Helicoprion

''Helicoprion'' is an extinct genus of shark-like cartilaginous fish in the order Eugeneodontiformes. Almost all ''Helicoprion'' fossils consist of spirally-arranged clusters of fused teeth, called "tooth whorls", which in life were embedded in the lower jaw. With the exception of the upper and lower jaws, the cartilaginous skeleton of ''Helicoprion'' is unknown. 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, study of the genus ''Helicoprion'' via morphometric analysis of the tooth whorls found that the genus contained only the species ''H. davisii, H. bessonowi'' and ''H. ergassaminon''.

Fossils of ''Helicoprion'' have been found worldwide, with the genus being known from Russia, Western Australia, China, Kazakhstan, Japan, Laos, Norway, Canada, Mexico, and the United States (Idaho, Nevada, Wyoming, Texas, Utah, and California). These fossils 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). 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 ''H. bessonowi''.

Description

Like other chondrichthyan fish, ''Helicoprion'' and other eugeneodonts had skeletons made of cartilage. Around the jaws this cartilage was mineralized, but the rest of the skeleton was likely unmineralized and quickly disintegrated once it began to decay. This makes drawing precise conclusions on the in-life appearance of ''Helicoprion'' difficult, but the body shape has been estimated from postcranial remains known from other members of its order, such as the Pennsylvanian to Triassic-age caseodontids '' Caseodus,'' ''Fadenia,'' and ''Romerodus.''

The caseodontids have a fusiform (streamlined, torpedo-shaped) body plan, with triangular pectoral fins. They have 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 the genus ''Romerodus'' had broad keels along the side of the body up to the caudal fin. ''Fadenia'' and the smaller ''Ornithoprion'' had at least five well-developed gill slits, possibly with a vestigial sixth gill. No evidence has been found of the specialized gill basket and fleshy operculum present in living chimaeroids. Based on the proportional size of caseodontoid tooth whorls, researcher Oleg Lebedev suggested that ''Helicoprion'' individuals with tooth whorls in diameter could reach in total length, rivaling the size of modern basking sharks. The largest known ''Helicoprion'' tooth-whorl, specimen IMNH 49382 representing an unknown species, reached in diameter and in crown height, and would have belonged to an individual or more in length.

Tooth whorls

Almost all ''Helicoprion'' specimens are known solely from "tooth whorls", which consist of dozens of enameloid-covered teeth embedded within a common logarithmic spiral-shaped root. The youngest and first tooth at the center of the spiral, referred to as the "juvenile tooth arch", is hooked, but all other teeth are generally triangular in shape, laterally compressed and typically serrated. 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 section of shaft that lacks the middle and upper portions of the tooth crown.

Cartilaginous skull

''Helicoprion'' specimens preserving more than tooth whorls are rare. The best-preserved specimen of ''Helicoprion'' is IMNH 37899 (also known as "Idaho 4"), referred to ''Helicoprion davisii''. It was found in Idaho in 1950 and was originally described in 1966 by paleontologist Svend Erik Bendix-Almgreen. A 2013 redescription by Tapanila and colleagues was accompanied by CT scanning, which allowed for more detailed study of the specimen's cartilage. CT scanning revealed a nearly complete set of upper and lower jaws, still in articulation and preserved in three dimensions. Alongside the tooth-whorl, the specimen preserves the palatoquadrates (forming the upper jaw), Meckel's cartilages (forming the lower jaw), and a robust block of cartilage bracing the tooth-whorl which has been identified as labial cartilage. All of these structures are mineralized and covered in prismatic calcified cartilage, as in modern cartilaginous fish. The specimen do 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 ''(after death) compression.

''Helicoprion'' had an autodiastylic jaw suspension, meaning that the inner edge of the palatoquadrate was firmly attached (but not fused) to the chondrocranium at two separate points. These two attachment points are the dome-shaped ethmoid process at the front of the palatoquadrate, and the flange-like basal process at its upper rear corner. Autodiastylic jaws are common in early holocephalans, though in modern animals they can only be found in embryonic chimaeriforms. Another well-preserved specimen, USNM 22577+494391 (nicknamed the "Sweetwood specimen"), has demonstrated that the inner surface of the palatoquadrate was covered with numerous small (~2 mm wide) teeth. The palatoquadrate teeth were low and rounded, forming a " pavement" that scraped against the tooth whorl. When seen from behind, the palatoquadrate forms a paired jaw joint with the Meckel's cartilage. No evidence is seen for an articulation between the palatoquadrate and the hyomandibula.

