Helicoprion

Helicoprion is an extinct genus of large shark-like cartilaginous fish that lived from the Early to the Middle Permian, about 290-270 million years ago. Helicoprion is a member of the Eugeneodontiformes, an extinct order of cartilaginous fish within the clade Holocephali, a group today represented only by chimaeras. It is also the type genus of the Helicoprionidae, a family of eugeneodonts characterised by distinctive tooth structures called tooth whorls. Helicoprion was first named in 1899 by Alexander Karpinsky on the basis of fossils discovered in Russia and Australia, the generic name meaning "spiral saw". Although numerous species were subsequently assigned to the genus, only H. bessonowi, H. davisii, and H. ergassaminon are recognized following a 2013 revision. The three species are distinguished by the shape and spacing of their tooth crowns.

Helicoprion is mainly known from its fused, spiral-shaped tooth whorls, which account for almost all documented fossils of the taxon. The position and function of these structures was long debated, but studies based on specimens preserving jaw cartilage indicate that they were positioned in the lower jaw and were specialised for grasping and slicing soft-bodied prey such as cephalopods. The whorl may also have aided in shelling or extracting the bodies of nautiloids and ammonoids. Based on the skeletal anatomy of smaller eugeneodonts, Helicoprion is estimated to have reached lengths between , with a general external appearance possibly comparable to that of tunas, swordfish, or mackerel sharks. Fossils of Helicoprion are known from marine deposits worldwide, indicating that it was pelagic and had a cosmopolitan distribution. The highest concentrations of these come from Idaho and Russia, which used to be covered by shallow seas where Helicoprion may have congregated.