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RE: Avian origins: new analysis confirms ancient beginnings



--- Janet m vandenburgh <van02@cox.net> schrieb:

> http://www.physorg.com/news121444505.html
> 
> Avian origins: new analysis confirms ancient
> beginnings
> 
> Did modern birds originate around the time of the
> dinosaurs' demise, or have
> they been around far longer? 

"...suggesting that they arose more than 100 million
years ago, not 60 million years ago, as fossils
suggest."

I wonder where they dug out *that* gem... they did
have to dig quite deep in any case. Even BCF can hold
more in its favor...

And even BMC states: "DNA points to missing bird
fossils"

But we don't know whether they *are* missing. And as
the news article *does* point out, it is unlikely that
we would easily recognize them as what they are, if
it's not nearly complete associated skeletons.

Supposing _Gallornis straeleni_ was neornithine (which
at present cannot be falsified) - I think there is no
mol-clock study that would withstand modern scrutinity
that suggests *that* early an age for the split.
(Actually, it is so fragmentary and little-suited to
large scale phylogenetic morph analyses that the
mol-clock data is at present the most practical
argument *against* it being neornithine).

And of course - since enantios split off from their
LCA with modern birds more than 125 MA ago and much of
what diverged in the latter's lineage basal to
Neornithes almost certainly also split off not too
late thereafter, the fossil record gives a window of
about 120-90 MA where the origin of Neornithes can be
placed with much comfort. May be earlier, may be
later, but eneither (especially the latter) is very
plausible at present.

In other words: the fossil record says "older than 100
Ma but not *very* much so? Pretty damn likely." We
apparently have a fair Mesozoic sample - of odds and
ends, but still - of most Neornithes the mol data says
we should have.

Yet this is a most interesting paper in that the
"Consensus" phylogeny has a few key points, and I
would have liked to see this dwelled upon more than
some flimsy straw man about a fossil record that may
just as well be unrecognized and a time of origin that
is at least not contradicted by the recognizable
record.

Cypselomorphs and Mirandornithes are recovered, as
well as the loon-tubenose-penguin clade (which ties in
ever so nicely with the fossil record) as well as a
"passerimorph" (or whatever) group containing
woodpeckers, rollers, etc. but not cuckoos. A
Metaves-Coronaves split is not recovered; rather here
is a basal polytomy among Neoaves which as certainly
as anyone can be certain about this must predate the
K-Pg boundary. Charadriiformes are not the basalmost
crown clade among Neoaves as they are in several other
studies.

Cypselomorphs stand out as oddballs (and
Falconiformes, which clade with Charadriiformes -
which IIRC does *occasionally* happen). In the latter
case, it is known that accipitrids are genomically
aberrant which may make their rate of molecular
evolution hard to compute; they certainly have a nasty
tendency to be not stable in molecular phylogenetic
analyses, except when they wind up in a polytomy
rather basal among Neoaves.

(And yet - the charadriiforms stand apart from all
other "higher waterbirds". As usual.)

For the cypselomorphs, something similar may apply;
they are unique among all major crown avian clades for
having a metabolism that (in extratropical taxa at
least) can switch to torpor mode[*] by default. The
range of conditions under which for example the
mitochondrial enzymes in a Ruby-throated Hummingbird
can function is very extreme among birds, probably one
of the most extreme among homoiotherms in general. It
is hard to imagine that this has *not*
influenced/restricted their evolution in some way.

It looks pretty robust all-around and may well be one
of those brilliant ones. Nowhere until now was the
fact that morphological distinctness postdates
population divergence postdates lineage divergence so
clearly illustrated. And much of the "supraordinal
clades" (or whatever you want to call them) gets
resolved amazingly well. Their interrelationships far
less so, but the charadriiforms, the
tubenose-and-allies group, etc had the K-Pg bottleneck
to cope with, which is likely to have screwed up
things to a point where the true phylogeny is hardly
recognizable on the molecular level.[**]

The provisional PDF version is here:
http://www.biomedcentral.com/content/pdf/1741-7007-6-6.pdf

Eike

[*] As far as I have grokked it, torpor metabolism is
a rather derived spinoff of poikilothermy, which
itself is plesiomorphic among amniotes as a whole
however and almost certainly was not present anymore
(except in hatchlings) even before Neornithes. It is
not a neoteny or an atavism in the strict sense (it
seems) as it is not simply the hatchling metabolism
being retained or the "reptile" metabolism reemerging;
hummers, swift, nightjars etc can switch to
poikilothermy if need be, but they are generally
homoiotherms, in the case of hummers with a very
fast-paced and robust metabolism operating at
extremely high (for an amniote at least) temperatures.

[**] That the tubenoses-and-allies still clade
suggests that either their internal divergence just
postdates the K-Pg boundary or that whatever the
bottleneck was, it was not strong enough to obscure
the relationships at least of *these* lineage. This is
a good candidate for the next major neoavian clade to
gain widespread recognition.


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