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Re: Genes show Neoaves branching before K/Pg extinction
David Černý <email@example.com> wrote:
> Yes, but that's exactly what I meant, too: if the ratite morphology
> originated more than once, the group diagnosed by the possession of
> that morphology is polyphyletic (it's a union of several monophyletic
> groups) rather than paraphyletic (a monophyletic group minus another
> monophyletic group).
The diagnosis of this ratite+tinamou clade (= crown palaeognathans)
depends on the topology (unless it has an apomorphy-based definition).
If the ratite morphology originated more than once, then this
morphology cannot diagnose the clade.
> Paraphyly and polyphyly cannot be distinguished
> using a tree topology alone. If you don't want to speculate about
> ancestral states, all that you can say is that ratites are
I would have thought you could indeed distinguish paraphyly from
polyphyly based on topology alone. The ancestral state is inferred
from the topology. The topology is itself based on the distribution
of character states among the included taxa.
> It could have happened (in fact, Phillips et al. 2010 showed that
> almost all ancestral state reconstruction methods, including the
> probabilistic ones, favor this scenario) -- it's just extremely
> improbable. Only three groups of vertebrates are known to have evolved
> flight separately, whereas the loss of flight has happened at least
> once in every major group of birds (paleognaths, galloanserines,
> "aquatic birds", charadriiforms, and landbirds), with several hundred
> independent losses in Rallidae alone (Steadman 1995).
I don't disagree with you here. In the case of tinamous, if they are
nested inside traditional ratites, it does seem more likely that
flightlessness arose several times in crown Palaeognathae.
David Marjanovic <firstname.lastname@example.org> wrote:
>>> 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?
> I think so, but that would require an absence of competitors and rather
> weird selection pressures.
Any weirder than what gave rise to flying avialans in the first place...?
>> ratite clade, and the same might be true for the South American
> You don't think *D.* is a rhea?
According to one study (Alvarenga, 2010) _Diogenornis_ is closer to
casuariids than to rheids.