Helicoprion

Helicoprion (from the Greek hélico meaning "spiral" and príōn meaning "saw"; lit. 'spiral saw')^[1]: 98 is an extinct genus of shark-like^[2] cartilaginous fish. Almost all Helicoprion fossils consist of spirally-arranged clusters of fused teeth, called "tooth whorls", which in life was positioned in the animal's lower jaw. Fossils of the upper jaws, lower jaws, and labial cartilage have been discovered, but the rest of the soft, cartilaginous skeleton of Helicoprion is unknown. The genus is a member of the extinct order Eugeneodontiformes, and many aspects of its anatomy can be inferred from other eugneodonts. The closest living relatives of Helicoprion and other eugeneodonts are a group of deep sea fish called chimaeras, though their relationship is very distant. In 2013, a revision of the genus found that Helicoprion contained only the species H. davisii, H. bessonowi and H. ergassaminon, although many other species have previously been named. These three species are distinguished by the shape and spacing of their tooth crowns, although these differences are only apparent in large individuals.

Helicoprion Temporal range: Cisuralian to Guadalupian (Artinskian to Roadian), 290–270 Ma PreꞒ Ꞓ O S D C P T J K Pg N
FHPR L2003-2, a Helicoprion davisii tooth-whorl from the Phosphoria Formation of Idaho, Utah Field House of Natural History
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Order:	†Eugeneodontiformes
Family:	†Helicoprionidae
Genus:	†Helicoprion Karpinsky, 1899
Type species
Helicoprion bessonowi Karpinsky, 1899
Other species
H. davisii (Woodward, 1886), originally Edestus davisii H. ergassaminon Bendix-Almgreen, 1966
Synonyms
Synonyms of H. davisii H. ferrieri (Hay, 1907), originally Lissoprion ferrieri H. jingmenense Chen, Cheng, & Yin, 2007 H. sierrensis Wheeler, 1939 Synonyms of H. bessonowi H. nevadensis Wheeler, 1939 Indeterminate species H. karpinskii Obruchev, 1953 H. mexicanus Mullerried, 1945 H. svalis Siedlecki, 1970 H.? clerci Karpinsky, 1916

Fossils of Helicoprion have been discovered from marine deposits worldwide, with the genus being known from Russia, Australia, China, Kazakhstan, Japan, Laos, Norway, Canada, Mexico, and the United States. Because of its wide distribution, Helicoprion is assumed to have been pelagic. The largest numbers of Helicoprion tooth whorls come from the Early Permian Phosphoria Formation of Idaho and Divya Formaton of Russia, and members of the genus may have congregated at these locations. Helicoprion was a predator, and its unusual tooth arrangement is thought to have been an adaptation for feeding on soft-bodied prey such as cephalopods and other cartilaginous fish. It may have used the tooth whorl to deshell hard-shelled cephalopods such as nautiloids and ammonoids. Before the cartilaginous jaws had been discovered and described in detail, many alternate hypotheses about the function of Helicoprion's whorl were proposed. The genus first appeared during the Early Permian, and went extinct during the Middle Permian.

Discovery and research history

Valid species

Cast of specimen TsNIGR 1/1865, a well-preserved H. bessonowi tooth-whorl from the Ural Mountains of Russia

While as many as 10 species of Helicoprion have been described and over 100 fossil specimens have been assigned to the genus, only three species are currently recognized as valid based on morphometric analyses: Helicoprion davidsii, H. bessonowi and H. ergassaminon. These species 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 H. bessonowi specimens found in the Ural Mountains of Russia. The remaining fossils are known from fossil deposits worldwide.^[3]

The type species,^[4] Helicoprion bessonowi, was first described based on six whorls from the Divya Gora quarry of Krasnaufimsk, Russia^[5][6] in an 1899 monograph by Alexander Karpinsky.^[3][7][5] These fossils were sent to Karpinsky the year prior by inspector A. Bessinov, after whom the species is named.^{[7][5][1]: 76–80} Although it was not the first Helicoprion species to be discovered, it was the first to be described from complete tooth whorls, and its discovery established that Helicoprion was a distinct genus.^[5] 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.^[6][8] In 1999, the holotype of H. bessonowi was stolen, but afterwards was shortly recovered with the aid of an anonymous fossil dealer.^[9][10]

