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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