Пермдік спираль тісті акула - Helicoprion

Genus of fossil fishes

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 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. Based on the proportional size of caseodontoid tooth whorls, Lebedev suggested that Helicoprion individuals with tooth whorls reaching in diameter could reach in length, rivaling the size of modern basking sharks. Tooth size increases away from the center of the spiral (abaxial), with the largest teeth possibly exceeding in length. The lower part of the teeth form projections that are shingled below the crown of the previous tooth. The lowest portion of the root below the enameloid tooth projections is referred to as the "shaft", and lies on cartilage that encapsulates the previous revolutions of the whorl. In a complete tooth whorl, the outermost part of the spiral terminates with an extended root that lacks the middle and upper portions of the tooth crown. A 2013 redescription by Tapanila and colleagues was accompanied by CT scanning, in order to reveal the cartilaginous remains in more detail. CT scanning revealed a nearly complete jaw apparatus, articulated in a closed position with three dimensional preservation. Alongside the tooth whorl, the specimen preserves a palatoquadrate (forming the upper jaw), Meckel's cartilage (forming the lower jaw), and a robust labial cartilage bracing the tooth whorl. All of these structures are composed of prismatic calcified cartilage, as with modern chondrichthyans. The specimen did not preserve a chondrocranium, the cartilaginous structure which would have housed the brain and sensory organs. The jaws are extensively laterally compressed (narrow) compared to living chondrichthyans, though this may at least partially be an artifact of post mortem compression. Another well preserved specimen, USNM 22577+494391 (the "Sweetwood specimen"), has demonstrated that the inner surface of the palatoquadrate was covered with numerous small (~2 mm wide) teeth. The relationship between Helicoprion and living chimaeras is very distant, but had been previously suspected based on details of its tooth anatomy. Upon naming H. bessonowi in 1899, Alexander Karpinsky reassigned E. davisii to Helicoprion. Karpinsky's identification of Edestus davisii as a species of Helicoprion would eventually be upheld by Curt Teichert, who described several more complete tooth whorls from the Wandagee Formation of Western Australia in the late 1930s. In 1907 and 1909, Oliver Perry Hay described a new genus and species of eugeneodont, Lissoprion ferrieri, from numerous fossils found in phosphate rich Phosphoria Formation on the border between Idaho and Wyoming. He also synonymized H. davisii with his new genus and species. However, Karpinsky separated the two species once more and transferred them to Helicoprion in 1911. H. ferrieri was initially differentiated using the metrics of tooth angle and height, but Tapanila and Pruitt (2013) considered these characteristics to be intraspecifically variable. As a result, they reassigned H. ferrieri as a junior synonym of H. davisii. Outside the Phosphoria Formation, H. davisii specimens have also been found in Mexico, Texas, and Canada (Nunavut and Alberta). H. davisii is characterized by its tall and widely spaced tooth whorl, with these becoming more pronounced with age. The teeth also noticeably curve forwards. Tapanila and Pruitt argued that the specimen was partially obscured by the surrounding matrix, resulting in an underestimation of tooth height. Taking into account intraspecific variation, they synonymized it with H. davisii. As a result, H. bessonowi serves as the type species for Helicoprion. H. bessonowi is primarily based on a number of specimens from Artinskian age limestone of the Divya Formation, in the Ural Mountains of Russia. H. bessonowi specimens are also known from the Tanukihara Formation of Japan and Artinskian age strata in Kazakhstan. One of two Helicoprion species described by Wheeler in 1939, H. nevadensis, is based on a single partial fossil found in a Nevadan mine by Elbert A. Stuart in 1929. This fossil, UNMMPC 1001, has been lost. 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.

Like H. davisii, Helicoprion ergassaminon is known from the Phosphoria Formation of Idaho. However, it is comparatively much rarer. H. ergassimon was named and described in detail within a 1966 monograph by Svend Erik Bendix Almgreen. Idaho, Utah, Wyoming, and Nevada. In 1924, Karpinsky separated H. clerci from Helicoprion and reclassified it under the new genus name Parahelicoprion, but it has been recently suggested that Parahelicoprion does represent a junior synonym of this genus.

