Helicoprion

This bizarre fossil was originally thought to be an ammonite, as the fossil looked like a spiraling, circular shell. However, after some examination, it was revealed that is wasn’t a shell, but a spiraling set of shark teeth, a “tooth whirl”. Unfortunately, due to a lack of a body (cartilage does not fossilize as well as bone), so a guessing game began. It was guessed to be on the shark’s dorsal fin, tail, or even its snout. Thankfully, a skull of a related shark, Ornithoprion, was found to have a tooth-whirl on its lower jaw. The tooth whirl likely contained all of the shark’s teeth that it would use in its life: its older teeth would be moved away to make room for its newer, better teeth. This does not solve the problem yet, however! The tooth whirl was then placed on the tip of the lower jaw but it turns out that that would actually slow down the shark! Perhaps the most accurate representation is one where the tooth whirl existed deep in its mouth instead.