I've tentatively identified this fossil as a lambeoceras. It's from an Ordovician limestone of the Red River formation. Unfortunately, I can't find much in the way of information on the lambeoceras! Anything in terms of environment, diet etc would be greatly appreciated.
The fossil in question (quite a large bugger, I might add): [img=http://img120.imageshack.us/img120/5576/uplambiodk6.th.jpg]
The fossil in question (quite a large bugger, I might add): [img=http://img120.imageshack.us/img120/5576/uplambiodk6.th.jpg]