Re: More on Argentavis

[Tim Williams <twilliams_alpha@hotmail.com>]

Michael Habib wrote:

> Granted, ostriches appear to be well above the maximum mass for a volant 
> bird (though not the limit for volant vertebrates as a whole, see 
> pterosaurs...), but many ratites are not.  Am I mistaken in saying that the 
> evidence suggests an increase to ostrich size *after* loss of volancy, 
> rather than a loss of flight in the specific lineage leading to ostriches 
> as size reached maximum launch limits?

I had the same thought.  Ratites are almost always recovered as a 
monophyletic group, and the most parsimonious explanation is that 
flightlessness is primitive for the clade.  Given the flight abilities of 
the tinamou, it may be that overall poor flight ability is primitive for 
palaeognath birds.  I agree that loss of volancy almost certainly occurred 
prior to an increase to ostrich size.

The basal part of the neornithean tree seems especially prone to both loss 
of flight and impressive increases in body size; as well as ratites, we also 
have dromornithids and gastornithids (both regarded as galloanseraeans, or 
at least basal neognaths).  It could be some ecomorphological 
pre-disposition to loss of flight + increase in body size; or it could just 
be opportunistic (i.e., lots of available ecological niches at around the 
same time); or just coincidence.

Cheers

Tim

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