| Parahelicoprion | |
|---|---|
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| The reassembledholotype specimen ofParahelicoprion 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 | |
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| Additional photograph of the holotype from another angle, with foremost teeth facing right | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Order: | †Eugeneodontiformes |
| Family: | †Helicoprionidae |
| Genus: | †Parahelicoprion Karpinsky, 1924 |
| Type species | |
| Helicoprion clerci Karpinsky, 1916 | |
| Other species | |
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| Synonyms | |
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Parahelicoprion is anextinctgenus ofshark-likecartilaginous fish that lived during theEarly Permian. The genus contains two species:P. clerci from theArta Beds of theUral Mountains ofRussia, andP. mariosuarezi from theCopacabana Formation ofBolivia. Members of the genus possessed a row of largetooth crowns on the midline of the lower jaw, known as a tooth whorl. The characteristics of this whorl are unique to fishes of the orderEugeneodontida, and more specifically the familyHelicoprionidae to whichParahelicoprion belongs. The genus name refers toHelicoprion, another eugeneodont from the Ural Mountains that bore a similar midline tooth arrangement.
Theholotype ofParahelicoprion clerci, which consists only of tooth fragments, was badly damaged by mining and is broken into several pieces. That of the BolivianP. mariosuarezi similarly consists of only nine partial teeth, the outer edges of all of which are broken off. Estimates of the extent of the complete whorl, body size, and ecology ofParahelicoprion are speculative as a result of its incomplete fossils, although it is assumed to have been very large, predatory, and potentiallypelagic. When first described,P. clerci was considered a species ofHelicoprion, although its initial describer,Alexander Karpinsky, later separated it into its own genus. It has since been suggested that this genus may indeed represent ajunior synonym ofHelicoprion or aparaphyletic,non-diagnostictaxon.
Thetype specimen ofParahelicoprion clerci was found in the Ural Mountains region of Russia,[1][2] instrata dated to theArtinskian stage of theCisuralianepoch (early Permian).[2][3][4] The fossil was embedded in amatrix of very fine-grainmarlstone,[1][5][6] and although it is exceptionally well-preserved it is broken into multiple fragments.[5] The first five portions of the specimen to be found, which consist of several teeth, were discovered incidentally by aminer and were badly damaged as a result poor handling during collection.[1][5][7] Additional fragments of the holotype were identified in the years following the taxon's description, all in similar condition and found within one square meter of the first find. These remains include a single partial tooth described in 1925 and a collection of three partial teeth purchased byGeorgii Fredriks and realized to correspond with the same fossil in 1926.[6][7] Several fragments of the tooth whorl were sold toprivate collectors and were never scientifically examined. Pieces of anichthyodorulite (fossilized fish spine) found in association have also been suggested to have come from the same individual as the teeth.[6] The rocks which produced the holotype specimen have, since its discovery, been defined as part of theDivjinskian Formation (alternatively spelled Divya Formation).[3][8] The exposure of the Divya Formation where the type specimen was found is near the town ofKrasnoufimsk,[1][2][5] and as of 1976 acast of theP. clerci holotype was housed at the Krasnoufimsk Museum.[9] The species is known only from the damaged holotype.[10]
The type specimen ofParahelicoprion was initially namedHelicoprion clerci by the RussiangeologistAlexander Karpinsky in 1916.[1][5] Karpinsky reassignedHelicoprion clerci to its own genus in a work published in 1924,[5] although he had informally referred to it asParahelicoprion in a series of publications two years prior and had even suggested it may warrant its own genus when he first named the species.[1][11][12] The given genus name derives from that of the relatedHelicoprion, meaning'coiled saw' or'spiral saw',[5][10][13] although due to the assumed shape ofP. clerci's tooth whorl Karpinsky considered this name technically unfitting.[5] The species name,P. clerci, honorsOnésime Clerc, who at the time of its description was the president of theUral Society of Natural Science Lovers.[1][2][5] Clerc was responsible for bringing the type specimen ofP. clerci to Karpinsky's attention.[6]
A second species,Parahelicoprion mariosuarezi, was described and tentatively assigned to the genus in 1986 by Dagmar Merino-Roda andPhilippe Janvier. This species, which is also based only on its type specimen (designated no. 6097, YPFB), was discovered in the Copacabana Formation ofYauri chambi, Bolivia, and was dated to theAsselian stage of the early Permian.[14][15] The holotype, a three-dimensional partial tooth whorl,[15] was found preserved in alayer ofcalcariousred sandstone. The species is named in honor of Dr. Mario Suarez-Riglos,[14] and the type specimen is currently housed in the collection of theNoel Kempff Mercado Natural History Museum.[16]
