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Re: Aurornis, new Jurassic avialan dinosaur from China
>From the Abstract of the _Aurornis_ paper
> The recent discovery of small paravian theropod dinosaurs with
> well-preserved feathers in the Middle–Late Jurassic Tiaojishan
> Formation of Liaoning Province (northeastern China) has challenged the
> pivotal position of Archaeopteryx, regarded from its discovery to be
> the most basal bird. Removing Archaeopteryx from the base of Avialae
> to nest within Deinonychosauria implies that typical bird flight,
> powered by the forelimbs only, either evolved at least twice, or was
> subsequently lost or modified in some deinonychosaurians.
Or... that "typical bird flight" only evolved in derived avialans,
post-_Archaeopteryx_. In which case, it doesn't matter where
_Archaeopteryx_ sits in maniraptoran phylogeny (i.e., closer to
deinonychosaurs or closer to extant birds). Either way, if powered
flight evolved late in avialan evolution, then _Archaeopteryx_ and
deinonychosaurs evolved prior to the origin of true flight.
> Thus our phylogeny is entirely consistent with the presence
> of a tetrapterygian condition (= four winged) and elongated rectrices
> in basal eumaniraptorans. We also postulate a single origin for typical
> forewing-powered flight, [snip]. These relationships are also consistent
> with the recent discovery of potentially four-winged flight surfaces in
> a range of Mesozoic basal birds.
Are "four-winged flight surfaces" necessarily consistent with "typical
bird flight"? This is something that deserves further study, and
should not simply be assumed. Those "wings" on the hindlimb might
have been used for maneuverability/control, until they were supplanted
by aerodynamic tail-fans. If basal eumaniraptorans had four wings,
and couldn't fly... how sure are we that four-winged basal birds like
sapeornithids and confuciusornithids could fly?
Matthew Martyniuk <email@example.com> wrote:
> I'm also noticing an increasing trend to call avialans, rather than
> Aves/the Archaeopteryx node, "birds". Which I guess is slightly less
> arbitrary than the Archie node. But I wonder if this is partly behind the
> desire to constantly re-define Avialae, making sure nothing with a sickle
> claw is let into the club. i.e. Velociraptor and Troodon must never be
> called birds, and Archaeopteryx must always be called a bird, so we need to
> re-define the clades in question with every new analysis that shifts their
> position (see also the recent Agnolin and Novas phylogeny which re-defined
> or re-named half the basal paravian clades!).
I agree. There is a discomfiting typological aspect to all this.
Sometimes Theropoda gets treated as a "bird factory", and every new
feature that pops up in maniraptorans gets viewed as related to flight
(or some kind of pre-flight aerial locomotion). Here's a case in
point, from the accompanying news article...
... _Aurornis_ is said to "probably used its wings to glide from tree
to tree". I don't see why. The skeletal anatomy says it lived on the
ground. It's only the phylogenetic placement of _Aurornis_ in Avialae
that leads to the presumption that _Aurornis_ lived in trees.
(Because the origin of flight must have been "trees-down"... right?
Who says? Not me.). Fortunately, the paper itself steers clear of
any ecological conclusions regarding the origin of flight.