinosaurs--somehow--without passing through an arboreal
stage. Overlooked in this debate is a third alternative, originally proposed
in 1911 by Othenio Abel, that some early dinosaurs were small, arboreal
animals, not terrestrial cursors, and that it is from such arboreal, birdlike
dinosaurs, or "dino-birds," that both birds and certain ground-dwelling,
cursorial theropod dinosaurs later descended. This scenario accounts for the
close relationship of birds and dinosaurs without invoking a miraculous
convergence hypothesis, and it accounts for the origin of avian flight
without invoking the equally miraculous appearance and accumulation of
flight-related anatomical features in animals not otherwise disposed to fly.
Thus it likely provides a better account of dinosaurian and avian evolution
than either the strict "trees-down" or "ground-up" scenarios.