I guess this is developing into the show your bison skulls thread. I have one that I traded a laptop for. It was dug from a slough in S. Dakota in the 60's. The archy who dug it and traded it to me said it had been injured in a fight with another bison that punched a hole in it's skull that eventually killed it.
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I have an alternate theory. The puncture wound is the perfect shape and size to have been caused by an iron arrowhead. It didn't initially kill the animal as there is an area of the skull surrounding the wound that shows signs of a massive infection that may have eventually killed it. I suppose a carbon date could determine if it's too old to have been hit by an iron arrowhead.