Miocene

USA

Excellent 0.2 inch tooth of a Hammerhead shark

Shark.

Sphyrna is the genus for all hammerhead sharks, living and extinct. It is known from at least the Oligocene. No fossil skeletons of hammerheads are known so scientists don’t really know when they evolved their unusually-shaped heads. 

Isolated teeth of hammerheads, sharpnose sharks (Rhizoprionodon), a couple of other related genera can be difficult to distinguish from one another.