Re: More on Argentavis

[Tim Williams <twilliams_alpha@hotmail.com>]

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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