Re: Back-evolution of limbs.

["Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>]

John Bois wrote:
>Why is it that birds--and only flying birds at that--are the only
>creatures to return to their former mode of locomotion, i.e., they may
>become secondarily flightless.
        Now, of course you realize that at least ichthyosaurs (if not sea
crocs and mososaurs) can be said to have truly reverted to their former mode
of locomotion (side-to-side undulations of the body). Never mind the huge
number of other taxa which have returned to the sea with different modes of
locomotion.
        To answer your real question, you have to look at habits (benthic,
nektic, etc.) rather than locomotory modes (cursorial, scansorial). What you
really want to know is, why do birds often return to a terrrestrial
existance? If you examine marine organisms, you will find switching between
habits within clades, and sometimes within species during ontogeny: many
benthic (seafloor) species have planktic (floating) larvae; if memory
serves, some crustaceans may persue benthic habits, but can go nektic
(swimmming); Cephalopods are believed to have started out benthic, gone
nektic, and now some have returned to a benthic existance (octopi?).
However, to my (very limited) knowledge, it does not appear that many nektic
(swimming is as close to flying as you get underwater, IMHO) clades
experience quite the return to the ocean floor that we see among birds.
        In terrestrial environments, five clades have colonized the air. Why
so few, especially when there are so many subaqueous "fliers"? Many animals
have a density close to that of water, making "taking off" under water a
relatively small problem. Further, being neutrally buoyant means never
having to drop back to the seafloor to rest. Birds, bats, pterosaurs, and
various flying insects had no such luck. They are negatively buouyant in
air.  Staying up takes energy (even soaring). You are constantly fighting
gravity because you are negatively buoyant. So flying creatures come down,
whether to rest, conserve energy, take advantage of the abundant resources
on the ground (far less abundant resources flying in the air than swimming
in the water...), or whatever. I do not know of any flying animal that never
comes down eventually. In order to facilitate this, flying creatures retain
a terrestrial locomotor ability of some sort.
        So then why don't we see more secondarily flightless bats,
pterosaurs, and insects, you ask? I cannot address the bugs, but as for bats
and pterosaurs, they do not have the separate flight and ambulatory
locomotor modules that birds do. Birds retain a facultative terrestrial
locomotor component ready for the first ground-based evolutionary
opportunity they find. Bats and pterosaurs would have to refashion their
limbs in order to become competitive terrestrial animals. Birds have a
double advantage in having evolved from bipeds who didn't need the forelimbs
to get around.  They can retain their flight ability as they begin to spend
more time on the ground. In order to become better at groundpounding, bats
and pterosaurs would have to give up flight ability. This could be
unfortunate over the delicate generations of evolutionary change. 
        Gatsey and coauthors have published some excellent work on avian
locomotor diversity and other questions pertaining to the evolution of avian
locomotion.
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    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
                    "...To fight legends." - Kosh Naranek