Helicoprion

Helicoprion Temporal range: 290–250 Ma PreЄ Є O S D C P T J K Pg N Early Permian to Early Triassic
Toothwhorl of H. bessonovi
Artist's reconstruction of H. bessonovi per latest hypothesis
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Order:	†Eugeneodontida
Family:	†Helicoprionidae
Genus:	†Helicoprion Karpinsky, 1899
Type species
Helicoprion bessonovi Karpinsky, 1899
Species
H. bessonovi Karpinsky, 1899 H. davisii Teichert, 1940 H. ferrieri Hay, 1907 H. ergasaminon Bendix-Almgreen, 1966 H. jingmenense Chen et al., 2007 H. mexicanus Mullerried, 1945 H. nevadensis Wheeler, 1939 H. sierrensis Wheeler, 1939

Helicoprion is a long-lived genus of extinct, shark-likeeugeneodontid holocephalid fish. Almost all fossil specimens are of spirally arranged clusters of the individuals' teeth, called "tooth whorls"— the cartilaginous skull, spine, and other structural elements have not been preserved in the fossil record, leaving scientists to make educated guesses as to its anatomy and behavior. Helicoprion first arose in the oceans of the early Permian290 million years ago, survived the Permian–Triassic extinction event, and eventually became extinct during the Early Triassic, 250 million years ago. Its name is Greek for "spiral saw". The closest living relatives of Helicoprion (and other eugeneodontids) are the chimaeras.

Comparisons with other eugenodontids suggest that Helicoprion may have grown to 3–4 m (9.8–13.1 ft) long on average, with large specimens up to 7.5 m (24.6 ft). However, in 2011, a tooth whorl from a Helicoprion was discovered in the Phosphoria site in Idaho. The tooth whorl measured 45 cm (18 in) in length. Comparisons with other Helicoprion specimens show that the animal that sported this whorl would have been 10 m (33 ft) in length, and another, even bigger tooth whorl that was discovered in 1980s (but was not published until 2013) which the discoverers dubbed IMNH 49382 or "Boise" was discovered at the same site. The whorl is incomplete, but in life it would have 60 cm (24 in) long and would have belonged to an animal that possibly exceeded 12 m (39 ft) in length, making Helicoprion the largest known eugeneodont. The scientists have also discovered that the two specimens belong to an unknown species of Helicoprion.

Until 2013, the only known fossils of this genus on record were their teeth, which were arranged in a "tooth-whorl" strongly reminiscent of a circular saw. As the skeletons of chondrichthyid fish are made of cartilage, including those of Helicoprion and other eugeneodonts, the entire body disintegrates once it begins to decay, unless exceptional circumstances preserve it. The tooth-whorl was not realized to be in the lower jaw until the discovery of the skull of a related genus of eugeneodont, Ornithoprion. The tooth-whorl represented all the teeth produced by that individual in the lower jaw; as the individual grew, the older, smaller teeth were moved into the center of the whorl by larger, newer teeth appearing. Models of Helicoprion's tooth-whorl have been made. In the 1994 book Planet Ocean: A Story of Life, the Sea, and Dancing to the Fossil Record, author Brad Matsen and artist Ray Troll describe and depict an example. They say that no teeth were in the animal's top row besides the crushing teeth for the whorl to cut against. The two envision the living animal to have a long and very narrow skull, creating a long nose akin to the modern-day goblin shark. According to their studies, the fossils that have been found are essentially a growth ring, as each set of new teeth pushes the previous set into the whorl. The images that Troll has devised are an educated guess at best.