Meckel's cartilage has an additional projection right before the joint with the palatoquadrate. This extra process, unique to ''Helicoprion'', likely served to limit jaw closure to prevent the whorl from puncturing the chondrocranium. Another unique characteristic of ''Helicoprion'' is that the preserved labial cartilage forms a synchondrosis (fused joint) with the upper surface of Meckel's cartilage. This joint is facilitated by a long facet on the upper edge of Meckel's cartilage. The labial cartilage provides lateral support for the tooth whorl, widening near the root of each revolution. By wedging into the palatoquadrate while the mouth is closed, the upper edge of the labial cartilage helps to spread out the forces used to limit the extent of the jaw closure. The rear portion of the labial cartilage has a cup-like form, protecting the developing root of the last and youngest revolution.

Scales

Tooth-like chondricthyan scales, specifically known as odontodes, have been found associated with ''H. bessonowi'' remains in Kazakhstan. They are broadly similar to scales of other eugeneodonts such as ''Sarcoprion'' and ''Ornithoprion''. The scales have a cap-shaped base with a concave lower surface. The crowns are conical and covered with serrated, longitudinal ridges. The scales may be monodontode (with one crown per base) or polyodontode (with multiple crowns resulting from the fusion of several odontodes). Compared to other eugeneodonts, the scales of ''Helicoprion'' are more strongly pointed.

Paleobiology

The unusual saw-like tooth whorl and the lack of wear on the teeth of ''Helicoprion'' implies a diet of soft-bodied prey, as hard-shelled prey would simply slip out of the mouth. Due to the narrow nature of the jaw, suction feeding is unlikely to have been effective, and ''Helicoprion'' is thought to have been a bite feeder. Biomechanical modelling by Ramsay ''et al''. (2015) suggests that the teeth in the whorl had distinct functions depending on where they were in the spiral. The frontmost teeth served to snag and pull prey further into the mouth, while the middle teeth spear, and the hind teeth served to puncture and bring prey further into the throat, with the prey being squeezed between the whorl and the two halves of the palatoquadrate. The labial cartilage served to buttress and provide support to the whorl.
''Helicoprion'' may have started with a large gape during initial prey capture, followed by smaller jaw opening and closing cycles to further transport prey into the mouth, as is done by modern bite-feeding sharks. While modern sharks shake their heads from side to side to facilitate sawing and cutting their prey, the teeth of ''Helicoprion'' would likely further cut the prey during the jaw opening, due to the arc-like path of the front teeth, similar to the slashing motion of a knife. ''Helicoprion'' likely used a series of rapid, forceful jaw closures to initially capture and push prey deeper into the oral cavity, followed by cyclic opening and closing of the jaw to facilitate sawing through prey.

Ramsay and colleagues further suggested that the whorl could have served as an effective mechanism for deshelling hard-shelled cephalopods such as ammonoids and nautiloids, which were abundant in Early Permian oceans. If a hard-shelled cephalopod was bitten head-on, the whorl could have served to pull the soft body out of the shell and into the mouth. During jaw closure, the palatoquadrates and tooth whorl combined to form a three-point system, equivalent to the set-up of an inverted three-point flexural test. This system was effective at trapping and holding soft parts to increase cutting efficiency and provide leverage against hard-shelled prey. At the three points of contact, the estimated bite force ranges between , with estimated bite stresses ranging from during initial prey contact. This large bite force may have allowed ''Helicoprion'' to expand its diet to vertebrates, as its jaw apparatus was more than capable of cutting through skeletal elements of unarmoured bony fish and other chondrichthyans.

Classification

Skull data from IMNH 37899 reveal several characteristics, such as an autodiastylic jaw suspension without an integrated hyomandibula, which confirm the placement of ''Helicoprion'' within the chondrichthyan subgroup Euchondrocephali. In contrast to their sister group Elasmobranchii (containing true sharks, rays, and kin), euchondrocephalans are primarily an extinct group. Living members of the Euchondrocephali are solely represented by the order Chimaeriformes in the subclass Holocephali. Chimaeriforms, commonly known as chimaeras or ratfish, are a small and specialized group of rare deep-sea cartilaginous fish. The relationship between ''Helicoprion'' and living chimaeras is very distant, but had been previously suspected based on details of its tooth anatomy.

