UCMP MYSTERY FOSSIL #37 is . . .
Helicoprion

Helicoprion was a very unusual Permian shark. This picture shows its tooth whirl, a coiled, and tooth-studded extension of the lower jaw. Reaching about ten feet in length, Helicoprion may have used the tooth spiral to disable its prey while hunting. You can see one artist’s reconstruction of Helicoprion here. Find out more about fossil sharks here.