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Parahelicoprion

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Not to be confused with Parahelicampodus or Paredestus.
Parahelicoprion
Temporal range: Early Permian (Asselian-Artinskian),
~298.9-283.5 Ma
The reassembled holotype specimen of Parahelicoprion clerci, with the foremost teeth facing left. Material labeled (A) was described in 1916, while material labeled (B) and (C) are associated fragments which were later assigned to the same specimen
Additional photograph of the holotype from another angle, with foremost teeth facing right
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Holocephali
Order: †Eugeneodontida
Family: †Helicoprionidae
Genus: †Parahelicoprion
Karpinsky, 1924
Type species
Helicoprion clerci
Karpinsky, 1916
Species
  • P. clerci Karpinsky, 1916
  • P. mariosuarezi Merino-Rodo & Janvier, 1986
Synonyms

Parahelicoprion (meaning 'near spiral saw' or 'near Helicoprion') is an extinctgenus of large shark-like cartilaginous fish that lived during the Early Permian. Two species of are known ; P. clerci from Arta Beds of the Ural Mountains of Russia, and P. mariosuarezi from the Copacabana Formation of Bolivia. Members of the genus possessed a row of large tooth crowns on the midline of the lower jaw, known as a tooth whorl. The characteristics of this whorl are unique to fishes of the order Eugeneodontida, and more specifically the family Helicoprionidae to which Parahelicoprion belongs. The genus name refers to Helicoprion ('spiral saw'), another eugeneodont from the Ural Mountains that bore a similar midline tooth arrangement.

The holotype of Parahelicoprion clerci, which consists only of tooth fragments, was badly damaged by mining. That of the Bolivian P. mariosuarezi consists of only nine partial teeth, all of which are missing their outer edges. Estimates of the extent of the complete whorl, body size, and ecology of Parahelicoprion are highly speculative as a result of its incomplete fossils, although it is assumed to have been very large, predatory, and potentially pelagic. When first described, P. clerci was considered a species of Helicoprion, although its initial describer, Alexander Karpinsky, later separated it into its own genus. It has since been suggested that this genus may indeed represent a junior synonym of Helicoprion or a paraphyletic, non-diagnostic taxon.

Discovery and naming

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Photograph of Onésime Clerc, for whom the species P. clerci is named (left), and the location of Krasnoufimsk, where the known remains of the species originated (right)

The type specimen of Parahelicoprion was initially named Helicoprion clerci by the influential Russian geologist Alexander Karpinsky in 1916. The fossil was found by a miner in the Ural Mountains region of Russia,[1][2] in strata dated to the Artinskian stage of the Cisuralian epoch (early Permian).[3][4] The rocks that produced P. clerci have subsequently been defined as part of the Divjinskian Formation (alternatively spelled Divya Formation)[3][5] and are composed of marlstone.[1] Karpinsky reassigned this specimen to its own genus in 1924,[6] although he had already informally referred to it as Parahelicoprion in a series of publications two years prior and had even suggested it may warrant its own genus when he first named the species.[7][8][1] His given genus name derives from the Greek prefix -para, meaning 'near' or 'beside', and from the name of the related Helicoprion,[9] itself meaning 'coiled saw' or 'spiral saw'.[10][11][12] In scientific nomenclature, -para is often used to denote similarity or relation.[13][14] The specific name, P. clerci, honors Онисим Клер (romanised as Onésime Clerc), who at the time of its description was the president of the Ural Society of Natural Science Lovers. The exposure of the Divya Formation where the type specimen was found is near the town of Krasnoufimsk,[1][2] and a cast of this material was subsequently housed at the Krasnoufimsk Museum.[15]

A second species, Parahelicoprion mariosuarezi, was described and tentatively assigned to the genus in 1986 by Dagmar Merino-Roda and Philippe Janvier. This species, which is based on a single large specimen (designated no. 6097, YPFB), was discovered in the Copacabana Formation of Yauri chambi, Bolivia, and was dated to the Asselian stage of the early Permian.[16][17] The holotype, a three-dimensional partial tooth whorl,[17] was found preserved in a layer of calcarious red sandstone. The species is named in honor of Dr. Mario Suarez-Riglos,[16] and the type specimen is currently housed in the collection of the Noel Kempff Mercado Natural History Museum.[18]

Description

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Fragments of the large spine-like organ which Karpinsky assigned to Parahelicoprion clerci.[19] This specimen has since been recognized as belonging to the form-genera Physonemus or Xystracanthus[20][21]

