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Matt Troutman wrote:

<True enough - of course no living bird has a long flexible tail. I
wonder how rigid the tail of Caudipteryx was?

  From the paper:
 "Almost two-thirds of the tail of NGMC-4-A is preserved as a straight
rod, but the vertebrae are not fused. The first six haemal spines are
elongate, rodlike structures. More posterior haemal spines decrease in
height, but expand anteriorly and posteriorly.">

  Tails of some of the Yixian dino-birds and one dino
(*Sinosauropteryx*) show that the flexibility was very limited. An
extreme case of rigor mortis is seen in two of the three specimens
where the tail is flexed almost parallel to the dorsal column, and one
where it is flexed so far as to overhang the skull, and in all cases
show that the base is very flexible, but becomes very stiff very fast,
much like a rod. *Coelophyis* skeletons in rigor mortis show the tail
quite flexed down it's length.

  Now, you addressed *Caudipteryx*, but a similar scenario is favored,
rigor mortis occurs in both specimens. That series of 12 vetrebrae (in
both specimens) shows that the centra are very well locked against
each other, enough to retard the disaccociation that occured in the
base vertebrae, flexible as Sino's and Coelo's. Meaning, in my
opinion, that the tail was stiff in life, at least for the caudal
half, while the cranial half was mobile, in greater degrees the
farther anterior you get.

  In reference to the haemal arches, these are morphologically similar
to the haemal arches of dromaeosaurs, where the rod-liek bones were
associated with more mobile, anterior vertebrae, and the skid-like
ones with the stiff posterior vertebrae.

  Oddly, the tail of *Protarchaeopteryx* is double-curved, and shows
the posterior caudals were mostly stiff, but capable of bending to a
limited degree.


  My opinion on the tail, anyway, is that the stiffened caudal portion
was the form that led to the pygostyle, with a middle scenario played
out in some unknown taxon who will have caudals fused together in a
manner similar to the mid-tail caudals of diplodocid sauropods.

Jaime A. Headden

Qilong, the website, at:
All comments and criticisms are welcome!

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