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Re: BAAD BCF



GO writes:

>Predator-scarce might be the best description. As far as I know, ratites were
>secondarily flightless (a theory disputed by a few ornithologists). The
>impression I get from examining the fossil record of large, flightless birds
>is that they appear rather suddenly without obvious ancestral forms, hang
>around for a while, then become extinct; and that the various groups of
>fossil forms are not particularly closely related to one another above the
>family level. To me this indicates sporadic, rapid-fire evolution in isolated
>environments. The absence of ancestral forms is evidence that they were small
>and not easily fossilized. As Greg Paul pointed out, this idea extrapolates
>nicely back into the Mesozoic to the theropods.
>

I'm not sure I accept this.  First of all there is no evidence that
ostriches (which may have evolved in Central Asia as Feduccia describes in
"The Age of Birds"), rheas, or emus (not to mention phorusrhacids,
diatrymids and dromornithids among non-ratites) evolved anywhere but on
continental land-masses with extant large terrestrial predators - do you
have any evidence that these areas were "predator-scarce"?  Also we
certainly have enough material of phorusrhacids, dromornithids and (if we
accept Feduccia's scenario) ostriches to show at least some evolutionary
trends such as increasing size etc.  Of course in the first case we even
have living volant survivors (or at least close relatives) in the form of
the seriemas (Cariamidae).

I suspect that in at least some of such cases the evolution of large size
and cursorial ability (as in living birds such as the larger bustards) may
have reached a stage before the loss of flight such that the evolution of
still larger size and further cursorial (or graviportal, or even predatory)
adaptations became more selectively advantageous than the retention of
flight which might have made such adaptations impractical.  This is quite
different from the scenario of insular release that has been used to explain
most cases of flightlessness in recent times (eg flightless insularrails,
ibises etc.).
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
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