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Re: Yixianosaurus (Theropoda) a basal paravian, not a basal maniraptoran (free pdf)



_Yixianosaurus_ is one of those maniraptorans that is caught in a
tug-of-war over whether the forelimbs were best adapted for predation
or for tree climbing.  The presence of large pennaceous feathers along
the forelimb (pennibrachium) led the authors to conclude that
_Yixianosaurus_ was perhaps capable of some form of aerial locomotion.
 The authors also argue that this feature is consistent with paravian
affinities for _Yixianosaurus_ "given that previous reports of
feathers of this type are restricted to oviraptorosaurs and
deinonychosaurs among non-avialan theropods..."  This no longer holds
true.  I guess the paper went to proof after the publication of a
putative pennibrachium in ornithomimids (Zelenitsky et al., 2012; DOI:
10.1126/science.1225376)


I also am not at all convinced by the argument in favor of
tree-climbing behavior in _Yixianosaurus_:

"The grasping manus of _Y. longimanus_ may have been used in
predation, as suggested by
Dececchi et al. (2012). However, grasping ability would also be useful
in carrying out other
types of behaviour, perhaps most notably arboreal climbing..."


Hmmm.  That depends what is meant by "grasping".  The manus was not
opposable, and was not adapted for one-handed grasping.  This is not a
deal-breaker; after all, many arboreal mammals do not have opposable
hands or feet, and they can grip branches perfectly well.  However,
what makes the paravian manus particularly maladapted for
branch-grasping is that the long fingers diverged during flexion.
This gave the long fingers a wide spread when the fingers 'closed'
around an object.  This makes sense if the paravian was grasping a
large object with both hands - such as big prey.  (Thus, it fits with
the inferred ecologies for dromaeosaurs such as _Velociraptor_ and
possibly _Deinonychus_, based on taphonomic evidence for attacks on
_Protoceratops_ and _Tenontosaurus_, respectively.)  But this type of
manus was ill-suited for wrapping the fingers around small or narrow
objects, such as tree branches.


Now, it may be that the kind of manus found in _Yixianosaurus_ was
suitable for trunk-climbing.  In that case, the trunk would be gripped
with both hands, as the animal scaled the trunk.  This qualifies as
scansorial behavior (in an ornithological sense).  Unfortunately many
studies that argue for "arboreal climbing" in non-avian dinosaurs are
vague on what is meant by "arboreal".  If small(ish) maniraptorans
like _Yixianosaurus_ climbed trees, what was the reason?  Was it to
access the tree crown, such as to climb or roost within the canopy?
Such "arboreal" behavior has been used to explain the presence of
aerodynamic limb feathers in other paravians (such as _Microraptor_),
because it allowed the paravian to glide from tree to tree, or from a
tree branch to the ground.  But if _Yixianosaurus_ did spend much of
its time in the canopy, it would need more than just trunk-climbing
(scansorial) abilities.


On the other hand (no pun intended), if the scansorial/arboreal
abilities of _Yixianosaurus_ were limited to climbing trunks, I don't
understand the purpose of large forelimb feathers in this context.
Trunk-climbing mammals (like colugos and many primates) can glide from
trunk to trunk; but these glides require a take-off impulse, which in
turn requires firm contact between the hindlimbs and the trunk.
Although the hands of certain non-avian maniraptorans have been
described as being adapted for trunk climbing, none have feet adapted
for grasping trunks.  (Admittedly, the hindlimbs are not known for
_Yixianosaurus_.  But I don't expect its feet to be any better at this
than any other non-avian maniraptoran... which is to say, not at all).


In short, I don't see any compelling reason for why the long, robust
arms and grasping hands of _Yixianosaurus_ were not simply adapted for
grabbing prey.  (Okay, they could have been used for other things,
like climbing, but this was opportunistic... not what they were
specialized for).  As for those forelimb feathers, they could have
been deployed for display (including threat postures) or some other
terrestrial behavior.  Whenever a small theropod turns up with
grasping hands and long limb feathers there is a tendency by some
people to regard it as an arboreal glider.  I think this is jumping
the gun.







Cheers

Tim


On Mon, Sep 23, 2013 at 2:22 PM, Ben Creisler <bcreisler@gmail.com> wrote:
> From: Ben Creisler
> bcreisler@gmail.com
>
> The English-language link for the new issue of Vertebrata PalAsiatica
> is not yet updated. However, the Chinese version is. Here is a new
> paper with the pdf link.
>
>
> XU Xing, Corwin SULLIVAN & WANG Shuo (2013)
> The systematic position of the enigmatic theropod dinosaur
> Yixianosaurus longimanus.
> Vertebrata PalAsiatica 51(3): 169-183
> http://www.ivpp.cas.cn/cbw/gjzdwxb/xbwzxz/201309/P020130923388748138363.pdf
>
>
>
> Yixianosaurus longimanus is a small theropod from the Lower Cretaceous
> of western Liaoning Province, China. It was originally suggested to be
> a derived maniraptoran, but this interpretation was challenged by a
> recent study that proposed instead that Y. longimanus was a basal
> maniraptoran. Given that the systematic position of this taxon will
> affect our understanding of such broad issues as the evolution of the
> theropod forelimb and plumage, it is important to carefully evaluate
> both systematic hypotheses and determine which is better supported.
> Here we review all available morphological features in Y. longimanus
> that appear informative with regard to its systematic position. We
> demonstrate that this small theropod is a basal paravian and most
> likely a basal deinonychosaurian, a result that conforms to the
> original interpretation of this specimen. The hypothesis that Y.
> longimanus is a basal paravian is consistent with the probable
> presence of pennaceous feathers in this taxon, and avoids implying a
> complicated evolutionary history for the maniraptoran forelimb.