Remember this? When I posted it I thought it might be a dinosaur tooth, but several commentators (Petern, Avalus, kestrel, Jazzlet) suggested it might be coral of some sort. It was Oggie, though, who took the time to look it up and told me it was

 think it is a rugose coral. Middle Ordovician to late Permian. Yet another victim of the PT extinction event – comment section Is this a Dinosaur Tooth?

Well, Oggie was absolutely right. I sent the photos off to The Royal Ontario Museum and they concur. Although they can’t say with certainty without seeing the piece in person, they suggest that it is horn coral, of the order rugosa from the Ordovician period. Mystery solved!

Thanks to everyone for your help and suggestions.