The first specimen of Helicoprion to be discovered (but not described as such) was WAMAG 9080,^[3] 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 a species of the genus Edestus: E. davisii, commemorating its discoverer, a gold prospector whose last name was Davis.^{[1]: 6–7 [11][note 1]} Upon naming H. bessonowi in 1899, Alexander Karpinsky reassigned E. davisii as another species of Helicoprion.^[5][10] In 1902, Charles R. Eastman instead referred H. davisii to his genus Campyloprion, but this proposal was never widely accepted.^[12] Karpinsky's identification of Edestus davisii as a species of Helicoprion would eventually be upheld by Curt Teichert, who discovered several more complete tooth whorls from the Wandagee Formation of Western Australia in 1939 and described them in 1940.^{[3][1]: 127–129 [13]} Outside of Western Australia and the Phosphoria Formation of Idaho, H. davisii specimens have also been found in Mexico, Texas, and Canada (Nunavut and Alberta).^[3]

IMNH 30900 ("Idaho 5"), a Helicoprion ergassaminon tooth-whorl from Gay Mine in Bingham County, Idaho.

Helicoprion ergassaminon is known from the Phosphoria Formation, and is very rare compared to H. davidsii, which it shared the environment with. 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.^[14] The type specimen of this species was formerly considered lost after being loaned to the Natural History Museum of Denmark, but was identified in 2017 and, in 2023, was returned to the collection of the Idaho Museum of Natural History.^[10][15]

Synonymous species

FMNH PF 74455, a H. davisii (=H. ferrieri) tooth-whorl from the Glass Mountains of Texas

In 1907 and 1909, Oliver Perry Hay described a new genus and species of eugeneodont, Lissoprion ferrieri, from three fossils from the Phosphoria Formation of Idaho and Wyoming. He also synonymized H. davisii with his new genus and species.^[3][16] However, Karpinsky separated the two species once more and transferred them to Helicoprion in 1911.^[17] H. ferrieri was initially differentiated using the metrics of tooth angle and height, but Tapanila and Pruitt (2013) considered these characteristics to be variable within a single Helicoprion species. As a result, they reassigned H. ferrieri as a junior synonym of H. davisii.^[3]

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.^[3] The other species, H. nevadensis, is based on a single partial fossil found in a Nevadan mine by Elbert A. Stuart in 1929.^[18] This fossil, UNMMPC 1001, has been lost. It was reported as having originated from the Koipato Formation, which Wheeler considered to be of Artinskian age. However, this formation is considered to be of Triassic age, and H. nevadensis more likely originated from a different, unknown locality. 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. Tapanila and Pruitt also determined that the distinguishing shaft range of H. sierrensis was well within the variation found in the species H. davisii.^[3]

Based on isolated teeth and partial whorls found on the island of Spitsbergen, Norway, researcher Stanisław Siedlecki described H. svalis 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,^[19] 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 72 mm (2.8 in), H. svalis is similar in size to the largest H. bessonowi, which has a maximum revolution height of 76 mm (3.0 in).^[3]

H. jingmenense was described in 2007 from a nearly complete tooth whorl (YIGM V 25147) with more than four revolutions 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.^[20] 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 (within a single species) variation, they synonymized it with H. davisii.^[3]

Other species

The partial tooth whorl of Parahelicoprion clerci, which was originally assigned to the genus

Several large whorls are difficult to assign to any particular species group, similarly to H. svalis. 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 56 cm (22 in) for the outermost volution (the only one preserved), but it is incompletely preserved and still partially buried.^[3] 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. Dmitry Obruchev described H. karpinskii from two teeth in 1953,^[21] although he provided no distinguishing traits for this species and it is now regarded as a nomen nudum.^[3] Various other indeterminate Helicoprion specimens have been described from Canada, Japan, Laos, Idaho, Utah, Wyoming, and Nevada.^[3][4][22]