Both Parahelicoprion species are very incompletely known,[22] and the only material which has been assigned confidently to the genus consists of fragments of the lower symphyseal (midline) tooth whorl.[23][15] While some authors have suggested the genus lacks defining, autapomorphous features,[16] the tooth crowns of both species of Parahelicoprion are noted to share extremely long, backwards-sweeping lower sections (referred to as "wings" or "ribs")[4][1] which extend nearly to the base of the tooth root, as well as curved denticles along the edges of the crowns.[4] In addition to tooth whorls, it has sometimes been suggested that large, forward-arching fin spines under the form-genus Physonemus (also classified as Xystracanthus) may have belonged to Parahelicoprion and related eugeneodonts,[3][21][19] with the Uralian species Xystracanthus grandis or its potential synonym Physonemus mirabilis suggested to correspond to Parahelicoprion clerci.[19][20][21] In a 2023 publication, researcher Serge Naugholnykh has asserted personal belief that these spines correspond to edestoids,[3] although sickle-like ichthyodorulites (fossilized fish spines) are believed by most modern researchers to represent the copulatory organs of symmoriiform fishes.[4][24][25] Members of the Eugeneodontida are today generally assumed to have lacked fin spines.[4][26]

Speculative life reconstruction of Parahelicoprion clerci, which conforms to that of P. mariosuarezi featured in Phillipe Janvier's Early Vertebrates (1996)[27]

The postcranial anatomy of eugeneodonts has been suggested to vary little between genera,[26] indicating that, like their smaller relatives, both species of Parahelicoprion possessed long, fusiform bodies with crescent-shaped caudal fins, and that they lacked pelvic and anal fins.[16][26] Paleontologist Philippe Janvier has reconstructed P. mariosuarezi (which he contributed to describing) with a proportionally very small, short tooth whorl situated at the tip of a pointed, greatly elongated pair of jaws,[27] an anatomy suggested in the animal's initial description based on the partial skull of Sarcoprion edax and the well-preserved fossils of the related caseodontoids.[16][26] In his 1916 description of Parahelicoprion clerci (then Helicoprion clerci), Karpinsky assumes the whorl of the species formed a large spiral,[1] and some subsequent authors have agreed that the shape of the whorl in life would have been indistinguishable from that of Helicoprion.[3][28]

Parahelicoprion clerci

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Three re-assembled crowns of the type specimen of P. clerci, photographed in lateral (1a) and transverse (1b) views
Additional fragments of the P. clerci holotype

The type specimen of Parahelicoprion clerci consists of several badly damaged tooth fragments, all found in association and believed to belong to a single tooth whorl. During preparation by Karpinsky in 1916, three of these fragments were glued together in order to reflect their in-life articulation,[1] and by 1926 these had again been assembled with several additional associated fragments discovered from the same locality.[29] The holotype specimen as recognized today spans 25 cm (10 in) across,[10] and consists of six partial tooth crowns.[16][10][26] Alexander Karpinsky believed that the material he described represented only a small portion of the tooth whorl as it was preserved, with the rest having been either destroyed or not collected during mining (a process which also degraded the surface texture of the specimen). No lateral dentition is confidently associated with this species,[8] although crushing teeth similar to those of Campodus may have been present.[30] The general shape of the reconstructed whorl fragments observed in Parahelicoprion clerci has been compared to that of the potential close relative Sarcoprion by various authors,[4][15][26] although ichthyologist Svend Erik Bendix-Almgreen has insisted that the material of the former is too poorly known for such comparisons to be made with confidence.[15]

Reconstructed teeth of P. clerci in lateral (1a) and transverse (1b, 2b) views, compared with the teeth of Helicoprion bessonowi (2a, 3).
Tip of the "wing" or "rib" of the tooth of P. clerci, displaying this species' unique arrangement of denticles (above) and crenulations (below)

The largest tooth crown tip (not including the serrated "rib") of the holotype measures 3.4 cm (1.3 in) in height, 3 cm (1.2 in) across, and 1.9 cm (0.75 in) wide at the widest point, while the smaller known tip measures 2.4 cm (0.94 in) in height, 2.9 cm (1.1 in) across and 1.5 cm (0.59 in) in width. This dramatic shift in size across a small portion of the whorl is different from that observed in Helicoprion bessonowi, in addition to the fact that the largest P. clerci tooth crown is significantly larger than that of the former.[1] The posterior edges of the crown tips thin to form sharp cutting blades.[1][8] The uppermost tips of the tooth crowns are uniquely rounded and blunt, rather than tapering like those of many other edestoids.[1] As in other members of its family, the teeth of the whorl angle forward in the mouth.[26] The "ribs" or "wings" (long segments which anchor to the root), which curve gently towards the back of the jaws, are extremely elongated and are lined with very deep, downward-angled serrations along the anterior edge and crenulations or corrugations along the sloping posterior edge.[1][4][26] The form of these serrations and crenulations is unique to this species.[1]

