Psittacosaurus bristles--description

[bh480@scn.org]

From: Ben Creisler bh480@scn.org
Here are some key passages from the text of Mayr et al. in 
the online article in Naturwissenschaften describing 
the "bristles" on the specimen of Psittacosaurus sp.:

Large parts of the integument are preserved as a thin 
layer of dark carbonized matter. At the shoulder, the skin 
structure corresponds well to that seen in skin 
impressions of the more advanced ceratopsian genus 
Chasmosaurus in that irregular rows of large, round, and 
plate-like scales are separated by numerous small, 
polygonal, and tubercle-like ones. At the limbs and the 
tail only the small tubercle-like scales are visible. ....
The most unusual feature, and the reason for the recent 
excitement among vertebrate paleontologists, is the 
presence of about 100 long bristle-like structures on the 
tail....These structures are restricted to a 235-mm long 
stripe along the dorsal surface of the proximal third of 
the tail (measured from the base of the most proximal to 
the base of the most distal "bristle"), extending over 14-
15 caudal vertebrae. They are not discernible anywhere 
else on the tail nor anywhere else on the specimen. The 
middle parts of the longest ones reach beyond the margin 
of the slab, but their overall length, from the base to 
the tip, can be estimated to be about 160 mm. At their 
base, they are about 1 mm wide; all of the preserved tips 
taper into a point. These "bristles" are preserved as a 
very thin brown layer which is somewhat brighter than most 
of the dark substance of the skin; however, they exhibit a 
completely different texture than the ossified tendons 
which are preserved along some of the tail vertebra of the 
specimen (and which are also found in other 
psittacosaurs). Under ultraviolet light they show the same 
fluorescence as the epidermal scales, which indicates that 
they might have been keratinized. .....
Because of diagenetic compression, the "bristles" are 
completely flat, but we assume that they were originally 
cylindrical and possibly tubular. Most of the "bristles" 
are more or less bent towards the caudal end of the tail 
and if they originally were flat and ribbon-like, one 
would expect to see some twisting of the shaft which 
cannot be observed.
Most "bristles" exhibit a dark stripe of varying width 
along at least a part of their midline, which possibly 
indicates the presence of a hollow lumen inside these 
structures, as in the filamentous structures of some 
theropods. However, the "bristles" of Psittacosaurus are 
oriented in parallel, whereas at least some of the 
filaments of theropods originate from a single point and 
form a radiating spray. Furthermore, they do not show any 
branching, which was recently reported for the filaments 
of the theropod genera Beipiaosaurus and Sinornithosaurus.
Virtually all of the exposed integument of the tail is 
from the right lateral side of the animal, which can be 
deduced from the fact that it is continuous with the 
integument that covers the bones in the abdominal region 
of the specimen. The "bristles" extend under this skin 
layer and nowhere lie above it, which indicates that they 
were attached to the dorsal midline of the tail only.
Careful preparation of some "bristles" revealed that these 
terminate directly at the vertebrae, at the tip of the 
neural spine in one case and at the prezygapophysis in the 
other. Since the more proximal caudal vertebrae have 
shorter and smaller neural spines than the more caudal 
ones, the "bristles" extend further into the skin in the 
proximal part of the tail.