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Bats and Archaeopteryx
To L. Smith: Thanks for the info on bats. I too was wondering about their
evolutionary development with regards to flight. I was also wondering if
there has been any evidence in their fossil record (scant as it is) to
show that they evolved from a gliding form (through the stages that you
list, ie. fingers becoming longer and incorporating into the wing structure)?
Also, have there been any studies done comparing the flight energy
needs/consumption between bats and birds? Don't bats require/expend more
energy than birds for flight?
RE. Archaeopteryx: Hmmm, let me try to rephrase my questions on this. My
question is simply one of whether the evolutionary pressure from an
arborial gliding existence would tend toward a bird-like wing development
or a flying squirrel-like skin-flap development. Extant arboreal gliders
all use the skin flap method. We have no modern analog to show that a
bird-like wing can be derived from this (or that a bird-like wing will
even evolve in an arboreal species). None of the modern gliding
animals is even in an interim state of development <towards> the wing
structure that birds have.
The tendency in extant arborial glider development has been to
develop characteristics that are already present in the animal (ie.
webbing between limbs/body or fingers, etc.). In which case, it seems
reasonable to assume that even if proto-Archie took to the trees at some
point, that the wing-type seen had already begun to develop somewhere
else. Archaeopteryx itself may have been a tree dweller (it probably
could have functioned equally well on the ground AND in the trees
since there are benefits to be derived in both environments [and
therefore a great pressure to develop the wing further]).
Another question is in regard of the tucked wing position. Extant
tree-dwelling gliders sprawl themselves out to bring their center of
gravity closer to the tree. They don't tuck their front limbs close to
the body. Tucking isn't likely to occur as long as you are climbing the
trunk of the tree. However, once you have started to develop a tucked
limb position, you can still hop up into the lower branches. Now, correct
me if I am wrong (or my facts out-of-date), but I seem to recall that
Archaeopteryx limbs were pretty-much tucked in (like a birds) but with a
greater range of motion. This would lead me to suspect that they had at
least evolved out of the trees, and (if Archies were tree dwelling) moved
there subsequent to the development of the wing-limb.
Now, IF the structures were already present prior to tree dwelling, they
most likely were used to the greatest advantage (which would include
stabilizers for sharp turns, added lift on jumps/hops, etc. All of which
are perfectly good flight precursers) while on the ground. Once in the
trees, the evolutionary advantage may have been ENHANCED and thus develop
into what we see today, but the beginnings, I believe, started on the ground.