Note the spiral teeth in the lower jaw

This is Helicoprion.

Helicoprion is an extinct genus of shark-like eugeneodont* fish. Almost all fossil specimens are of spirally arranged clusters of the individuals' teeth, called "tooth whorls", which in life were embedded in the lower jaw. As with most extinct cartilaginous fish, the skeleton is mostly unknown.

And here is a fossil of the dentition:

More at the link, and some informed commentary at the via.  You learn something every day.

* Eu+gene+odont = true origin teeth