Large Bone ID Help

[TC82]

I recently bought this piece of fossil bone at a bookstore in Tucson AZ. My guess is that someone had originally bought it at the Tucson gem and mineral show and then sold or traded it at the bookstore.  I don't know anything about its original geographic origin.  I had a friend test it with his Geiger counter and it gives off a fair amount of radiation compared to the background.  I thought it maybe could be a plesiosaur fin bone because of the way it flairs out but I honestly have no idea.  Any help identifying what species this fossil bone came from would be greatly appreciated.  

[Harry Pristis]

Scraps of bone don't typically show up at the Tucson shows because of low value/high transport cost.  This reminds me of a bovid proximal metatarsal, but who can say for sure!

 

 

[TC82]

I can definitely see the resemblance.  I found a picture of a bison antiquus metatarsal and I put them at same scale side by side.  The one I have seems larger and less symmetrical where it flairs out at the bottom.  Any chance it could be a bone from another part of the bison, or could it be some kind of dinosaur metatarsal?  Also are bison fossils typically radioactive?  It was fairly radioactive at approx. 300 counts/min.