In 1916, Karpinsky named the species H. clerci for several fragments of a very large tooth whorl found in the Divya Formation.^{[1]: 121 [23][24]} In 1924, Karpinsky separated H. clerci from Helicoprion and reclassified it under the new genus, Parahelicoprion.^[25][26] P. clerci has been suggested to represent a junior synonym of Helicoprion by paleontologist Serge Naugolnykh,^[23] although other authors retain this species in its own genus.^[27] In 1922, Karpinsky named the new species Helicoprion ivanovi, from Gzhelian (latest Carboniferous) strata near Moscow.^[28] However, this species was subsequently removed from Helicoprion and placed as a second species of the related eugeneodont Campyloprion,^[29] and more recently it has become the type species of the genus Karpinskiprion.^[10][27]

Description

Skeletal diagram of H. bessonowi (top) and life restoration of H. davisii (bottom)

Like other chondrichthyan fish, Helicoprion had a skeleton made of cartilage. Around the jaws this cartilage was mineralized,^[6][14] but the rest of the skeleton was likely completely unmineralized and quickly disintegrated once it began to decay.^[30] 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,^[6][31][32] such as the Pennsylvanian to Triassic-age caseodontid eugeneodonts Caseodus, Fadenia, and Romerodus.^[30][33]

Eugeneodonts are characterized by 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).^[30] This general body plan is shared by active, open-water predatory fish such as tuna, swordfish, and lamnid sharks.^[6][34] 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.^[30] Fadenia and the smaller Ornithoprion had at least five well-developed gill slits, possibly with a vestigial sixth gill.^[31][33][35] No evidence has been found of the specialized gill basket and fleshy operculum present in living chimaeroids.^[30][33] Based on the proportional size of caseodontoid tooth whorls, researcher Oleg Lebedev suggested that Helicoprion individuals with tooth whorls 35–40 cm (14–16 in) in diameter could reach 5–8 m (16–26 ft) in total length, comparable to the size of modern basking sharks.^[6] The largest known Helicoprion tooth-whorl, specimen IMNH 49382, reached 56 cm (22 in) in diameter and 14 cm (5.5 in) in crown height, and would have belonged to an individual 7 m (23 ft) or more in length.^[3][32][36] Paleontologist Leif Tapanila has suggested a total length of 9.7–12 m (32–39 ft) for the largest Helicoprion individuals.^[10]

Tooth whorls

Diagram of the teeth within the spiral

Almost all Helicoprion specimens are known solely from "tooth whorls", which consist of dozens of enameloid-covered teeth embedded within a single logarithmic spiral-shaped root. The youngest and first teeth at the center of the spiral, referred to as the "juvenile tooth arch", are hooked, but all other teeth are generally triangular in shape, laterally compressed and typically serrated.^[36] Tooth size increases away from the center of the spiral, with the largest teeth possibly exceeding 10 cm (3.9 in) in height. The lower part of the teeth form projections 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 jaw cartilage that covers 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.^[3]

The three species of Helicoprion differ only in the anatomy of their tooth-whorls. Each species is differentated by features of the upper, middle and lower sections of the tooth crowns, which are apparent only after the 85th tooth of the spiral. H. davisii can be differentiated from other Helicoprion species by tall, widely spaced tooth crowns with forward-curved tooth tips. H. bessonowi had comparatively short, closely-spaced tooth crowns, backward-curved teeth, a narrow shaft and widely angled tooth bases. H. ergassaminon is roughly intermediate in anatomy between H. bessonowi and H. davisii, with tall but closely spaced crowns and tooth bases angled similarly to H. davidsii.^[3]

Cartilaginous skull

IMNH 37899 ("Idaho 4"), a Helicoprion davisii tooth-whorl from the Phosphoria Formation which preserves the articulated jaw cartilages

