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Re: Molecular clocks and avian diversification



> > A nice way of putting what a biomedical researcher
> > seeking major founding would without doubt have
> called
> > "...will revolutionize our understanding of..."
> 
> Where's the revolution?

Molecular dating finally being based on sound science.
birds are quite state-of-the-art in the matter; other
branches of evo-bio are not really arrived at relaxed
clocks yet.

> Instead, let me doubt the dates. A Permian date for
> the basal divergence of 
> the crown of Archosauria is suspect. 

Most heartily agree.

> An Early
> Cretaceous date for the basal 
> divergence of Neornithes is highly suspect. And so

Not necessarily as deep as suggested, true. But
possible (though not really likely). There might have
been an ornithuran family/clade which was to become
the ancestors of all living birds, and if there were
genera/subclades in it which have distinct living
descendants today, you should get such a pattern.
Neornithes were the mega-wallflowers of Ornithurae
even, until c.80 mya maybe.

> > Well, as I understand it, there was (and and I'm
> > leaving paleognaths out here!) the anseriform
> lineage
> > which existed as very distinct by the C/T boundary
> (as
> > indicated by Vegavis),
> 
> What's so "very distinct" about it?

Vegavis was a full-fledged member of a crown-group
family (which later became extinct due to other
reasons)
 
> > The "Galloanseres" I
> > find weakly supported by evidence at present; they
> may
> > be an artifact of traditional cladistics being
> limited
> > to dichotomous splits* and an underlying ancestral
> > morphology largely obliterated by the odd
> adaptational
> > specialization.
> 
> So you think there was a trichotomous split?!?

Not really, I think that trichotomous vs dichotomous
is a matter of perspective. Evolutionary lineages do
not necessarily have a thickness of 1 as cladograms
might suggest.
At any rate, Galloanserine snyapomorphies could have
a) evolved in parallel (not too likely I guess) and at
any rate are b) obscured by adaptational changes,
making them features somewhat tough to interpret. The
Galliformes are particularly interesting: anything
with a gamefowl lifestyle seems to be so much prey in
the Late Cretaceous ecosystem. There were terrestrial
theropod predators, airborne enantiornithine predators
aplenty.

> I have not yet seen evidence for this. The low
> diversity of "graculavids" --  
> the Cretaceous ones all come from New Jersey --
> doesn't argue for it...

They were not a monophyletic group. And "graculavid"
(as form taxon) birds do not come just form NJ, only
the ones that were placed in the presumed "family"
were (or at least from the general area). Vegavis
would have been placed in the Graculavide (Presbyornis
was).

EG Ciconiiformes (both S/A and traditional) are
apparently paraphyletic, and possibly deeply so.
Paleogene radiation suggests a Late Cretaceous split
as more likely. A "transitional shorebird" ecotype
goes well with either stork or heron (which are
possibly as distant as crown neognaths can be)
ancestors.

Neornithine seabirds from the Maastrichtian ought to
have been found if there was appreciable diversity.
Landbirds, ditto. More material than Tytthostonyx,
Polarornis, and other wretched stuff at least. There
was some diversity of "transitional shorebirds", but
we know they're not wader ancestors, nor apparently
closely related.

> Don't confuse cladogenesis ( = any halfway permanent
> split of a population 
> in two) and speciation. Those aren't the same...
> except under some species 
> concepts...

I don't, but speciation is necessary for cladogenesis.
Any lineage should have passed through a point where
it was a lower-level taxon in the Linnean system, e.g.
a genus or species. Some lineages may never be much
more; Ichthyornis makes a possible case.

> Cladistics is the method to find phylogenetic trees,
> nothing more. It does 
> not tell you how to classify.

Hmmm... a drawing of the evolutionary relationships of
these ducks would help. The result is something that
cannot be represented by a dichotomous tree, not if
you permit trichotomy either. A result that cannot
come from a cladistic analysis, hence: cladistics does
not help in this case.

This is not cladogenesis, granted - but it is a case
for what may happen at nodes because (without further
research) all active speciation events are possible
nodes in a phylogeny.

Assuming A. bavarica was a distinct species and is
correctly assigned to genus (i.e. representation of a
distinct but adjoining or possibly barely overlapping
gene pool), it might have given rise to a distinct
clade of sternal-keel-muscled flyers, whereas A.
lithographica could have resulted in a clade of
coracoid- or whatnot-based flyers, all because of the
small difference in thoracal skeleton.

Or: what were the first-diverged (as opposed to
last-common) ancestors of 2 sister clades? Certainly
living, breathing animals. Sister species? Usually. In
hopothetical descendants of Anas sensu stricto,
anything from vagrants to half a genus (That's what I
meant in the "trichotomy" bit on Galloanseres above:
gamfowl and waterfowl are apparently early
divergences).

Regards,

Eike



        

        
                
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