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Re: Oviraptorosaur tail forms and functions



Denver Fowler <df9465@yahoo.co.uk> wrote:

> .. And given the very short metatarsus, that would make the legs of
>  Velociraptor & Deinonychus (et al) even shorter functionally. Did they have 
> any
> stride length at all, or just hopped.


Yes, because the effective hindlimb of birds is typically limited to
below the knee, the distal hindlimb (i.e., below the knee) tends to be
more elongated in order to maintain stride length.  It was on the
basis of its elongated distal hindlimb (plus some dodgy 3D modelling)
that _Caudipteryx_ was interpreted as having a forward center of mass
and avian-style stride generation (Jones et al., 2000).  If
_Velociraptor_ and _Deinonychus_ (et al) had a knee-based locomotor
style, the proportionally short metatarsus would make stride length
very short indeed!


There are other osteological correlates of avian-style knee-based
(rather than hip-based) stride generation.  Firstly, in modern birds
the femur tends to be proportionally shorter and more robust, in
keeping with its new role (Christiansen and Bonde, 2002).  Secondly,
an ossified knee cap acts to stabilize the bent knee (Makovicky &
Zanno, 2011).  Neither of these characters are evident in non-avialan
theropods - including deinonychosaurs, _Caudipteryx_ and
_Archaeopteryx_.  IMHO _Confuciusornis_ is the most basal known
theropod to show evidence of having an avian-style posture.


David Marjanovic <david.marjanovic@gmx.at> wrote:

>> However, I wonder if the tail was a little 'over-engineered' for display
>> purposes alone.
>
> I agree, but...
>
>> Perhaps the tail was used as a weapon instead, and swung
>> side-to-side to clobber any predators that came too close.
>
> In that case I'd expect it to be more like in ankylosaurids: with an 
> elongate, stiff "handle" and an actual club instead of a mere pygostyle.


The ankylosaurid tail is extremely specialized.  The sauropod
_Shunosaurus_ also has a tail club, but no specialized "handle".  In
any case, I wasn't picturing the oviraptorosaur tail as a specialized
weapon, more as a crude way of fending off small(er) predators that
got too close.  If so, the fusion of the distal caudals might be a way
to reinforce the tip of the tail, rather than being a weapon in its
own right.


I know this idea is highly speculative.  But I'm not wholly convinced
that the oviraptorosaur pygostyle necessarily served to anchor an
elaborate array of tail feathers.  Fusion of the most distal caudal
vertebrae might simply be a byproduct of the truncation of the tail.
Similarly, there is no evidence that pygostyle formation in basal
avialans was initially associated with attachment of a tail fan -
although the pygostyle was certainly recruited for this purpose in
more derived avialans.






Cheers

Tim