Helicoprion specimens preserving more than tooth whorls are rare. The best-preserved specimen of Helicoprion is IMNH 37899 (nicknamed "Idaho 4"), which is referred to H. davisii. It was found in Idaho in 1950 and was originally described in 1966 by paleontologist Svend Erik Bendix-Almgreen.^[14] 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 compression after death.^[31][37]

Restoration of the skull of Helicoprion, based on IMNH 37899

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.^[31] Autodiastylic jaws are common in early holocephalans, though in modern animals they can only be found in embryonic chimaeriforms.^[38] 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.^[36] The palatoquadrate teeth were low and rounded, forming a "pavement" that scraped against the tooth whorl.^[37] When seen from behind, the palatoquadrate forms a paired jaw joint with the Meckel's cartilage. No evidence is seen for articulation between the jaws and the hyomandibula, which helps to suspend the jaws of modern sharks.^[31]

The Meckel's cartilage of Helicoprion 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 skull. 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 of the tooth-whorl.^[31][37][36]

Scales

Tooth-like chondrichthyan 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.^[6][35] 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 extending from a single base). Compared to other eugeneodonts, the scales of Helicoprion are more strongly pointed.^[6]

Classification

Skull data from IMNH 37899 reveal several characteristics, such as an autodiastylic (two-jointed) jaw suspension with a non-suspensory hyomandibula, which confirm the placement of Helicoprion within the chondrichthyan subclass Holocephali (or the more broadly defined Euchondrocephali). Holocephalans are primarily an extinct group, and the only extant representatives of the group are the specialized, deep-sea Chimaeriformes (also called ratfish).^[39] The relationship between Helicoprion and living chimaeras is very distant, but had been previously suspected based on details of its tooth anatomy.^[31]

Helicoprion can be characterized as a member of Eugeneodontiformes, an order of holocephalans that lived from the Devonian to Triassic periods and are defined by their tooth-whorls along the midline of the jaw and autodiastylic skulls.^[30][31][36] Within the Eugeneodontiformes, Helicoprion is placed within the Edestoidea, a group of eugeneodonts with particularly tall and angled symphyseal 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.^[30][36] The other family, which contains Helicoprion, is sometimes called Agassizodontidae,^[30][36] but is today referred to as Helicoprionidae; a name which was first used 70 years prior and which has priority.^[6] Helicoprionids have large, cartilage-supported whorls with strongly arched shapes, and did not shed their teeth; instead, their tooth whorls continually add new teeth with bases inclined forwards at the top of the whorl.^[6][30] Helicoprion is unique among members of its family in possessing a closed (complete) spiral whorl which revolves around itself multiple times, while the tooth whorls of other genera form an incomplete, open spiral.^[3][40] As most eugeneodonts are based on fragmentary tooth remains, relationships within the group remain unclear.^[27][36] A cladogram illustrating the group's possible relations, drawn from Rainer Zangerl's 1981 volume of the Handbook of Paleoichthyology,^[30] is provided below.

Eugeneodontida (=Eugeneodontiformes)	Caseodontoidea (=Eugeneodontoidei) Caseodontidae Fadenia Erikodus Ornithoprion Caseodus Romerodus Eugeneodontidae Bobbodus Eugeneodus Gilliodus Edestoidea Agassizodontidae (=Helicoprionidae) Campyloprion Parahelicoprion Helicoprion Agassizodus Toxoprion Sarcoprion Edestidae Syntomodus Lestrodus Edestus Helicampodus Parahelicampodus

Paleobiology and whorl function

Whorl reconstruction and function

Cross-section of the jaw of Helicoprion: Interior revolutions of the whorl not shown

Due to the narrow nature of Helicoprion's jaws, suction feeding is unlikely to have been effective, and Helicoprion is thought to have been a bite feeder. Biomechanical modelling in a 2015 paper by Ramsay and colleagues suggests that the teeth in the whorl had distinct functions depending on where they were in the spiral. The foremost teeth served to snag and pull prey further into the mouth, while the middle teeth spear and the hind teeth puncture and bring prey further into the throat. The prey would be squeezed between the whorl and the palatoquadrates during feeding. The labial cartilage served to provide support for the whorl. 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.^[37]

