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Re: flying Archie
An alternative "bottleneck" evolutionary path to the gradual terrestrial
transition scenario; abandonment of terrestrial locomotion entirely
(perhaps/probably for an aquatic environment), and then after the avian
bodystyle was well-established, re-invasion of the terrestrial environment.
Flight superiority well in wing, so to speak. No worries about center-of-mass
issues relative to the hindlimbs during "remodeling" there.
----- Original Message ----
From: Tim Williams <email@example.com>
Sent: Sunday, September 24, 2006 1:44:16 PM
Subject: Re: flying Archie
In basal birds (including _Archaeopteryx_), predator avoidance (or evasion)
could have been achieved in the old-fashioned way: Running away. The thing
about a long bony tail is that it does help keep the center of mass
posteriorly, whereas shortening the tail (and expanding the sternum and
pectoral musculature) shifts the center of mass forward of the hips. Modern
birds have had to do a complete 'makeover' of the hindlimb in order to
locomote on the ground, with stride generation principally occuring at the
distal femur, not the hip.
I don't know at what stage the femur was decoupled from stride generation in
avian evolution, but there must have been a transitional phase from the
primitive theropod style to the derived avian style. During this
transition, running on the ground might have been difficult, and birds may
not have yet reached the point where a vertical takeoff was do-able. This
could have been one selective pressure that kept _Archaeopteryx_ in its
plesiomorphic coleurosaurian body plan. There are "costs" associated with
the modern avian body plan, and I suspect Archie represents a compromise: it
kept the benefits of a coelurosaurian osteology (including the primitive
terrestrial locomotor style), while the plumage became dedicated to aerial
Don Ohmes wrote:
>Not local eco-stuff... also, implied in your post is the idea that A. may
>have been more optimal for tree to tree flight than modern birds, and birds
>sacrificed that to obtain long-distance flight. Highly doubtful, right?
I personally don't believe that _Archaeopteryx_ was optimal for tree-to-tree
flight. The reason I say this is that its perching abilities were minimal
to nonexistent. However, I do believe that _Archaeopteryx_ was adapted for
flying over short distances.
Heinz Peter Bredow wrote:
>I think that this concentration on Archaeopteryx regarding the origin of
>flight is arbitrary.
I don't think it is. Chronologically and morphologically, _Archaeopteryx_
is the first known theropod to show evidence of powered flight. Thus, until
we have evidence to the contrary, then we must assume that _Archaeopteryx_
at least approximates an early stage in the evolution of flight. It
certainly may not have been directly ancestral to later birds; and I don't
deny that it is possible that _Archaeopteryx_ is a bizarre aberrant form
that is a complete red herring to avian evolution (as you suggest). But the
latter implies that we know a *lot* less than we think we do about avian
evolution, which is a sobering thought.