Helicoprion bessonowi

Helicoprion is a genus of extinct, shark-like eugeneodontid 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 lived in the oceans of the early Permian 290 million years ago, with species known from North America, Eastern Europe, Asia, and Australia. The closest living relatives of Helicoprion (and other eugeneodontids) are the chimaeras.

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 been 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. When compared with other eugeneodontids, it is thought that the Helicoprion could have reached a size of about 7 meters long. Tooth-whorl 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 the Helicoprion 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 Trolldescribe and depict an example of such a model. They proposed that no teeth were present in the animal's top jaw besides the crushing teeth for the whorl to cut against. The two envision the living animal to have a long and very narrow skull…

Helicoprion species proliferated greatly during the early Permian. Fossils have been found in the Ural Mountains, Western Australia, China (together with the related genera Sinohelicoprion and Hunanohelicoprion), and Western North America, including the Canadian Arctic, Mexico, Idaho, Nevada, Wyoming, Texas, Utah, and California. More than 50% of Helicoprion specimens are known from Idaho, with an additional 25% being found in the Ural Mountains. Due to the fossils' locations, the various species of Helicoprion may have lived off the southwestern coast of Gondwana, and later, Pangaea.

H. bessonowi Helicoprion was first described by Alexander Karpinsky in 1899 from a fossil found in Artinskian age limestones of the Ural Mountains. Karpinsky named the type species Helicoprion bessonowi; Oliver Perry Hay originally described the species. This species can be differentiated from others by a short and narrowly spaced tooth whorl, backward-directed tooth tips, obtusely-angled tooth bases, and a consistently narrow whorl shaft. One of two Helicoprion species described by Wheeler in 1939, H. nevadensis, is based on a single partial fossil found in 1929 by Elbert A Stuart. It was reported as having originated from the Rochester Trachyte deposits, which Wheeler considered to be of Artinskian age. However, the Rochester Trachyte is in fact Triassic, and H. nevadensis likely did not originate in the Rochester Trachyte, thus rendering its true age unknown. Wheeler differentiated H. nevadensis from H. bessonowi by its pattern of whorl expansion and tooth height, but Leif Tapanila and Jesse Pruitt showed in 2013 that these were consistent with H. bessonowi at the developmental stage that the specimen represents. Based on isolated teeth and partial whorls found on the island of Spitsbergen, Norway, H. svalis was described by Stanisław Siedlecki in 1970. The type specimen, a very large whorl, was noted for its narrow teeth that apparently are not in contact with each other. However, this seems to be a consequence of only the central part of the teeth being preserved, according to Tapanila and Pruitt. Since the whorl shaft is partially obscured, H. svalis cannot be definitely assigned to H. bessonowi, but it closely approaches the latter species in many aspects of its proportions. With a maximum volution height of 72 millimetres (2.8 in), H. svalis is similar in size to the largest H. bessonowi, which has a maximum volution height of 76 …