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Help me identify a mystery vertebra.



 This summer, as part of our fieldwork in Dinosaur Provincial Park, I
collected a most unusual centrum that defies identification, so I thought
I'd throw it out to you all for any ideas you may have. As I have no formal
training, I have difficulty describing things technically so please bear
with me.

 The fossil was collected from Bonebed 50, near the base of the Dinosaur
Park Formation, making it about 76 million years old. The specimen consists
of a centrum only- the neural arch was unfused indicating the immaturity of
the animal when it died (this is confirmed by the "spongy" juvenile bone
texture. The bone surface looks like typical ornithischian bone, it is
definately not theropod (at least any theropod bone texture we've seen).
Measurements: max. height 27 mm.; max. length 19 mm.; max. width 27.5 mm.
Both endplates of the centrum are slightly concave. The neural arch bases
are basically simple unsculptured depressions. The spinal cord depression is
hourglass-shaped, but the "waist" is not very pinched, more columnar. I
cannot see any indication of a rib or chevron facets. There are a few,
small, random blood vessel openings on the sides of the centrum. The ventral
surface is distinctive and may help identify this specimen. I cannot tell
anterior from posterior on the bone so I've arbitrarily picked one end as
anterior. On the anterior ventral midline is a low, rounded, but prominent
keel about 7 mm. in length, ie. it only occurs over a little under half the
length of the centrum. Flanked on both sides of this keel and set back a few
mm. posteriorly are two tear-drop shaped ridges, but with the posteriorly
directed point of the teardrop missing where the ridge blends into the rest
of the centrum. The rounded part of the two teardrop-shaped ridges are set
15.5 mm. apart (centre to centre).

 The only thing this vaguely resembles (but definately is not) is the
cervical centrum of CHAMPSOSAURUS with its ventral keel and two cervical rib
facets. There is little question this is a dinosaur bone, yet nether Phil
Currie or I have ever seen anything like it. In some ways it resembles one
of those disk-like proximal caudal vertebrae found in hadrosaurs, but the
presence of the ventral keel and two teardrop-shaped accessory keels are
baffling. I cannot see these ridges as some complex chevron attachment
point. Since we now know of protoceratopsians coming from the Park, we are
considering it might be from one of those, but we have neither the
comparative material or detailed knowledge to make a final decision. Any
ideas?? Another possibility is pachycephalosasurid, but again we lack
comparative material/know how. I won't be at SVP this fall, but might have
someone hand carry it there for perusal. 

 Many thanks,

 Darren Tanke
    
 Darren Tanke, Technician I, Dinosaur Research Program, Royal Tyrrell Museum
of Palaeontology, Drumheller, Alberta, Canada. 
 Interested in all aspects of centrosaurine ceratopsians, paleopathology,
recent and fossil dento- and osteopathy; senior editor on annotated
bibliography of extinct/extant vertebrate dental pathology, osteopathy and
related topics (9,911 entries as of Sept. 14, 1996). 

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