The teeth are described by Karpinsky as being coated in enamel,[1] although this substance has subsequently been identified as enameloid or vitrodentine in other helicoprionids.[26][31] The interior of the teeth was composed of a spongy form of dentin, identified by Karpinsky as vasodentin (a form common amongst fishes).[1][8]

Parahelicoprion mariosuarezi

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The holotype whorl of Parahelicoprion mariosuarezi larger than that of P. clerci.[16][10] Merino-Rodo and Janvier (1986) state that it is likely nearly complete, with the exception of the cutting edges along the upper part of the whorl being broken off.[16] Unlike the type species, serrations and crenulations are absent along most of the crown, but several large, pointed denticles do protrude along the lingual (lower) edges of the third to ninth tooth crowns. This species also preserves a single denticle-bearing parasymphyseal tooth (a form of lateral dentition) along the edge of the whorl, a tooth type which is not known from P. clerci. The holotype of P. mariosuarezi shows the smallest crown was positioned anteriorly at the very front of the preserved portion of the whorl and was significantly smaller than the next crown in the sequence,[4][16] which suggests that the whorl was short and did not form a helical spiral. If the type of P. mariosuarezi is nearly complete as suggested in its description, then in life the whorl, which as preserved possesses only nine tooth crowns,[16] bore far fewer crowns than related genera such as Helicoprion (which may have had between 130-180).[11][32][33] Despite being unpreserved, the outer cutting edges of the teeth in P. mariosuarezi are thought to have conformed in shape and orientation to those of the type species, due to the preserved portions of the crowns being similar in shape and thickness.[16]

Estimated length

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A high-end length estimate of the closely Helicoprion bessonowi, based on the proportions of caseodonts such as Romerodus and Caseodus. It has been proposed that Parahelicoprion may have exceeded H. bessonowi in size[1]

While the known tooth crowns of the genus are considered to be among the largest of the eugeneodonts, believed to be up to 15 cm (5.9 in) in height when complete,[34] estimates of Parahelicoprion's total body size are controversial.[35] Karpinsky himself did not provide body length estimates in his 1916 description, but did note that the tooth crowns of P. clerci were very large, and that the animal must have been similarly large to accommodate them.[1] Philippe Janvier refers to the potentially even larger P. mariosuarezi as "huge" in his book Early Vertebrates (1996),[27] and in Doug Perrine's 1999 book Sharks and Rays it is stated that Parahelicoprion "... might have been over 100 ft (30 m) in lengths - perhaps the largest fish of all time", based on an estimate proposed by paleontologist Richard Lund.[34] A similar length was suggested by author and illustrator Richard Ellis in his 2003 book Aquagenesis: The Origin and Evolution of Life in the Sea. Ellis states that, in spite of the poor quality of the known material, "... unless it [Parahelicoprion] was an animal with a gigantic head or outlandishly oversized teeth, it had to have been a monster, at least 100 feet long and maybe more." This conclusion was reached based on comparisons between the height of the teeth in Otodus megalodon, which the teeth of Parahelicoprion are said to exceed in size.[10] Body length estimates for eugeneodont fishes published in academic literature range up to 11 meters (36 ft).[35]

Oleg Lebedev, a researcher from the Palaeontological Institute of the Russian Academy of Sciences, estimated in a 2009 publication that the closely related Helicoprion bessonowi may have been between 5–8 meters (16–26 ft) in total body length; a measurement based on assumptions about its head-to-whorl proportions in comparison to the related caseodonts.[35][32] Later studies have proposed a most plausible total length of up to 7 meters (23 ft) for both Helicoprion and other, similarly large edestoids.[35][36] Due to the fragmentary nature of their fossils, however, it has been considered unreasonable by some researchers to give precise total length estimates for Helicoprion, Parahelicoprion, or any other members of Edestoidea.[23][35] Though their exact maximum sizes have not been determined, both species of Parahelicoprion were likely among the largest animals in their respective environments,[3][27] and they and other edestoids likely represented the some of the largest fishes of the Paleozoic era.[37][36][38]

