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



Do the feet have to actually grasp (i.e., more or less conform to the
contour of) a tree trunk to provide purchase? I'm thinking of bears -
admittedly not a great analogy, anatomically - but so far as I know,
they climb by pressing the forepaws inward toward a tree trunk in a
clamping action, while using the hindpaws to scramble up by digging
the claws into the bark. IIRC their hind feet don't need to rotate
much in order to provide some of the thrust while the forepaws do the
vice-like grasping that provides stability. I would think a small
maniraptoran could more easily hook its hind claws into nooks and
crannies in bark than a large mammal ever could.

Just an informal thought that seemed like it might be worth sharing.

On Mon, Sep 23, 2013 at 3:00 AM, Tim Williams <tijawi@gmail.com> wrote:
> _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.