Re: Oviraptorosaur tail forms and functions

[Tim Williams <tijawi@gmail.com>]

Michael Habib <biologyinmotion@gmail.com> wrote:

> I see what you're getting at, but do we really have a good threshold for
> being "over-engineered"?  Why can't a muscular, mobile tail basically just
> be a display structure?


It may well be just be a display structure.  I hope it is the case
that large oviraptorosaurs sported large and elaborate "fans".  My
argument (such as it is) is that the evidence is not compelling - at
least as far as derived oviraptorosaurs are concerned.  Across
non-ornithuromorph theropods, there is no correlation between the
presence of a pygostyle and a tail "fan" (= a radial arrangement of
feathers).

True, caudipterygids have large distal tail feathers (rectrices) that
appear to have been used for display.  _Similicaudipteryx_ has a
pygostyle; _Caudipteryx_ does not (although the authors argue that the
last 5 vertebrae were "tightly integrated into a functional unit";
Barsbold et al. (2000) call this a "stiffened rod").  So we have
evidence that some basal oviraptorosaurs used the terminal rectrices
for display.

However, the basal therizinosaur _Beipiaosaurus_ has a pygostyle, but
does not preserve any elongate tail feathers - even though filamentous
integument is preserved along the tail.  The individual specimen
appears to be immature - so it's possible the display feathers came
late in ontogeny?  But it's also possible that _Beipiaosaurus_ just
didn't have them.

Many non-ornithuromorph birds (confuciusornithids, several
enantiornitheans) have a pygostyle, but no tail fan - although they do
show terminal feathers (often streamer-like) that appear to have been
used for display, associated with the pygostyle.


>  If it did have a proper tail fan (understood that
> it's uncertain) than maybe brooding, as well, I suppose.  Not trying to be
> too argumentative, but I think it's easy to assume that big powerful
> structures have to be related to feeding or combat, and that's not always
> the case.  Imagine how much musculature goes into moving the display
> feathers on a bird of paradise, for example.


Please, be argumentative.  :-)

I see your point.  But unless a derived oviraptorosaur turns up with
some sort of tail-feather "fan" attached to its pygostyle, I don't
think we can argue that the enlarged caudal musculature and/or the
pygostyle were used for display.  IMHO inferring the existence of a
"fan" in derived oviraptorosaurs, based on the presence of large
terminal rectrices in basal oviraptorosaurs, is tenuous.  This
hypothesis may well be correct.  However, I think the link between a
pygostyle and a tail-feather fan is currently weak.  Fusion of the
distal caudals could have occurred for any number of reasons, rather
than simply serving to anchor a specialized display structure made up
of elongated rectrices.





Cheers

Tim