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Re: Terrestriality is a bias



Dinogeorge@aol.com wrote:
> 
> ... these features also occur in nonavian theropods, often simultaneously, 
> which
> is difficult to account for unless there was once an arboreal stage in their
> evolution.

Arboreal or Scansorial? Of course defining where one ends and the other
begins may be difficult when you're dealing with the grey area in
between. However, what I was arguing was that bird ancestors were
probably more scansorial rather than fully arboreal, hence a fully
upright stance was maintained and the legs were never incorperated into
the flying structures.

Why? Probably because they needed their legs for getting about on the
ground. In general the more arboreal a creature is, the less time it
spends on the ground. Take lemurs for example. Most of them (all?) are
extremely arboreal. Some species find it impossible to walk on the
ground, and have to resort to a weird sideways hop. If such lemurs ever
develop into actively flying creatures there is nothing to stop them
incorperating the hind legs into the flight surfaces (assuming the
forelimbs become the main means of locomotion). Therefore you might
expect something more along the lines of a bat or pterosaur.

It seems that birds are unique amongst actively flying vertebrates in
that the hind legs have been kept quite separate from the flight
surfaces. I suspect this is no accident. I think that early flying birds
were decended from creatures that spent as much time on the ground as in
the air, to the point that their hind legs remained important
structures. Hence, bird flight could have developed from both the ground
up AND trees down at the same time, which would explain why neither
theory has gained strongly against the other.

"SCANSORIALITY FOREVER!"

-- 
____________________________________________

        Dann Pigdon
        GIS Archaeologist
        Melbourne, Australia

        Australian Dinosaurs:
        http://home.alphalink.com.au/~dannj
        http://www.geocities.com/dannj.geo
____________________________________________