| Helicoprion | |
|---|---|
 | |
| Information | |
| Range | Western Australia |
| Estimated Population | ~0 |
| Scientific Classification | |
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Chondrichthyes |
| Order | Eugeneodontida |
| Family | Helicoprionidae |
| Genus | Helicoprion |
| Species | H. bessonowi |
| Conservation Status | |
 extinct | |
Helicoprion is an extinct genus of shark-like eugeneodont fish. Almost all fossil specimens are of spirally arranged clusters of the individuals' teeth, called "tooth whorls", which in life were embedded in the lower jaw. As with most extinct cartilaginous fish, the skeleton is mostly unknown. Fossils of Helicoprion 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). 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, systematic revision of Helicoprion via morphometric analysis of the tooth whorls found only H. davisii, H. bessonowi and H. ergassaminon to be valid, with some of the larger tooth whorls being outliers.
Fossils of Helicoprion have been found worldwide, as the genus is known from Russia, Western Australia, China, Kazakhstan, Japan, Laos, Norway, Canada, Mexico, and the United States (Idaho, Nevada, Wyoming, Texas, Utah, and California). 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 the species H. bessonowi.
Like other chondrichthyan fish, Helicoprion and other eugeneodonts had skeletons made of cartilage. As a result, the entire body disintegrated once it began to decay, unless preserved by exceptional circumstances. This can make it difficult to draw precise conclusions on the full body appearance of Helicoprion. However, the body shape can be estimated via postcranial remains known from a few eugeneodonts. Eugeneodonts with preserved postcrania include the Pennsylvanian to Triassic-age caseodontoids Caseodus, Fadenia, and Romerodus.
These taxa have a fusiform (streamlined, torpedo-shaped) body plan, with triangular pectoral fins. There is a single large and triangular dorsal fin without a fin spine, and a tall, forked caudal fin which externally appears to be homocercal (with two equally-sized lobes). This general body plan is shared by active, open-water predatory fish such as tuna, swordfish, and lamnid sharks. Eugeneodonts also lack pelvic and anal fins, and judging by Romerodus, they would have had broad keels along the side of the body up to the caudal fin. Fadenia had five well-exposed gill slits, possibly with a vestigial sixth gill. There is no evidence of the specialized gill basket and fleshy operculum present in living chimaeroids. The largest known Helicoprion tooth whorls were about 40 cm (16 in) in diameter. Based on the proportional size of caseodontoid tooth whorls, this may indicate that Helicoprion individuals could reach 5 to 8 meters (16.4 to 26.2 feet) in length, rivaling the size of modern basking sharks.