Classification

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Whorl segments of P. clerci illustrated in lateral (3) and transverse (4) views, compared with those of the related eugeneodonts Campodus, Helicoprion, and Edestus[8]

When first named Parahelicoprion was considered a member of the family Edestidae, which at that time also encompassed genera such as Helicoprion and Campodus.[8] While the relation and classification of edestids and helicoprionids was variable throughout the 20th century,[15][26][39] Parahelicoprion is today considered to be a member of the family Helicoprionidae within the monophyletic order Eugeneodontida (alternately spelled Eugeneodontiformes),[4][30] which is itself a member of the subclass Holocephali or Euchondrocephali.[4][37][40] The helicoprionids (described as agassizodontids by some authors)[26] are defined by possessing tooth whorls with forward angled, blade-like tooth crowns and, in many genera, tooth roots which are completely fused. Members of the group also possessed a laterally positioned pavement of flattened crushing teeth. Other aspects of the group's jaws and skull are only rarely preserved, and nothing is known of their postcranial anatomy.[4][26]

Helicoprion bessonowi (left) and Sarcoprion edax (right), both of which are considered close relatives of Parahelicoprion.[4] Exhibit displayed at the Moscow Paleontological Museum

In a 1925 publication, Karpinsky suggested that P. clerci may represent a directly intermediate, transitional form between the "primitive" genus Campodus and the more derived Helicoprion,[19] a conclusion agreed with by Egil Nielsen in his 1952 description of Sarcoprion and Parahelicampodus.[41] Svend Erik Bendix-Almgreem, in a 1976 paper, suggested that Parahelicoprion may have been related to a radiation of whorl-toothed cartilaginous fish unrelated to the helicoprionids and edestids. This lineage was proposed to also consist of Erikodus, Fadenia, and Sarcoprion,[15][26] with P. clerci considered a possible member due to similarities with the latter taxon.[15] Rainer Zangerl (1981) considered Parahelicoprion to be the sister taxon to Campyloprion in his morphological analysis of all (then known) members of the newly proposed order Eugeneodontida, which united the edestids, helicoprionids, and caseodonts.[26] Following Zangerl's analysis, paleontologists Dagmar Merino-Rodo and Philippe Janvier concluded in their 1986 description of P. mariosuarezi that the genus Parahelicoprion may lack defining derived characteristics, which puts its status as a monophyletic group into question and complicates the matter of assigning new species.[16] In a 2018 publication, paleontologist Serge Naugholnykh proposes that P. clerci specimens simply represent especially large individuals of Helicoprion and that the two genera are synonymous,[3][28] although subsequent papers have continued to recognize Parahelicoprion as a valid genus of helicoprionid eugeneodont.[22][30][42]

Paleobiology and paleoecology

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Ecology and habitat

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A map of Earth as it appeared during the Cisuralian epoch

Parahelicoprion was a carnivore that inhabited marine environments.[16][17][43] It has been suggested to have been the apex predator of its ecosystem,[3] with the blade-like serrated tooth whorl being adapted for cutting and battering prey.[1][10] While multiple feeding styles are thought to have been present among different genera of edestoids,[32][30] it has been hypothesized that members of the Helicoprionidae were molluscivorous and fed primarily on ammonoid and coleoid cephalopods,[32][44] with smaller cartilaginous fish potentially constituting a portion of their diet as well.[44]

The teeth of Artiodus prominens, a euselachian stem-group shark known from the Arta Beds of Krasnoufimsk[5]

The Artinskian deposits of Krasnoufimsk, or the Arta Beds, were deposited in a shallow sea basin between the Boreal and Tethys oceans.[3] These fossil beds are made up of the lower (older) Divjinskian or Divya Formation and the upper (younger) Sarginskian Formation, and are composed predominantly of marls and limestones.[3][5][45] During the early Permian, reef habitats made up of bryozoans and rugose corals were present, which were inhabited by a diverse assemblage of trilobites,[5] goniatite and nautiloid cephalopods,[45] and fishes.[3] In addition to Parahelicoprion clerci, the Divya Formation yields a large variety of chondrichthyan fossils, including the remains of euselachian sharks, hybodonts, symmoriiformes, petalodonts and cochliodonts.[5] The similar helicoprionid Helicoprion bessonowi is also known from numerous tooth whorls collected in the Divya Formation.[3][5][33]