BothParahelicoprion species are very incompletely known,[19] and the only material which has been assigned confidently to the genus consists of fragments of the symphyseal (midline) tooth whorl.[9][20] While some authors have suggested the genus lacks defining,autapomorphous features,[14] the tooth crowns of both species ofParahelicoprion are noted to share extremely long, backwards-sweeping, recessed lower sections (sometimes referred to as "wings" or "ribs")[4][1][5] which extend nearly to the base of thetooth root, as well as curveddenticles orserrations along the edges of these wings.[4] In addition to tooth whorls, it has been suggested that large, forward-arching fin spines under theform-genusPhysonemus (also classified asXystracanthus) may have belonged toParahelicoprion and related eugeneodonts,[3][18][6] with the Uralian speciesXystracanthus grandis or its potential synonymPhysonemus mirabilis suggested to correspond toParahelicoprion clerci.[6][17][18] In a 2018 publication, researcher Serge Naugholnykh asserted personal belief that these spines correspond to edestoids,[3][21] although sickle-like spine fossils are believed by other researchers to represent thecopulatory organs ofsymmoriiform fishes or,[4][22][23] alternatively, of undescribed holocephalans.[24][25] It is generally assumed that members of the Eugeneodontiformes such asParahelicoprion lacked fin spines.[4][26][27]
In life, the tooth whorl ofParahelicoprion was positioned symphyseally (along the midline) in the lower jaw.[4][5][14] In his 1916 description ofP.clerci (thenHelicoprion clerci), Karpinsky assumed the whorl of the species formed a very large spiral,[1] and Serge Naugholnykh has subsequently agreed that the shape of the complete whorl would have been indistinguishable from that ofHelicoprion.[3][21] In a 1924 publication, however, Karpinsky calculated that the complete whorl ofP. clerci likely consisted of no more than 20 tooth crowns and did not form a spiral, although he considers the precise number of tooth crowns unknown.[5] The description ofP. mariosuarezi suggests that it likely possessed a very short tooth whorl situated at the tip of a greatly elongated pair of jaws,[14][28] based on both the partial skull ofSarcoprion edax and the well-preserved fossils of the relatedcaseodonts.[14] Various other authors have agreed with aSarcoprion-like interpretation,[9][26][29]: S1 but have acknowledged that the shape of the complete whorl is not known and that the material is too fragmentary for such comparisons to be confidently made.[9][29]: S1 Thepostcranial anatomy of eugeneodonts has been suggested to vary little between genera,[26] indicating that, like their smaller, better known relatives, both species ofParahelicoprion possessed long,fusiform bodies with crescent-shapedcaudal fins, and that they lackedpelvic andanal fins.[14][26]
The known remains ofParahelicoprion clerci consist of badly damaged tooth fragments,[5][7] all belonging to a single tooth whorl.[7][10] During preparation by Karpinsky in 1916, three of these fragments were glued together in order to reflect their in-life articulation,[1][6] and by 1926 these had again been assembled with additional associated material.[7] The holotype specimen as recognized today spans 25 cm (10 in) in profile view and consists of six articulated tooth crowns,[10][14][26] as well as an associated smaller tooth fragment.[5] The surface texture of the teeth has been badly damaged by mining, and Karpinsky considered it likely that parts of the fossil had been destroyed or never collected.[1][5] Nolateral dentition is confidently associated with this species,[12] althoughcrushing teeth similar to those ofCampodus orOrodus may have been present.[7][6][30]
The teeth increase significantly in size towards the rear of the whorl,[7] with the largest tooth crown tip (not including the serrated "rib") of the holotype measuring 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 smallest 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.[1] The morphology of the teeth also change depending on their position, with the teeth farthest forward being rounder and more asymmetrical than those at the back, which are nearly completely symmetrical in profile.[7] The posterior edges of the foremost crown's tips thin to form sharp cutting blades,[1][7][12] which are not serrated.[5] In the rear crown tips, both the posterior and anterior edges of the tooth are blade-like.[7][6][10] The uppermost apexes of all the known tooth crowns are uniquely rounded and blunt, rather than tapering to a point like those of many other edestoids.[1][5] 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) curve gently towards the back of the jaw and are extremely elongated.[4] The wings are lined with very deep, downward-angled serrations along the anterior edge and crenulations or corrugations along the sloping posterior edge,[1][4][26] both of which are absent from the tips of the crowns.[5] The form of these serrations and crenulations is unique to this species.[1][5] The root ofParahelicoprion's whorl is described as entirely fused, with no gaps or separations between segments.[5][12]
