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Who's aquatic lifestyle? was: Re: Cite correctlly (Re: Limbs and niche partitioning)



Garrison L Hilliard writes:
> It is HIGHLY doubtful that an upright stance would be an adaptation to an
> aquatic enviroment, so I seriously doubt that would have led to bipedal,
> flying archosaurs. Simply put, the most common traits gained by an aquatic
> lifestyle are a. webbed digits, leading to b. flippers, ending in c. no
> limbs at all.
> (snip)

You mean that's the most well documented trait lineage known to us.  There
is also the possibility that gliding flight evolved not from jumping out of
trees, but off cliffs into the water.  When a duck lands in water, it slows
down it's ungainly body's descent using it's webbed feet as a partial
lifting surface to manipulate how it comes down (watch ducks landing in the
park and you will see their outspread feet pulsing against the wind).  if a
proto-bird landed in the water, lacking the specialised buoyant body ducks
have and assuming that the basal body plan is with a torso that is greater
dorsoventrally (up and down) than it is laterally (side to side), it would
plunge through the water and have to fight to keep it's head above water.
This would mean it's force of propulsion would be at a diagonal going down
and back (caudo-ventral?).  It would not be swimming like the precursors to
seals, whales, dolphins or any other kind of ex-terrestrial obligate swimmer
which has an essentially round torso in cross-section and can therefore get
the best results through water bringing their limbs behind them and
propelling from the rear, thus making it possible to go through your
webs-flippers-no limbs route.  I'm not even sure if it could move it's
hind-limbs behind it (after all this thread did originate talking about the
resultant functional morphology of a stiff-legged group.  Perhaps someone
with more experience of working with limb mobility in the lab than I could
be of assistance here?  Even if they could, propulsion from directly behind
on a torso designed like that would cause the organism to travel forwards
and down - fatal to anything requiring air to breathe.  To sum up, there is
an alternative line of events open to anyone who cannot functionally
implement yours and that is long toes-web feet-strong hind limbs-bipedalism.
You never know it might even solve the fore-hind limb decoupling question
too.
Yours sincerely,
                         Samuel Barnett