Jaw motion of Helicoprion, after Ramsay and colleagues (2015)

Helicoprion may have started with a wide gape during 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. Helicoprion likely used a series of rapid, forceful jaw closures to initially capture and push prey deeper into the mouth, followed by repeated opening and closing of the jaw to saw through prey.^[37]

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 1,192 and 2,391 newtons (268 and 538 lb_f), with estimated bite stresses ranging from 397 to 797 million N/m² (57,600 to 115,600 psi) during prey contact. Due to its high bite force, Helicoprion may have also been able to eat vertebrates, in addition to cephalopods.^[37]

Historical whorl reconstructions

Earliest reconstructions

Alexander Karpinsky's 1899 hypothesis of the placement of the tooth whorl on H. bessonowi.

Hypotheses for the placement and identity of Helicoprion's tooth whorls were controversial since it was first discovered. Henry Woodward, 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 chondrichthyan fish. William Davies agreed, specifically comparing it to the jaws of the Permian petalodont Janassa bituminosa. Researcher J.S. Newberry alternatively 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 and at the time sometimes assigned to Ptychodus).^[11]

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.^[5]

Debates over the identity of Helicoprion's tooth whorl continued 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.^[41] Shortly after his original monograph, Karpinsky published the argument that the whorl represented a curled, scute-covered tail akin to that of Hippocampus (seahorses).^[42] 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.^[43] Paleoichthyologist A.S. Woodward (unrelated to Henry Woodward) followed this suggestion with the hypothesis that each whorl represented a tooth battery from a gigantic shark.^[44] G. Simoens illustrated Karpinsky's various proposals and used histological data to adamantly argue that the whorls were toothed structures placed within the mouth.^[45] In 1911, Karpinsky illustrated the whorls as components of the dorsal fins.^[17] Reconstructions similar to those of Karpinsky (1899) were used in Russian publications and textbooks as late as 2001.^[6]

Later reconstructions

An outdated illustration of H. bessonowi, with the skull based on a figure published by Janvier (1996).

By the mid-20th century, the tooth whorl was generally accepted to be positioned in the lower jaw. 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.^[6][31]

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.^[14]

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.^[46] 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.^[6][47]

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 symphyseal 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.^[48] This reconstruction was criticized for the overly intricate and potentially ineffective design of such a structure, if solely used to assist swallowing.^[49]

Lebedev (2009) found more support for a reconstruction similar to those of Bendix-Almgreen (1966) and Troll (1994).^[6] 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 eugeneodont 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 (ichthyoteuthophage) have reduced dentition, often restricted to the tip of the lower jaw.^[6] Lebedev's reconstruction approximates modern views on Helicoprion's anatomy, though the hypothetical long jaw has been superseded by CT data.^[31]

Paleoecology

The wide geographic distribution of Helicoprion suggests it was a pelagic animal which may only have migrated into shallower waters periodically.^[6] During the Artinskian, the Phosphoria Formation represented a deep inland sea with an anoxic, muddy bottom. Based on the large number of Helicoprion specimens discovered in the formation, the Phosphoria Sea may have been either a nursery for breeding Helicoprion, or potentially a hunting ground.^{[1]: 116–117}

Helicoprion is believed to have been an apex predator.^[23] The taxon's diet likely consisted primarily of cephalopods such as belemnoids, nautiloids and ammonoids, and it also may have hunted cartilaginous and unarmored bony fish as well.^[6][37]

Extinction

Although the genus is sometimes characterized as dying out during the Permian-Triassic extinction event, no Helicoprion specimens are known from the Late Permian,^{[1]: 249–251} and the youngest Helicoprion fossils instead date to the Guadalupian or Middle stage of the Permian.^[3][10][40] The disappearance of the genus is not associated with a larger extinction event, and has been suggested to have been a "background extinction".^{[1]: 249–251}

Notes

1. The full name of Mr. Davis is unknown