The histology of the holotype dentition is suggested to have originally been well-preserved, although on the exterior of the teeth the damage is severe enough that it impedes histological study.[5] The outer coating of the teeth, which is described by Karpinsky as beingenamel,[1][5] has been abraded away across most of the tooth fragments.[5] This substance has alternatively been identified asenameloid orvitrodentine in other helicoprionids, rather than true enamel.[4][26][31] The interior of the teeth remains in excellent condition, and was composed of a spongy form ofdentin identified by Karpinsky asvasodentin (a form common among fishes).[1][12] Unlike other eugeneodonts,trabecular dentin (aporous form found in chondrichthyans) was likely absent from the teeth.[6] The interior structure of the known specimen could not be studied in detail, however, as Karpinsky did not want to damage the specimen through cross-sectioning.[5]
The holotype whorl ofParahelicoprion mariosuarezi is larger than that ofP. clerci.[10][14] 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.[14] Unlike the type species, serrations and crenulations are absent along most of the crown, but several large, pointeddenticles do protrude along the wings 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 fromP. clerci. The holotype ofP. 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][14] The whorl possesses only nine tooth crowns,[14] far fewer than those of related genera such asHelicoprion, which had between 130 and 180 tooth crowns.[13][32][33] Despite being unpreserved, the outer cutting edges of the teeth inP. 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. The rear portion of the whorl is described as being fused.[14]
While the known tooth crowns of the genus are considered to be among the largest of the eugeneodonts, estimates ofParahelicoprion's total body size are controversial.[29] Karpinsky himself did not provide body length estimates in his descriptions,[29]: S1 but did note that the tooth crowns ofParahelicoprion clerci were significantly larger than those of any then-knownHelicoprion specimens and suggested that the animal must have been very large to accommodate them.[1][5] Philippe Janvier refers to the potentially even largerParahelicoprion mariosuarezi as "huge" in his bookEarly Vertebrates (1996),[28] and in Doug Perrine's 1999 bookSharks and Rays it is stated thatParahelicoprion "... might have been over 100 ft (30 m) in lengths – perhaps the largest fish of all time", based on an estimate proposed bypaleontologist Richard Lund.[34] A similar length was suggested by author and illustratorRichard Ellis in his 2003 bookAquagenesis: The Origin and Evolution of Life in the Sea. Ellis states that, in spite of the fragmentary nature 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 inOtodus megalodon, which the teeth ofParahelicoprion are said to exceed in size.[10] Lengths of 11–13 meters (36–43 ft) have more recently been suggested forParahelicoprion by online sources,[35][29]: S1 although these numbers originate from non-academicamateur researchers and are not supported byscientific literature.[29]: S1
Oleg Lebedev, a researcher from the Palaeontological Institute of the Russian Academy of Sciences, estimated in a 2009 publication that the closely relatedHelicoprion 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 with the relatedcaseodonts.[29][32] A later study has proposed a most plausible total length of up to 7 meters (23 ft) for bothHelicoprion andParahelicoprion, and suggests that previous overestimations ofParahelicoprion's size were the result of assuming that the fossils were very small fragments of aHelicoprion-like spiral, rather than being nearly complete.[29]: S1 Due to the fragmentary nature of the known material, it has been considered unreasonable by some researchers to give precise total length estimates forHelicoprion, Parahelicoprion, or any other members of Edestoidea.[20][29] Though their exact maximum sizes have not been determined, both species ofParahelicoprion were likely among the largest cartilaginous fish of thePaleozoic era.[36][37][38]
When first named,Parahelicoprion was considered a member of the familyEdestidae, which at that time also encompassed genera such asHelicoprion andCampodus.[6][12] While the relation and classification of edestids and helicoprionids was variable throughout the 20th century,[9][26][39]Parahelicoprion is today considered to be a member of the family Helicoprionidae within themonophyletic order Eugeneodontiformes (alternately spelled Eugeneodontida),[4][30] which is itself a member of the subclass Holocephali orEuchondrocephali.[4][37][40] The helicoprionids (also called 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 alaterally positioned pavement of flattened crushing teeth. Other aspects of the group's jaws and skulls are only rarely preserved, and nothing is known of their bodies.[4][26]
