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In a message dated 9/24/99 8:19:56 PM EST, NJPharris writes:
<< The total decoupling of fore- and hindlimbs in avian locomotion implies to
me that the two sets were already completely decoupled before there was any
development of the arm towards a wing condition (i.e. that pre-flying birds,
whenever they might have lived, were already completely bipedal). >>
This is possible, but then you're left with the problem of explaining why
bipedality occurred in dinosaurs in the first place. I don't think
explanations such as "bipedality helped them run faster," etc., hold any
water, since if bipedality conferred this kind of advantage, there would be
lots more bipedal vertebrates besides birds & dinosaurs and a few specialized
mammals (humans, kangaroos). Climbing up a vertical substrate gives the fore
and hind limbs different functions (the forelimbs pull while the hind limbs
push, for example), which readily promotes decoupling of the limbs for their
different functions. Complete decoupling of fore and hind limbs for flight
may simply be the result of having self-supporting flight structures, i.e.,
feathers, that do not have to be stretched between the limbs like a membrane
usually does. Bipedality and flight likely evolved together, with neither
behavior appreciably preceding the other: More primitive climbers would have
given rise to "less bipedal" forms (e.g., ornithischians) than the more
derived fliers would have (e.g., theropods).
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