Helicoprion
(Varies by Species)

Helicoprion was a shark-like fish with one of the most bizarre features in the fossil record — a spiral whorl of teeth in its lower jaw, coiling inward like a circular saw. For over a century after its discovery, palaeontologists debated where exactly this tooth whorl sat in the animal's body, with suggestions ranging from the snout to the tail. It is now understood to have been positioned at the back of the lower jaw, used to slice through soft-bodied prey like a biological buzzsaw!

 

The image above is one of many interpretations — the exact orientation of Helicoprion’s tooth whorl is still debated and reconstructions vary. It is highly likely that the whorl actually curved upward into the mouth rather than downward.