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Re: Genes show Neoaves branching before K/Pg extinction



David Černý <david.cerny1@gmail.com> wrote:

>> We show that rheas are sister to the kiwis, emu and cassowaries, and confirm 
>> ratite paraphyly
>
> Polyphyly. Ratites could be paraphyletic only if the last common
> ancestor of all paleognaths were a ratite, too -- a large flightless
> bird with a keel-less sternum.


This topology, with tinamous nested inside traditional ratites, likely
means that the ratite morphology (especially characters associated
with flighlessness) arose multiple times in Palaeognathae.  If so,
instead of Ratitae being the sister group to Tinamidae, the Ratitae is
now paraphyletic relative to the Tinamidae.  This is probably what the
authors meant.


>  In that case, tinamous would have to be
> secondarily volant, and that is a bold proposal indeed to put it
> mildly.


Apparently it's happened in phasmids:

Whiting et al. (2003) Loss and recovery of wings in stick insects.
Nature 421: 264-267 doi:10.1038/nature01313


So it can happen in at least one group of insects.  But could it
happen in birds?  Could something like the modern kagu (which has
muscles that are too weak for powered flight, but it still glides)
give rise to powered fliers millions of years in the future?


Let's say that the most recent common ancestor of crown palaeognaths
(ratites + tinamous) was indeed secondarily flightless.  If most of
the flight equipment was there, but "downgraded" (reduced sternum and
carina, shorter wings, enlarged scapula-coracoid angle, etc), perhaps
it could be re-mobilized in order to be fully volant again, if
circumstances changed.  (Meanwhile, other palaeognath lineages simply
continued to lose their flight-related characters, until they reached
the point of no return.)


Of course, multiple losses of flight by ratites does seem more likely
than tinamous regaining the ability to fly from a flightless ancestor.
 But as unlikely as it seems, we just don't know enough about tinamou
evolution to automatically discount the possibility that modern
tinamous are secondarily volant.









Cheers

Tim