Hypotheses for the placement and identity of Helicoprion's tooth whorls were controversial from the moment it was discovered. Woodward (1886), who referred the first known Helicoprion fossils to Edestus, discussed the various hypotheses concerning the nature of Edestus fossils. Joseph Leidy, who originally described Edestus vorax, argued that they represented the jaws of "plagiostomous" (chondrichthyan) fish. William Davies agreed, specifically comparing it to the jaws of Janassa bituminosa, a Permian petalodont. On the other hand, J. S. Newberry suggested that the jaw like fossils were defensive spines of a stringray like fish. Woodward eventually settled on E. D. Cope's argument that they represented pectoral fin spines from fish similar to "Pelecopterus" (now known as Protosphyraena). Shortly after his original monograph, Karpinsky published the argument that the whorl represented a curled, scute covered tail akin to that of Hippocampus (seahorses). This proposal was immediately criticized by various researchers. E. Van den Broeck noted the fragility of the structure and argued that it was most well protected as a paired feeding apparatus in the cheek of the animal. A. S. Woodward (unrelated to Henry Woodward) followed this suggestion with the hypothesis that each whorl represented a tooth battery from a gigantic shark. G. Simoens illustrated Karpinsky's various proposals and used histological data to adamantly argue that the whorls were toothed structures placed within the mouth. In 1911, Karpinsky illustrated the whorls as components of the dorsal fins. The other publication was Bendix Almgreen's monograph on Helicoprion. His investigations reinterpreted the tooth whorl as a symphyseal structure wedged between the meckelian cartilages, which were separated by a gap at the front. A pair of cartilage loops, the symphyseal crista, seems to develop as a paired extension of the jaw symphysis where the meckelian cartilages meet at the back of the jaw. Each loop arches up before curling back inwards, tracing over the root of the tooth whorl. The largest and youngest teeth form at the symphysis near the back of the jaw. Over time they are carried along the symphyseal crista, spiraling forwards, then downwards and inwards. The series of teeth accumulate into a spiraling structure, which is housed within the cavity defined by the symphyseal crista. The lateral and lower edges of the tooth whorl would have been obscured by skin during life. According to Bendix Almgreen, the most likely use of the tooth whorl was as a tool for tearing and cutting prey against the upper jaw. A 1996 textbook by Philippe Janvier presented a similar reconstruction, albeit with sharp teeth at the front of the upper jaw and rows of low crushing teeth in the back of the jaw. In 2008, Mary Parrish created a new reconstruction for the renovated Ocean Hall at the Smithsonian Museum of Natural History. Designed under the direction of Robert Purdy, Victor Springer, and Matt Carrano, Parrish's reconstruction places the whorl deeper within the throat. This hypothesis was justified by the argument that the teeth supposedly had no wear marks, and the assumption that the whorl would have created a drag inducing bulge on the chin of the animal if located in a symphysial position. They envisioned the tooth whorl as a structure derived from throat denticles and designed to assist swallowing. This would hypothetically negate the disadvantages the tooth whorl would produce if positioned further forward in the jaw. This reconstruction was criticized for the overly intricate and potentially ineffective design of such a structure, if solely used to assist swallowing. Lebedev (2009) found more support for a reconstruction similar to those of Bendix Almgreen (1966) and Troll (1994). A tooth whorl found in Kazakhstan preserved radial scratch marks; the whorl was also found near several wide tuberculated teeth similar to those of the putative caseodontoid Campodus. Lebedev's reconstruction presented a cartilage protected tooth whorl in a symphysial position at the front of the long lower jaw. When the mouth was closed, the tooth whorl would fit into a deep longitudinal pocket on the upper jaw. Both the pocket in the upper jaw and the edges of the lower jaw would have been lined with dense rows of Campodus like teeth. This was similar to the situation reported in related helicoprionids such as Sarcoprion and Agassizodus. As for Helicoprion's ecology, it was compared to modern cetaceans such as Physeter (the sperm whale), Kogia (dwarf and pygmy sperm whales), Grampus (Risso's dolphin), and Ziphius (Cuvier's beaked whale). These fish and squid eating mammals have reduced dentition, often restricted to the tip of the lower jaw. Lebedev's reconstruction approximates modern views on Helicoprion's anatomy, though the hypothetical long jaw and Campodus like lateral dentition has been superseded by CT data.