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In a message dated 95-12-07 06:52:50 EST, Robert.J.Meyerson@uwrf.edu (Rob
>My biggest reservation for BCF is the unlikelyhood of the events. =
> Essentially, what is being said is that an animal gains the power of=
> flight, then gives it up again. What's the adaptive value in that??? I=
> mean, no one talks about secondarily flightless pterosaurs. Once
> learned to fly, a whole new set of niches opened up for exploitation, and=
> it seems to me that the wouldn't be able to reverse the trend until the=
> evolutionary momentum had run down. Evolution may be random usually; but=
> once a trend has started, the group is usually committed to that trend to=
> the end.
But flightlessness occurs time and again among modern bird lineages! And
these include some of the best-developed fliers in the animal kingdom. How
much more likely would secondary "flightlessness" then have been in lineages
that were not nearly as highly derived fliers as modern birds? In lineages of
climbing and gliding animals?
Actually, as the central avian lineage approaches modern birds, the rate of
appearance of flightless branches seem to decrease. There were
hesperornithiforms, _Patagopteryx_, perhaps alvarezsaurids and avimimids.
Compare this to the wealth of herrerasaurians, ceratosaurians, carnosaurs,
dromaeosaurids, oviraptorosaurs, ornithomimosaurs, and so forth, which BCF
asserts branched off mainly before the advent of powered flight. Your thesis
may be more correct than you think.
Regarding secondarily flightless pterosaurs and bats: it's entirely possible
that they existed but haven't yet been found. I've heard intriguing rumors,
but nothing concrete yet. Meanwhile, you might want to give some thought to
how you might identify a flightless pterosaur in the fossil record.