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Re: A long-tailed bird with a pygostyle



> Lü, Junchang & Lianhai Hou, 2005.  A possible long-tailed bird with a
> pygostyle from the late Mesozoic Yixian Formation, western Liaoning,
China.
> Acta Geologica Sinica 79 (1): 7-10.

Lu kindly sent me the pdf.  It's just a distal tail (12 free caudals, five
fused into a pygostyle, two chevrons).  Not named.  The characters Lu and
Hou use to call it a bird are largely unconvincing.  The last free caudals
are as elongate as enigmosaurs'.  The pygostyle of Nomingia is also composed
of five vertebrae.  The 'pits' on the centra aren't compared to any bird
(and are unreported in Mesozoic birds as far as I know), and are said to be
a possible autapomorphy of this taxon in the text.  The laminar pygostyle
and procoelous distal centra are said to be similar to Iberomesornis, but
the free caudals of that taxon are amphicoelous (Sereno, 2000) and Nomingia
has a dorsal crest on its pygostyle too, though not as tall.  Such tall
crests are also absent in basal avebrevicaudans like Sapeornis and
Confuciusornis though, and this specimen is clearly not a (adult)
pygostylian.  The foramen between the first two neural arches of the
pygostyle doesn't support an avian relationship, since it's not present in
Mesozoic birds (e.g. Sinornis, Iberomesornis), and only reported in
Struthio.

But this doesn't mean it's not a bird.  I entered it into my matrix
(assuming an original caudal count of ~30 or less, as with other pygostyled
maniraptorans) and it came out sister to Beipiaosaurus in 99% of the trees,
and always in Therizinosauroidea.  It codes identically to Beipiaosaaurus
(and other therizinosauroids, if unknown in Beipiaosaurus) except for its
distally procoelous caudals.  It's closer to Beipiaosaurus than to
Neimongosaurus and Alxasaurus based on the elongate distal prezygapophyses.
The pygostyle makes it different from Caudipteryx, while the apparent lack
of transverse processes that far back makes it different from Nomingia.
Birds differ by having short prezygapophyses, except for a juvenile
enantiornithine specimen.  Some young enantiornithines do lack pygostyles
("Cathayornis" caudatus; embryo of Zhou and Zhang, 2004), so a stage with a
small pygostyle like the present specimen is conceivable.  However, even the
largest Yixian bird (Sapeornis) has a tail a third the size of this
specimen, so it being a juvenile bird seems doubtful.  Then again, that
might explain the mysterious pitting.  In conclusion, neither identification
(segnosaur, oviraptorosaur, basal avebrevicaudan) seems particularly well
founded, and I might recommend Maniraptora incertae sedis.

Mickey Mortimer
Undergraduate, Earth and Space Sciences
University of Washington
The Theropod Database - http://students.washington.edu/eoraptor/Home.html