Helicoprion

Helicoprion Temporal range: 310–250Ma PreꞒ Ꞓ O S D C P T J K Pg N Late Carboniferous to Early Triassic
Toothwhorl of H. bessonovi
Artist's reconstruction of H. bessonovi per latest theory
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Holocephali
Order:	†Eugeneodontida
Family:	†Agassizodontidae
Genus:	†Helicoprion Karpinsky, 1899
Type species
Helicoprion bessonovi
Species
H. bessonovi H. davish H. ferrieri H. ergasaminon H. jingmenense H. mexicanus H. nevadensis H. sierrensis

Helicoprion is a long-lived genus of extinct, shark-like^[1] eugeneodontid holocephalid fish. Almost all fossil specimens are of spirally arranged clusters of the individuals' teeth, called "tooth whorls." Helicoprion first arose in the oceans of the late Carboniferous 310 million years ago, survived the Permian–Triassic extinction event, and eventually became extinct during the Early Triassic, 250 million years ago. The closest living relatives of Helicoprion (and other eugeneodontids) are the chimaeras.

Palaeobiology

Nature of the tooth-whorl

Up 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. It was not until the discovery of the skull of a related genus of eugeneodont, Ornithoprion, that it was realized that the tooth-whorl was in the lower jaw. The tooth-whorl represented all of the teeth produced by that individual in the lower jaw, in that as the individual grew, with the older, smaller teeth being moved into the center of the whorl by the appearance of larger, newer teeth. Models of Helicoprion‍ '​s tooth-whorl have been made. Author of 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 there were no teeth 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. Helicoprion‍ '​s true physical identity remained hidden in 280-million-year-old rocks.^[2] Comparisons with other eugenodontids suggest that Helicoprion may have grown up to 3–4 metres (9.8–13.1 ft) long.

For over a century, it was not certain where the tooth-whorl was located in the lower jaw. Older reconstructions placed the whorl in the front of the lower jaw. A 2008 reconstruction, created by Mary Parrish under the direction of Robert Purdy, Victor Springer and Matt Carrano for the Smithsonian, places the whorl deeper into the throat,^[3] although other studies did not accept this conclusion.^[4][5] A 2013 study based on new data places the tooth-whorl at the back of the jaw, where the tooth-whorl occupied the entire mandibular arch.^[6]

In the article, "Helicoprion in the Anthracolithic (Late Paleozoic) of Nevada and California, and its Stratigraphic Significance", author Harry E. Wheeler describes another Helicoprion fossil, based on the species H. sierrensis, collected by J. H. Menke, that once resided in the University of Nevada, Mackay Museum in 1931. (The fossil's current location is not known) According to Meller, Helicoprion’s mouth is its most distinct trait. The mouth consists of a whorl separated into 3 ¼ volutions. The biggest diameter is about 170 millimetres (6.7 in). The whorls have a separation of about 1 millimetre (0.039 in) in the first volution, and it goes to about 8 millimetres (0.31 in) at the largest whorl displayed. The specimen has a total of about 32 teeth in the first volution, 36 in the second and 41 in the last. The teeth at the end of the first volution are about 7 millimetres (0.28 in) long and measure about 2.4 inches (61 mm) width reaching about 40 millimetres (1.6 in) long and 9.5 wide at the end of the third. The teeth are symmetrically opposed to one another.^[7]

Additionally, other extinct fish, such as onychodontiformes, have analogous tooth-whorls at the front of the jaw, suggesting that such whorls are not as big of an impediment to swimming as suggested in Purdy's hypothesis. While no complete skulls of Helicoprion have been officially described, the fact that related species of chondrichthyids had long, pointed snouts suggests that Helicoprion did as well.

Distribution

Fossils of Helicoprion species first appear in Upper Carboniferous marine strata, proliferate greatly during the Permian, and eventually disappear during the Early Triassic. Fossils have been found in the Ural Mountains, Wandagee Mountain of Western Australia, China^[8] (together with the related genera Sinohelicoprion and Hunanohelicoprion), and Western North America, including the Canadian Arctic, Mexico, Idaho, Nevada, Wyoming, Texas, Utah, and California. Due to the fossils' locations, it is speculated that the various species of Helicoprion lived off the southwestern coast of Gondwana, and later, Pangaea.

Species

H. bessonowi

Helicoprion was first described by Alexander Karpinsky in 1899 from a fossil found in Artinskian age limestones of the Ural mountains.^[9] Karpinsky named the type species Helicoprion bessonowi. Oliver Perry Hay originally described the species

H. ferrieri

Helicoprion ferrieri jaw, Wolfcampian, Glass Mountains, Texas

Helicoprion ferrieri was originally described as a species of the genus Lissoprion in 1907, from fossils found in the Phosphoria Formation of Idaho. An additional specimen, tentatively referred to H. ferrieri was described in 1955. That specimen was found in Wolfcampian age quartzites exposed on China Mountain, six miles southeast of Contact, Nevada. The 100 millimetres (3.9 in) wide fossil consists of 1 ¾ whorls and about 61 preserved teeth. Due to weathering the rest of the fossil was lost and the preserved section is distorted from slippage of the host rock.^[9]

H. jingmenense

Helicoprion jingmenense was described from of a nearly complete tooth whorl with 4¹⁄₃ volutions (part and counterpart) found in the Lower Permian Qixia Formation of Hubei Province, China. The specimen is very similar to H. ferrieri and H. bessonowi, though, it differs from the former by having teeth with a wider cutting blade, and a shorter compound root, and differs from the latter by having fewer than 39 teeth per volution.^[10]

H. nevadensis

One of two Helicoprion species described by Harry, E Wheeler in 1939, H. nevadensis is based on a single partial fossil found in 1929 by Elbert A Stuart.^[11]

Gallery

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  Outdated reconstruction of Helicoprion bessonovi

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  Helicoprion ferreri, alternative restoration of eugeneodontid "shark" with short rostrum

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  Lower jaw of Helicoprion bessonovi

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  Spiral denture of Helicoprion

References

External links

Wikimedia Commons has media related to Helicoprion.

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