Re: Pygostyle-like structure from Beipiaosaurus

["Jaime Headden" <ja_headden@qilong.8m.com>]

David Marjanovic (david.marjanovic@gmx.at) wrote:

<However, those of *B.* and *N.* are apparently equally extensive -- 5 fused 
vertebrae.>

  Hmm. In *Nomingia,* there are only four sequential vertebrae in its 
pygostyle. In*Beipiaosaurus,* the specimen in question has
only four complete vertebrae with what appears to be a fifth at the end that is 
not complete, so four is the basic common value, not
five.

<In *B.*, the 2 preceding vertebrae are also fused to each other.>

  This might be a pathology associated with stress: there is a tubercle on the 
margin between centra, quite below the position where
the transverse process is on other vertebrae, suggestive that the fusion of 
elements occured as a pathological response to physical
stresses in the tail or injury, as occurs in some sauropod tails and in 
*Tyrannosaurus.*

<The sutures are less visible than in *N.*. And based on the apparently 
separate last sacral, the specimen may be immature, so
perhaps the pygostyle would still have grown. *Caudipteryx* is mentioned as 
lacking a pygostyle.>

  Which is a good thing; however, it may have an [unfused] mechanical 
equivalent of a pygostyle, in that the last few vertebrae do
appear to form a stiffened element, even if they are unfused. This has been 
raised before as what the difference is between a
pygostyle and a pygostyle-like structure, i.e., do the vertebrae have to be 
fused for form a stiffened, pygostylar complex? If the
answer is no, then *Caudipteryx* and dromaeosaurids have a "pygostyle."

<I don't know if the preparation is good enough to tell if there were 
rectrices. According to the text, the preservation may be too
bad anyway.>

  The slab preserves a large set of fibrous complexes that (despite Ruben et 
al.) cannot be anything but extra-integumentary
structures like "hair." These are composed around the tail terminus unevenly, 
and may have formed a sort of fan, even if it wasn't
stiffened with rachides....

  Hmm ... just to note: the tail terminus of the slab is preserved out of 
series than with the remainder of the tail, and is at an
angle of around 40 degrees to the proximal segment of the tail, suggestive of 
two things: really good dessication (possible) and
that it belongs to another individual than the rest of the tail (maybe less 
likely, as only really small taxa seem to preserve
together in the lake sediments...).

  Cheers,

  Jaime A. Headden

  Little steps are often the hardest to take. We are too used to making leaps 
in the face of adversity, that a simple skip is so
hard to do.  We should all learn to walk soft, walk small, see the world around 
us rather than zoom by it.

  "Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)