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Re: Back-evolution of limbs.

> 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.

There's a group of marsupials known as tree kangaroos (I kid you not) 
which are fully arboreal.   They live in northern, tropical Australia 
and New Guinea.  Tree kangaroos evolved from ground-living, jumping 
kangaroos.  These ground-living, jumping kangaroos may have evolved
from arboreal ancestors.  Macropods (kangaroos, wallabies, potoroos, 
tree kangaroos) belong to a clade of marsupials known as the 
Diprotodonta, most of whom are arboreal.  It seems reasonable to 
assume that diprotodonts are primitively arboreal, and macropods came 
down to the ground.  Tree kangaroos returned to the trees.

Interestingly, Tim Flannery has found a species of tree kangaroo in 
New Guinea that spends much of its time on the ground, but clearly 
has an arboreal ancestry.   Hence, we have a ground-dwelling 
marsupial that evolved from a tree-dwelling marsupial that evolved 
from a ground-dwelling marsupial that (presumably) evolved from a 
tree-dwelling marsupial.

Evolution can be so wonderfully confusing.

>         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.
> ----------------------------------------------------------------------------
>     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
>                   "...To fight legends." - Kosh Naranek