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Re: More on Argentavis



Evelyn Sobielski wrote:

Ratites are a peculiar case, their flightlessness is
probably very highly convergent (based on place and
time of fossil record of paleognaths).

There is some disagreement on this. However, Cooper et al. (2001)'s mitochondrial phylogenetic study support "a Late Cretaceous vicariant speciation of ratite taxa, followed by the subsequent dispersal of the kiwi to New Zealand." This implies that dispersals were land-based, with the ancestors of the ostrich and elephant-bird each crossing the Kerguelen Plateau to Indo-Madagascar and (for the ostrich lineage) on to Eurasia; and kiwi ancestors crossing over to NZ from Australia via the Norfolk Ridge or Lord Howe Rise some time later (65-72 Mya). This means that flightlessness *could* be primitive for ratites, although I agree that it is not direct proof that this was the case.


Reference

Cooper, A., Lalueza-Fox, C., Anderson, S., Rambaut, A. and Austin, J. (2001). Complete mitochondrial genome sequences of two extinct moas clarify ratite evolution. Nature 409: 704-707.

Essentially, ratites seem to be "big chicks", insofar
as that their morphotype seems at least partially due
to neoteny. That is, their acquiring flightlessness
was probably a more rapid process than in other
flightless birds which simply reduced the flight
apparatus.

Yes, that's indeed possible.

The known flightless galliforms (a mere handful of
spp, all subrecent I think) on the other hand
generally became flightless due to size increase.

I can think of only a few extinct flightless galliforms, including _Sylviornis neocaledoniae_ (Sylviornithidae) and the megapodes _Megavitiornis altirostrum_ and (probably) _Megapodius amissus_. _Sylviornis_ and _Megavitiornis_ were certainly pretty darn big. As these were found on Pacific islands (New Caledonia, Fiji), this suggests that island gigantism may indeed be responsible for the large size of _Sylviornis_ and _Megavitiornis_. But they might have become flightless before they became so big, rather than the other way round. _M. amissus_ is about the same size as some flighted megapodes, but the wings are shorter.


Cheers

Tim

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