More specifically, ''Helicoprion'' can be characterized as a member of Eugeneodontida, an order of shark-like euchondrocephalans that lived from the Devonian to Triassic periods. Eugeneodonts have simple, autodiastylic skulls with reduced marginal dentition and enlarged whorls of blade-like symphysial teeth on the midline of the jaw. Within the Eugeneodontida, ''Helicoprion'' is placed within the Edestoidea, a group of eugeneodonts with particularly tall and angled symphysial teeth. Members of the Edestoidea are divided into two families based on the style of the dentition. One family, the Edestidae, has relatively short tooth blades with roots that incline backwards.

The other family, which contains ''Helicoprion'', is sometimes called Agassizodontidae, based on the genus ''Agassizodus''. Other authors, though, prefer the family name Helicoprionidae, which was first used 70 years prior to Agassizodontidae. Helicoprionids (or agassizodontids) have large, cartilage-supported whorls with strongly arched shapes. Helicoprionids do not shed their teeth; instead, their tooth whorls continually add new teeth with bases inclined forwards at the top of the whorl. As most eugeneodonts are based on fragmentary tooth remains, concrete phylogenetic relationships within the group remain unclear.

History and species

Three species of ''Helicoprion'' are currently considered valid via morphometric analyses, differing in the proportions of the upper, middle and lower sections of the tooth crown. These differences are only apparent in adult individuals past the 85th tooth of the spiral.

''H. davisii'' and synonyms

The first specimen of ''Helicoprion'' to be described was WAMAG 9080, a 15-tooth fragment of a tooth whorl found along a tributary of the Gascoyne River in Western Australia. Henry Woodward described the fossil in 1886 and named it as the species ''Edestus davisii,'' commemorating the man who discovered it. Upon naming ''H. bessonowi'' in 1899, Alexander Karpinsky reassigned ''E. davisii'' to ''Helicoprion''. In 1902, Charles R. Eastman referred ''H. davisii'' to his new genus '' Campyloprion'', but this proposal was never widely accepted. 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 forward.In a 1939 publication, Harry E. Wheeler described two new species of ''Helicoprion'' from California and Nevada''.'' One of these, ''H, sierrensis,'' was described from a specimen (UNMMPC 1002) found in glacial moraine deposits in Eastern California, likely originating from the Goodhue Formation. Tapanila and Pruitt determined that the distinguishing shaft range of ''H. sierrensis'' was well within the variation found in ''H. davisii''.

''H. jingmenense'' was described in 2007 from a nearly complete tooth whorl (YIGM V 25147) with more than four volutions across a part and counterpart slab. It was discovered during the construction of a road passing through 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. 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''.

''H. bessonowi'' and synonyms

''Helicoprion bessonowi'' was first described in an 1899 monograph by Alexander Karpinsky. Although it was not the first ''Helicoprion'' species to be described, it was the first known from complete tooth whorls, demonstrating that ''Helicoprion'' was distinct from ''Edestus''.
Also printed as eprinted (1899). St. Petersburg: C. Birkenfield. . ./ref> 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. It can be differentiated from other ''Helicoprion'' species 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 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 Tapanila and 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 with specimen number PMO A-33961, was noted for its narrow teeth that apparently are not in contact with each other, but 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 , ''H. svalis'' is similar in size to the largest ''H. bessonowi'', which has a maximum volution height of .

In 1999, the holotype of ''H. bessonowi'' was stolen, but afterwards was shortly recovered with the aid of an anonymous fossil dealer.

''H. ergassaminon''

Like ''H. davisii, H. ergassaminon'' is known from the Phosphoria Formation of Idaho, though it is comparatively much rarer. ''H. ergassaminon'' was named and described in detail within a 1966 monograph by Svend Erik Bendix-Almgreen, and the holotype specimen ("Idaho 5"), bears breakage and wear marks indicative of its usage in feeding. ''H. ergassaminon'' is also represented by several other specimens from the Phosphoria Formation, though none of these show wear marks. This species is roughly intermediate between the two contrasting forms represented by ''H. bessonowi'' and ''H. davisii'', having tall but narrowly spaced teeth. Its teeth are also gently curved, with obtusely angled tooth bases. The type specimen of this species was formerly considered lost, but following its rediscovery in 2023, it has been returned to the collection of the Idaho Museum of Natural History.

