Helicoprion

Helicoprion (meaning "spiral saw") is an extinct genus of shark-like cartilaginous fish in the order Eugeneodontiformes. Almost all Helicoprion fossils consist of spirally-arranged clusters of fused teeth, called "tooth whorls", which in life were embedded in the lower jaw. With the exception of the upper and lower jaws, the cartilaginous skeleton of Helicoprion is unknown. The closest living relatives of Helicoprion (and other eugeneodonts) are the chimaeras, though their relationship is very distant. The unusual tooth arrangement is thought to have been an adaption for feeding on soft-bodied prey, and may have functioned as a deshelling mechanism for hard-bodied cephalopods such as nautiloids and ammonoids. In 2013, study of the genus Helicoprion via morphometric analysis of the tooth whorls found that the genus contained only the species H. davisii, H. bessonowi and H. ergassaminon.

Fossils of Helicoprion have been found worldwide, with the genus being known from Russia, Western Australia, China, Kazakhstan, Japan, Laos, Norway, Canada, Mexico, and the United States (Idaho, Nevada, Wyoming, Texas, Utah, and California). These fossils are known from a 20 million-year timespan during the Permian, period from the Artinskian stage of the Cisuralian (Early Permian) to the Roadian stage of the Guadalupian (Middle Permian). More than 50% of the fossils referred to Helicoprion are H. davisii specimens from the Phosphoria Formation of Idaho. An additional 25% of fossils are found in the Ural Mountains of Russia, belonging to H. bessonowi.