Edestus.

Edwardchristhoper.blogspot.com - Edestus is a mysterious genus of edestid shark. Some of the other edestids are Sarcoprion, Ornithoprion, Parahelicoprion, Helicoprion, and Campyloprion. There are five species of edestus: E. giganteus, E. heinrichi, E. mirus, E. minor, and E. vorax. Edestus giganteus is the most mysterious species of edestus, because people have only found one fossilized row of teeth.

This fossil of Edestus shows what the pinking-shear-like saw looked like. No one really knows where in the jaw the pinking shears went---if one went out of the lower jaw, if one went out of the upper jaw, or if there was one on the lower jaw and one on the upper jaw. Edestus may have used its saw to cut prey in half.

The jaw isn't the only mysterious part of Edestus, because no one knows what Edestus's body looked like---if it was long, tapering and eel-like, like the body of a Xenacanth shark, or if it was more shark-like, like the body of a modern day shark.

Source : http://www.lifebeforethedinosaurs.com