The Copacabana Formation represented a shallow marine habitat, somewhat older than the Arta Beds and dated to the boundary between the Carboniferous and the Permian. Of the formation's two strata which preserve fish fossils, Parahelicoprion is known only from the upper (younger) layer, while the majority of observed species come from a slightly older layer below it. Among these species, actinopterygians (ray-finned fish) and holocephalans have been identified. The known fossils of holocephalans include remains of a large petalodont similar to Megactenopetalus as well as the tooth-plates of cartilaginous fishes similar to Lagarodus or Helodus, both believed to be bottom dwelling durophages. Teeth and scales belonging to bony fish in the family Platysomidae are also known, and isolated teeth suggest that cladodont sharks (Identified by Merino-Rodo and Janvier as Cladodus) were also present in the environment.[16][43] The teeth of jawless, fish-like vertebrates called conodonts are abundant, and it is from these index fossils that the age of the formation has been determined. Marine invertebrates have also been found at the site, and include bivalves, brachiopods, trilobites, crinoids and bryozoans. While lower fish-bearing strata of the Copacabana Formation are believed to represent a benthic reef community, Merino-Rodo and Janvier suggest that the sandstones which compose the upper fish-bearing layer and which preserve the whorl of Parahelicoprion may have formed in an even shallower, intertidal habitat, and that the type of P. mariosuarezi represents a stranded animal.[16] In spite of the difference in time and proposed habitat, Janvier has subsequently described and illustrated the aforementioned species as coexisting.[27]

Extinction

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The latest known occurrence of the genus is the Artinskian stage of the Permian,[5] and it is assumed to have been extinct by the end of the Cisuralian.[42] Karpinsky has suggested that the disappearance of the seaway connecting the Arctic and Tethys oceans was directly responsible for the extinction of the Uralian edestoids.[1] Alongside Parahelicoprion, many cartilaginous fish genera of the Divya Formation disappear from the fossil record at the close of the Artinskian stage.[5]