Other material

Several large whorls are difficult to assign to any particular species group, ''H. svalis'' among them. IMNH 14095, a specimen from Idaho, appears to be similar to ''H. bessonowi'', but it has unique flange-like edges on the apices of its teeth. IMNH 49382, also from Idaho, has the largest known whorl diameter at for the outermost volution (the only one preserved), but it is incompletely preserved and still partially buried. ''H. mexicanus'', named by F.K.G. Müllerreid in 1945, was supposedly distinguished by its tooth ornamentation. Its holotype is currently missing, though its morphology was similar to that of IMNH 49382. In the absence of other material, it is currently a ''nomen dubium''. Vladimir Obruchev described ''H. karpinskii'' from two teeth in 1953. He provided no distinguishing traits for this species, thus it must be regarded as a ''nomen nudum''. Various other indeterminate ''Helicoprion'' specimens have been described from Canada, Japan, Laos, Idaho, Utah, Wyoming, and Nevada.

In 1922, Karpinsky named a new species of ''Helicoprion'', ''H. ivanovi'', from Gzhelian (latest Carboniferous) strata near Moscow. However, this species has subsequently been removed from ''Helicoprion'' and placed as a second species of the related eugeneodont ''Campyloprion''. In 1924, Karpinsky separated ''H. clerci'' from ''Helicoprion'' and reclassified it under the new genus name ''Parahelicoprion'', but ''Parahelicoprion'' has recently been suggested to represent a junior synonym of this genus.

Historical reconstructions

Earliest reconstructions

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 stingray-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''; then associated with ''Ptychodus'').

Karpinsky's 1899 monograph on ''Helicoprion'' noted that the bizarre nature of the tooth whorl made reaching precise conclusions on its function difficult. He tentatively suggested that it curled up from the upper jaw for defensive or offensive purposes. This was justified by comparison to the upper tooth blades of ''Edestus'', which by 1899 had been re-evaluated as structures belonging to the jaw.

Debates over the identity of ''Helicoprion'''s tooth whorl were abundant in the years following Karpinsky's monograph. In 1900, the publication was reviewed by Charles Eastman, who appreciated the paper as a whole, but derided the sketch of the supposed life position of the whorl. Though Eastman admitted that the teeth of the whorl were very similar to those of other chondrichthyans, he still supported the idea that the whorl may have been a defensive structure embedded into the body of the animal, rather than the mouth. 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. Reconstructions similar to those of Karpinsky (1899) were common in Russian publications as late as 2001.

Later reconstructions

By the mid-20th century, the tooth whorl was generally accepted as positioned in the lower jaw of the animal. Though this general position was suspected almost immediately in the aftermath of Karpinsky's monograph, it was not illustrated as such until the mid-1900s. Around that time, an artist known only as "F. John" depicted ''Helicoprion'' within a set of "''Tiere der Urwelt''" trading cards. Their reconstruction presented the tooth whorl as an external structure curling down from the lower jaw of the animal. Similar downward-curling reconstructions have also been created by modern paleontologists and artists such as John A. Long, Todd Marshall, and Karen Carr. The utility of the tooth whorl in this type of reconstruction was inferred based on sawfish, which incapacitate prey using lateral blows of their denticle-covered snouts.

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 accumulates 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.

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 a reconstruction based on the information gleaned by Bendix-Almgreen (1966). They proposed that no teeth were present in the animal's upper jaw, besides crushing teeth for the whorl to cut against. The two envisioned the living animal to have a long and very narrow skull, creating a long nose akin to the modern-day goblin shark. 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.

References

External links

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{{Taxonbar, from=Q133376

Agassizodontidae
Cisuralian first appearances
Permian extinctions
Prehistoric fish of Australia
Permian fish of Asia
Prehistoric fish of Europe
Prehistoric fish of North America
Fossil taxa described in 1899
Fossils of the United States
Paleozoic life of Alberta
Paleozoic life of British Columbia
Paleozoic life of Nunavut
Cisuralian life
Artinskian life
Kungurian life
Guadalupian life
Roadian life
Permian cartilaginous fish
Prehistoric cartilaginous fish genera