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In a 1925 publication, Karpinsky suggested thatP. clerci may represent a directly intermediate, transitional form between the "primitive" genusCampodus (based on material now assigned toAgassizodus)[26] and the more derivedHelicoprion,[6] a conclusion agreed with by Egil Nielsen in his 1952 description ofSarcoprion andParahelicampodus.[41] Svend Erik Bendix-Almgreem, in a 1976 paper, suggested thatParahelicoprion may have been part of a radiation of whorl-toothed cartilaginous fish unrelated to the helicoprionids andedestids. This lineage was proposed to also consist ofErikodus,Fadenia, andSarcoprion,[9][26] withP. clerci considered a possible member due to similarities with the latter taxon.[9] In 1981, Rainer Zangerl consideredParahelicoprion to be thesister taxon toCampyloprion in his morphological analysis of all (then known) members of the newly proposed order Eugeneodontiformes, which united the edestids, helicoprionids, and the caseodonts.[26] Following Zangerl's analysis, paleontologists Dagmar Merino-Rodo and Philippe Janvier concluded in their 1986 description ofP. mariosuarezi that the genusParahelicoprion may lackautapomorphic (derived) characteristics, which puts its status as amonophyletic group into question and complicates the matter of assigning new species.[14] In a 2018 paper, paleontologist Serge Naugholnykh proposed thatP. clerci simply represents an especially large individual ofHelicoprion and that the two genera are synonymous,[3][21] although subsequent papers have continued to recognizeParahelicoprion as a valid genus of helicoprionid eugeneodont.[19][30][42]
Parahelicoprion was a carnivore that inhabited marine environments.[14][15][43] It has been suggested to have been theapex predator of its ecosystem,[3] with the blade-like serrated tooth whorl being adapted for cutting prey.[1][5][6] 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 weremolluscivorous and fed primarily onammonoid andcoleoidcephalopods,[32][44] with smaller cartilaginous fish potentially constituting a portion of their diet as well.[44]
The Artinskian deposits of Krasnoufimsk, or the Arta Beds, were deposited in a shallow sea basin between theBoreal andTethys oceans.[3][5] These fossil beds are made up of the lower (older) Divjinskian or Divya Formation and the upper (younger)Sarginskian Formation, and are composed predominantly ofmarls andlimestones.[3][8][45] During the early Permian,reef habitats made up ofcrinoids,sponges,[6]bryozoans andrugose corals were present, which were inhabited by a diverse assemblage oftrilobites,[8]goniatite andnautiloid cephalopods,[45] and fishes.[3] In addition toParahelicoprion clerci, the Divya Formation yields a large variety of chondrichthyan fossils, including the remains ofeuselachian sharks,hybodonts,petalodonts,cochliodonts andcladodonts.[6][8] The similar helicoprionidHelicoprion bessonowi is also known from numerous tooth whorls collected in the Divya Formation.[3][8][33] Fossilized teeth collected from the Divya show no indication ofpost-mortem wear,[5] indicating calm water conditions.[6] Karpinsky had compared the site's preservational environment to theBurgess Shale and regarded the fossils found there as being very well preserved.[5][6]
The Copacabana Formation represented a shallow marine habitat, somewhat older than the Arta Beds and dated to the boundary between theCarboniferous 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) andholocephalans have been identified. The known fossils of holocephalans include remains of a large petalodont similar toMegactenopetalus as well as the tooth-plates of cartilaginous fishes similar toLagarodus orHelodus, both believed to be bottom dwellingdurophages. Teeth and scales belonging tobony fish in the familyPlatysomidae are also known, and isolated teeth suggest thatcladodont sharks (Identified by Merino-Rodo and Janvier asCladodus) were also present in the environment.[14][43] The teeth ofjawless, fish-likevertebrates calledconodonts are abundant, and it is from theseindex fossils that the age of the formation has been determined. Marine invertebrates have also been found at the site, and includebivalves,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 ofParahelicoprion may have formed in an even shallower,intertidal habitat, and that the type ofP. mariosuarezi represents the remains of an animal which stranded on the shore.[14] In spite of the difference in time and proposed habitat, Janvier has described and illustrated the aforementioned species coexisting.[28]
The latest known occurrence of the genus is the Artinskian stage of the Permian,[8] 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 theUralian edestoids. Although he does not suggest a specific cause of extinction elsewhere,[1][5] he does suggests that the large size and extremely specialized dentition of the genus made it difficult for it to adapt to environmental change.[6] AlongsideParahelicoprion, many other cartilaginous fish genera of the Divya Formation disappear from the fossil record at the close of the Artinskian stage.[8]
