What it was
A relative of sharks from about 310 million years ago. It grew a curved blade of teeth at the front of the upper jaw, and a second at the front of the lower. Two blades, sticking out past its face.
The blades
The biggest is 43 centimeters. Across its life it grew more than forty teeth on each blade, shedding them in whole pieces.
Still argued
Did the blades shear sideways, or did the animal slash with its whole head? Scientists still argue about it.