I have been trying to imagine what the creature looked like that this fossilized jaw belonged to. It is some sort of scolecodont (maybe an Oenonites) and was found east of Louisville, Kentucky at an Ordovician Period road cut.
While I have referred to it as a worm it might be more like a modern lamprey or some sort of eel. If it were a worm, you could take it fishing but not need a hook. The jaws on this animal are quite menacing. So just put the scolecodont on the fishing line and let it grab the fish as they swim by. Okay, I am joking about this.
The picture is not magnified and the fossil is about 5 mm long which appears to be quite large for one of these fossils.
Here is my rough drawing of what a creature would look like. It is totally off base but having not seen any of the fossils of the soft scolecodont bodies I have just my imagination as a reference. My interpretation is the creature was some sort of parasite that latched onto its prey with the grappling jaws and then sucked nutrients from its host.
UPDATED: Thanks to Howard for pointing out that my original title was redundant. Scolecodont has a Latin meaning of "worm jaw". I have adjusted the title. :)
Here is a new drawing of a worm that is based on the Nereis virens clam worm. I changed its color around and went with more earth tones. Of course, that worm reminds me of a centipede. Apparently, the clam worm eats other worms and algae.