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Re: More on Argentavis



--- Tim Williams <twilliams_alpha@hotmail.com>
schrieb:

> 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." 

Problem is that basically each dataset provides a
different tree. Even different sequence-based trees
don'tr eally agree well. Also, fossil evidence beats
inference from any kind of data hands-down. It's rare
that a fossil say "ain't so", but if it does, that's
that.

Any hypothesis must deal with _Palaeotis_ and possibly
_Remiornis_, the lithornithids, etc (even if not
ratites, they're too close for comfort). Ideally, it
must also find a place for _Limenavis_ and deal with
assorted touch-me-nots like  USNM 336103, _Wyleyia_,
_Paleocursonis_, etc (the latter may be easy as it
could just as well be non-avian).

> 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.

Yes, these are the guys I thought of (+ _Leipoa
gallinacea_ though I'm not 100% sure whether this was
flightless - at least in females). The island
megapodes example is interesting, because there is a
narrow size range containing flying and flightless
forms. It seems to boil down to what evolved faster:
wing reduction or size increase. FWIW, megapodes are
hyperprecocious and able to fly for hundreds of meters
after emerging from the mound. Obviously an
autapomorphy, but not one that would evolve easily in
a lineage already prone to becoming flightless.

In any case, in _L. gallinacea_, _Megavitiornis_ and
_Sylviornis_ we have evolution towards giant size and
flightlessness on large to largish landmasses and in
the presence of terrestrial predators (mekosuchine
crocs and squamates, and in Australia mammals), all in
Subrecent times, which is decidedly odd. The ancestry
of _Sylviornis_ would surely shed light on this (and
other) issues (as some osteological features are quite
bizarre) but sadly, the hypodigm is zero.

(As an aside, many waterfowl are seasonally flightless
due to their moult pattern. May or not be relevant to
this case insofar as it means that they have to deal
with flightlessness when they are still
ontogenetically able to fly)

Eike


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