Evolution of flight

[Dann Pigdon <dannj@alphalink.com.au>]

I've been away for a while so I might be
covering old ground, so
if so I beg tolerance.
        I have always been suspicious of
'transitionary' forms. The
whole idea suggests that creatures
evolve towards something, and
that just smacks of creationism (oops,
the 'C' word. Sorry).
        All of the earliest fossil pterosaurs
and bats that I know
of already seem to show advanced
adaptations to flight. I know that
the fossil record is fragmentary and not
the whole picture, but
it still seems suspicious that few
transitionary forms have been
found. Archaeopteryx has been touted as
such, but I'll get back to
that.
        I have always prefered the punctuated
equilibrium (or saltation)
approach to evolution, that things only
change when they absolutely
have to, and then relatively quickly. If
this is the case then the
development of flight may have been
quite rapid (in geological terms
at least). This makes sense. A gliding
animal is good at gliding
and does not have to change. A flying
animal is also good at flying.
Something in between is not particularly
good at either, and risks
being out competed by either more
advanced flyers or remanants of
its gliding anscestory. Therefore to me
a long evolutionary process
seems out of the question. If I were to
develop a detailed hypothesis
of the evolution of flight I would
probably call it something
catchy like 'The band-aid theory': the
quicker the change is made
the better.
        Perhaps the issue with birds is
complicated by the many
bird-like theropods that co-existed with
them. It is easy to order
birds and theropods (lets assume they
are separate for now, not
something I would usually do) into a
nice progression, with primitive
theropods at one end, advanced birds at
the other, and varying degrees
of 'birdy' theropods in between. People
have done this sort of
thing with homonid skulls before, lining
them out on a table from
the most robust to the most gracile, and
suggesting that the line up
represents an evolutionary sequence. It
may look good, but it's
not necessarily so. I suspect that many
of the bird-like theropods
are only very distantly related to
birds. Archaeopteryx has been
described as a transitionary form
between dinosaur and bird,
probably because the presence of
feathers was once taken as proof
of its avian status. Yet archae may not
even belong to the line that
resulted in modern birds and may belong
on the dinosaur side of
the dino-avian divide (such a divide
being purely arbitrary of course).
The growing evidence of feathered
non-avian dinosaurs makes this
quite likely.
        The point to all this? Simply that we
can argue about what
is bird and what is dinosaur, and which
came first, and how flight
evolved, and so on, however if the
evolution of flight was rapid then we
may never have the fossil evidence to
prove any of it.
        Please excuse the rave. It's been a
while and I've had
withdrawal symptoms.
-- 
____________________________________________________
        Dann Pigdon
        Melbourne, Australia

        Dinosaur Reconstructions:

http://www.geocities.com/capecanaveral/4459/
        Australian Dinosaurs:
        http://www.alphalink.com.au/~dannj
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