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_Sinosauropteryx_ fibers

As I visited the San Francisco Zoo today, I was struck by a brief
discussion of polar bear fur anatomy.  I'd actually encountered this
information before, but now I had _Sinosauropteryx_ and _Mononykus_ in
mind, so I began to make a hypothetical connection.

Polar bears, it seems, have black skin and clear, hollow fur.  The
transparent fur conducts the warmth of the arctic sunlight to the
heat-absorbent black skin, much as man-made fiber optics convey light from
one end of a transparent fiber to the other.  So polar bear fur is cold on
the surface, but the animal's skin is warm.  Moreover, the hollow fibers
trap air inside, providing additional insulation.  I find these adaptations
most remarkable, but it is the implications of the "hollow fibers" that I
will discuss here.  (Unfortunately, I have no references; please correct me
if I'm misrepresenting the functional morphology of polar bear fur).

Such hollow fibers bring to mind the fibers preserved on the
_Sinosauropteryx_ fossils, which were interpreted by Philip Currie to have
been hollow structures (based on photomicroscopy of cross-sectioned
fibers), as he attempted to demonstrate on October 10, 1997 at the annual
SVP meeting in Chicago.  He projected a slide of a fossil fiber
cross-section, which looked something like a donut.  Perhaps the simplest
feathers began as something roughly analogous to polar bear fur, an
integument composed of hollow fibers which held air inside for the purpose
of insulation.  Such a stage would have been advantageous to small
endothermic theropods, and would appear to be a plausible first step toward
the much more complex feathers of flying birds.  It may be significant that
the fibers are found on <this> animal, one of the smallest (non-avian)
dinosaurs known.  I am further intrigued by comparisons with the fibers
found on the diminutive _Mononykus_, but, having missed Mary Schweitzer's
SVP presentation, I will leave it to others to comment.

Ralph Miller III <gbabcock@best.com>