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>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 lineage=
>that were not nearly as highly derived fliers as modern birds? In lineages =
>climbing and gliding animals?
Yes, but these secondary-flightless birds appear *after* the dinosaurs go=
extinct (when it was "safe" to do so). Since there were a whole crop of=
niches that were left vacant by this demise, there would be a new=
evolutionary pressure to revert to a ground form.
>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 thesi=
>may be more correct than you think.
To argue against myself (using hesperornis as the only true bird in the=
above group), one could say that it evolved from an early form of seabird=
that found an adaption to a fully aquatic form useful (a seagoing dinosaur?=
>Regarding secondarily flightless pterosaurs and bats: it's entirely possibl=
>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.
This would depend on (and perhaps prove) whether pterosaurs were quadrepedal=
or bipedal. Beyond this, we would probably see stronger limbs (fore and=
aft), and a serious reduction of the wing finger. Also, the wing/arm might=
show greater flexibility (although this one is rather weak), but it would=
still retain a wing-like morphology. We would also see a loss of the crest=
in the pterodactyloids, since there would be no value of keeping it. We=
would be most likely to spot these on islands, where the pressure to fly is=
not as great (using the dodo as an example).