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Re: Oviraptorosaur tail forms and functions



Anthony Docimo <keenir@hotmail.com> wrote:

> I would think that oviraptorosaurs wouldn't need a large fan-tail...as their 
> heads already make an ideal display structure.   (On the other hand, having 
> the head and a fan-tail both being
> display structures, might provide the same sort of advantage that eyespots 
> and false heads serve in fish and reptiles)


Both bony and integumental structures offered the opportunity for
display, so many oviraptorosaurs likely employed both.  Besides, some
oviraptorosaurs lacked cranial crests (e.g. basal oviraptorosaurs,
_Conchoraptor_, _Ingenia_).


> Perhaps additional comparisons are needed?  You mentioned three groups within 
> the Therapoda who have a pygostyle - does the feature appear outside the 
> Theropoda?
> (from what I remember last I saw a specimen in a museum, ankylosaurs have a 
> similar-looking structure in their own tails; yes/no?)


The pygostyle is only found in maniraptorans.  But even here it has an
erratic phylogenetic distribution: one therizinosaur
(_Beipiaosaurus_); a couple of oviraptorosaurs; and birds in the clade
Pygostylia (which includes modern birds).  In modern birds the
pygostyle serves as part of a sophisticated apparatus to control the
tail fan, and is critically important in flight.  Within pygostylian
birds, an aerodynamic tail fan is limited to the line that includes
modern birds (Ornithuromorpha).  Enantiornithes have no aerodynamic
tail fan (apart from _Shanweiniao_, which might have had a sort of
tail-fan), with some enantiornithean birds having streamer-like tail
feathers (as did confuciusornithids), or no apparent rectrices at all.


I wonder if we're according too much importance to terminal fusion of
the tail in many theropods.  In most non-ornithuromorph birds, the
pygostyle does not appear to have served any specific function;
terminal fusion of the tail vertebrae might actually reflect a *lack*
of function.  I see no reason to assume that the presence of a
pygostyle in oviraptorosaurs automatically indicates the presence of a
tail fan.  Although derived ornithuromorphs have a pygostyle that
supports a tail fan, here it serves a highly refined aerodynamic
function.  In other pygostylian birds, there is no evidence that the
pygostyle is associated with a tail fan.  It may be that terminal
fusion of the vertebrae was initially adventitious, and associated
with overall shortening of the tail skeleton.  Ditto for
oviraptorosaurs and _Beipiaosaurus_.


I'm happy to be proved wrong on this.  Such as if a _Nomingia_ or
_Citipati_ skeleton turns up with a magnificent tail fan!






Cheers

Tim