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Re: Moa-Tinamou Clade Found Within Ratites



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


> Another reason is that the biogeographic distribution of paleognaths
> in combination with the relatively young age of the clade (supported
> by several molecular dating analyses)


I admit to being confused on this point.  If neognaths were present at
the end of the Cretaceous (e.g., _Vegavis_ and _Austinornis_, both
regarded as galloanserans), how can palaeognaths be a "young" clade?
Palaeognathae must be at least as old as these.  I may be
misunderstanding what you mean by your reference to a relatively young
age for the palaeognaths - if so, I apologize in advance.


> requires extensive transoceanic dispersal that flightless birds wouldn't be 
> capable of.


This requires not just the ability to fly; it also requires flight
abilities good enough for long-distance overwater dispersal.  This
includes the ancestors of kiwi and elephant birds.

The Miocene apterygid _Proapteryx_ is based on an incomplete femur and
a tentatively referred quadrate - not much to go on in terms of
whether it was volant or not, except to say that the inferred size of
this bird is consistent with it being volant.  But it's possible that
the kiwi lineage (Apterygidae) was always small, and became flightless
at quite a small body size.  (Interestingly, _Proapteryx_ lived
alongside crocodilians and terrestrial mammals in early Miocene New
Zealand.)


> Johnston
> (2011) claimed that the Notopalaeognathae topology is compatible with
> a strictly vicariant scenario (although he still favored multiple
> losses of flight over a single re-acquisition of flight in tinamous),
> but his hypothesis relied on incorrect positions of moa and
> elephantbirds and an unrealistically old estimate for the age of
> paleognaths.


I think we have to be careful here.  I'm reluctant to use words like
"correct" and "right" in the context of any phylogeny - including
molecular-based phylogenies.  We cannot "know" the true phylogeny - we
can only use the evidence available to try and discern the
phylogenetic signal.  On that score, molecular-based analyses are no
silver bullet.  They have their own problems, and are underpinned by
assumptions that in the future may be shown to be wanting.  This
applies both to the gross topology and to the divergence dates
('molecular clock') of a phylogeny.  Even retroposon insertions have
their bugs (e.g., Churakov et al., 2009; 10.1101/gr.090647.108 w.r.t.
placental evolution).

Long story short: I don't think palaeognath phylogeny is set in stone.
 This includes the position of tinamous vis-a-vis ratites.  I have to
say that, based on the corpus of evidence, that it looks increasingly
like multiple losses of flight occurred across Palaeognathae.  But one
or two key fossils could confirm or overturn this hypothesis.


> It may be even more important to find out where lithornithids -- some
> of which were highly volant -- belong within the (pan-)paleognath
> tree. Dyke & van Tuinen (2004) and Worthy & Scofield (2012) recovered
> _Lithornis_ as a stem-ratite and a stem-paleognath, respectively, but
> since they also found monophyletic "ratites", their results probably
> can't be trusted.


This seems to imply that no morphological-based analysis can be
trusted, at least where palaeognaths are concerned.  If so, and the
shared similarities between "ratites" is a consequence of convergence,
then we need basal/volant members of the individual lineages to
recover palaeognath interrelationships using morphology.


> Johnston (2011) and most recently Mitchell et al.
> (2014) supported a sister-group relationship between _Lithornis_ and
> tinamous, but I wonder if that's not just a result of the high degree
> of overall similarity between the two taxa.


That's okay if the high degree of shared similarity is synapomorphic.
If _Lithornis_+Tinamidae are nested inside the ratites, then it makes
it less likely that the shared similarities are plesiomorphic for
Palaeognathae.






Cheers
Tim