Living sharks have many rows of teeth & constantly shed worn ones. Helicoprion had a single row of sharp teeth in its lower jaw & only a few flat crushing teeth in its upper jaw. All of its worn teeth were retained in a bizarre whorl, with new large teeth erupting from the back of the mouth.

This Permian aged fossil is from a phosphorus mine in Idaho. The tiniest teeth in the center of the whorl are the oldest teeth. That's because this shark never shed worn teeth. When the shark was young it had tiny teeth so what you are seeing is a kind of fossilized growth record. Rumors abound of a giant five foot spiral. This one is eighteen inches across.