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diagnostics of Theropoda



 
 
A few years ago, a rancher in eastern Montana asked me to identify a 
fragment of a dinosaur bone that he had found on his ranch, which is 
situated on the Hell Creek formation.  I was in a hurry to leave, but I 
looked at it, took a few notes, and off-handedly said that it is the distal 
end of a phalanx of a theropod.  I forgot about the incident until recently,
when a friend and I were talking about what would be the minimum amount 
of information needed to diagnose a manus or pedal phalanx down to the level 
of, say, Suborder level.  I made the claim that, with theropods, it is easy,
because theropod phalanges are hollow, and there are usually (but not 
always) co-lateral ligament pits on the distal end of the bone.  Theropod 
phalanges also have well-developed, "pully-like" distal articular surfaces.  
My friend said that the ranchers's bone could have come from any of a variety 
of Hell Creek ornithopods, theropods, or even from the finger of a very large 
croc. Is it possible to unequivocally identify a non-ungual phalanx as comming
from a theropod?  What criteria would be diagnostic?  What about crocodyles?
 
Here are my notes on this phalanx:
 
Transverse width of phalanx at distal end = 31 mm.
 
Dorsoventral width of phalanx at distal end (diameter of "pully's") = 32 mm.
 
Nearly circular co-lateral ligament fossae.
 
Diameter of co-lateral ligament fossae = 16 mm.
 
Depth of co-lateral ligament fossae = 10 mm.
 
Distal articular surface is "pully-like" in shape.
 
Medulary cavity is totally open and is slightly triangular in 
  cross-section.
 
    Any ideas?