See also

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References

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  1. ^ a b c d e f g h i j k l m n o p q r Karpinsky, Alexander Karpinsky (27 April 1916). "On a new species of Helicoprion (Helicoprion clerci, n. sp.)". Bulletin de l'Académie Impériale des Sciences de Saint Pétersbourg (in Russian) (6): 701–708 – via Biodiversity Heritage Library.
  2. ^ a b "Notes". Nature. 98 (2447): 54–55. 21 September 1916 – via Biodiversity Heritage Library.
  3. ^ a b c d e f g h i j k l Naugolnykh, S.V. (2018). "Artinskian (Early Permian) Sea Basin and Its Biota (Krasnoufimsk, Cis-Urals)". Stratigraphy and Geological Correlation. 26 (7): 734–754. Bibcode:2018SGC....26..734N. doi:10.1134/S0869593818070080. S2CID 135304766.
  4. ^ a b c d e f g h i j k l Ginter, Michał; Hampe, Oliver; Duffin, Christopher J. (2010). Handbook of paleoichthyology: teeth. München: F. Pfeil. pp. 117–126. ISBN 978-3-89937-116-1.
  5. ^ a b c d e f g h Ivanov, Alexander; Duffin, Christopher; Naugolnykh, Serge (2017). "A new euselachian shark from the Early Permian of the Middle Urals, Russia". Acta Palaeontologica Polonica. 62. doi:10.4202/app.00347.2017. ISSN 0567-7920.
  6. ^ Karpinsky, Alexander (1924). "Helicoprion (Parahelicoprion n.g.) clerci". Записки Уральского общества любителей естествознания (Notes of the Ural Society of Natural Science Lovers). 39: 1–10.
  7. ^ Karpinsky, Alexander (1922). "Helicoprion Ivanovi, n. sp". Bulletin de l'Académie des Sciences de Russie (Bulletin of the Russian Academy of Sciences) (in Russian): 369–379 – via Google Books.
  8. ^ a b c d e f Karpinsky, Alexander (1922). "Замечания о зубных сегмента х Edestidae и об и х ориентировке (Notes on the dental segments of Edestidae and their orientation)". Bulletin de l'Académie des Sciences de Russie (Bulletin of the Russian Academy of Sciences) (in Russian): 379–388 – via Google Books.
  9. ^ "para- | Etymology of the prefix para". etymonline.com. Retrieved 22 October 2024.
  10. ^ a b c d e f Ellis, Richard (2003). Aquagenesis: the origin and evolution of life in the sea. New York, N.Y: Penguin. pp. 120–121. ISBN 978-0-14-200156-1.
  11. ^ a b Eastman, C. R. (1900). "Karpinsky's genus Helicoprion: a review". The American Naturalist. 34 (403): 579–582. doi:10.1086/277706. JSTOR 2453848 – via JSTOR.
  12. ^ Ewing, Susan (2017). Resurrecting the shark: a scientific obsession and the Mavericks who solved the mystery a 270 million year old fossil. New York London: Pegasus Books. p. 70. ISBN 978-1-68177-343-8.
  13. ^ "Para Definition and Meaning". Merriam-Webster. Retrieved 23 October 2024.
  14. ^ Mutter, Raoul J.; Neuman, Andrew G. (2008). "New eugeneodontid sharks from the Lower Triassic Sulphur Mountain Formation of Western Canada". Geological Society, London, Special Publications. 295 (1): 9–41. Bibcode:2008GSLSP.295....9M. doi:10.1144/SP295.3. ISSN 0305-8719.
  15. ^ a b c d e f g Bendix-Almgreen, Svend Erik (1976). "Palaeovertebrate faunas of Greenland". Geology of Greenland: 557–559. doi:10.22008/GPUB/38226.
  16. ^ a b c d e f g h i j k l m n o Merino-Rodo, Dagmar; Janvier, Philippe (1986). "Chondrichthyan and actinopterygian remains from the Lower Permian Copacabana Formation of Bolivia". Geobios. 19 (4): 479–493. Bibcode:1986Geobi..19..479M. doi:10.1016/S0016-6995(86)80005-5. S2CID 131213667.
  17. ^ a b c "PBDB Taxon". paleobiodb.org. Retrieved 15 October 2024.
  18. ^ "Colleciones Cientificas". museonoelkempff.org. Retrieved 11 October 2024.
  19. ^ a b c d Karpinsky, Alexander (1925). "Sur une nouvelle trouvaille de restes de Parahelicoprion et sur relations de ce genre avec Campodus" [On a New Find of Remains of Parahelicoprion and on the Relations of This Genus with Campodus]. Soc. Geol. Belg. Livre Jubilaire (in French). 5 (1): 125–137 – via HathiTrust.
  20. ^ a b Zidek, Jiri (1977). "Oklahoma Paleoichthyology Addendum to Part V" (PDF). Oklahoma Geology Notes. 37 (5): 151–156 – via Oklahoma Geological Survey.
  21. ^ a b c Baird, Donald (1957). "A Physonemus Spine from the Pennsylvanian of West Virginia". Journal of Paleontology. 31 (5): 1011–1018. JSTOR 1300575 – via JSTOR.
  22. ^ a b Sansom, Ivan J. (2024-01-24). "Supplementary material from "The skeletal completeness of the Palaeozoic chondrichthyan fossil record"". Royal Society Open Science. doi:10.6084/m9.figshare.c.7041560.v1.
  23. ^ a b Maisey, John G.; Bronson, Allison W.; Williams, Robert R.; McKinzie, Mark (2017-05-04). "A Pennsylvanian 'supershark' from Texas". Journal of Vertebrate Paleontology. 37 (3): e1325369. Bibcode:2017JVPal..37E5369M. doi:10.1080/02724634.2017.1325369. ISSN 0272-4634.
  24. ^ Lund, Richard (1986-03-07). "On Damocles serratus , nov. gen. et sp. (Elasmobranchii: Cladodontida) from the Upper Mississippian Bear Gulch Limestone of Montana". Journal of Vertebrate Paleontology. 6 (1): 12–19. Bibcode:1986JVPal...6...12L. doi:10.1080/02724634.1986.10011594. ISSN 0272-4634.
  25. ^ Maisey, John G. (2009-03-12). "The spine-brush complex in symmoriiform sharks (Chondrichthyes; Symmoriiformes), with comments on dorsal fin modularity". Journal of Vertebrate Paleontology. 29 (1): 14–24. Bibcode:2009JVPal..29...14M. doi:10.1671/039.029.0130. ISSN 0272-4634.
  26. ^ a b c d e f g h i j k l m n Zangerl, Rainer (1981). Handbook of Paleoichthyology Volume: 3A: Chondrichthyes 1 (1st ed.). Verlag Dr. Friedrich Pfeil. pp. 74–94. ISBN 978-3899370454.
  27. ^ a b c d e Janvier, Philippe (1996). Early vertebrates. Oxford science publications. Oxford : New York: Clarendon Press ; Oxford University Press. p. 22. ISBN 978-0-19-854047-2.
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