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RE: Cretaceous feathers




Eike wrote:


> Well thanks for the "frond" - I have been looking for
> a word to distinguist the moveable tail fans of modern
> birds from what often gets called "fans" but is
> something different, found in non-avians. You are
> right of course about the pygostyle - I was thinking
> of _Nomingia_ which has one - and quite possibly a
> "frond" too, as it was a) close enough to
> _Caudipteryx_ and b) it is hard to imagine for what
> else such a bone should evolve (that would mean that
> the non-avian pygostyle evolved because it improved a
> pre-existing or incipient "frond" by making it
> structurally more robust)


This sounds perfectly reasonable.  However, Gatesy (in the Ostrom Symposium 
volume) believes that the rectrices associated with the pygostyle are actually 
derived from the mid-tail, and did not begin as distal rectrices.  Under this 
scenario, as the tail skeleton shrunk during avian evolution, and the proximal 
caudals were incorporated into the synsacrum and the distal caudals were 
incorporated into the pygostyle, all that was left were the rectrices in 
between.  These rectrices (the only ones left standing after their brothers had 
been eaten up by the synsacrum and the pygostyle) were later incorporated into 
the rectricial fan.


As you said in a previous message, confuciusornithids have a pygostyle, but no 
rectrices.  In fact, it has been argued (by Rayner, I think, and maybe others) 
that the tail of confuciusornithids played absolutely no role in flight.  If 
the origin of the pygostyle has nothing to do with rectricial attachment or 
support, then there's no reason to assume that the "pygostyle" of _Nomingia_  
supported any rectrices either.  The "pygostyle" of _Beipiaosaurus_ does not 
support any rectrices, just the more rudimentary filamentous structures.  So 
this may have been true for _Nomingia_ too.  (BTW, some authors prefer not to 
refer to the fused distal caudals of non-avian theropods as a "pygostyle", and 
reserve the term solely for the derived avian structure.)  


Since _Caudipteryx_ and _Protarchaeopteryx_ have a distal rectricial "frond", 
the formation of a pygostyle-like structure by _Nomingia_ may actually have 
eliminated those rectrices attached to the distal caudals.  In this case, if 
rectrices were associated with the pygostyle in _Nomingia_, then they may have 
originated from more proximal tail segments.  But this doesn't seem to have 
happened in _Beipiaosaurus_, which has neither a rectricial frond, nor a 
rectricial fan.  


Cheers

Tim
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