Stethacanthus

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Stethacanthus
Temporal range: 382.7–298.9  Ma
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Late Devonian to Late Carboniferous
StethacanthusesDB 2.jpg
Restoration of S. altonensis and S. productus
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Order:† Symmoriiformes
Family:† Stethacanthidae
Genus:† Stethacanthus
Newberry, 1889
Type species
Stethacanthus altonensis
St. John and Worthen, 1875 [1]
Other species
  • S. concavusGinter, 2018 [2]
  • S. gansuensisWang et al., 2004 [3]
  • S. neilsoni
  • S. praecursorHussakof & Bryant, 1918 [4]
  • S. resistens
  • S. thomasi
  • S. productusNewberry, 1897 [5]
Synonyms

Stethacanthus is an extinct genus of shark-like cartilaginous fish which lived from the Late Devonian to Late Carboniferous epoch, dying out around 298.9 million years ago. Fossils have been found in Australia, Asia, Europe and North America.

Contents

Etymology

Stethacanthus comes from the Greek στῆθος (stēthos), meaning "chest", and ἄκανθος (akanthos), meaning "spine" or "thorn". The name refers to the distinctive anvil-shaped first dorsal fin and spine displayed by mature males of the genus. [6]

Description

Stethacanthus had different sizes depending on species, S. altonensis had length about 1.5 metres (4.9 ft), while S. productus reached 3 metres (9.8 ft). [7] In many respects, it had a shark-like appearance. However, it is best known for its unusually shaped dorsal fin, which resembled an anvil or ironing board. Small spikes (enlarged versions of the dermal denticles commonly covering shark skin) covered this crest, and the ratfish's head as well. [8] The crest may have played a role in mating rituals, aided in clamping to the belly of larger marine animals, or been used to frighten potential predators. [9]

Like other members of Stethacanthidae, Stethacanthus had unique pelvic girdles, single-crowned and non-growing scales, a pectoral fin composed of metapterygium with an accompanying ‘whip’ attached and a distinctive first dorsal fin and spine, termed the spine-brush complex. The neurocranium had a narrow suborbital shelf, a broad supraorbital shelf, a short otico-occipital division, large orbits, [10] and cladodontic teeth that aligned precisely. In addition to these features, Stethacanthus also had male pelvic claspers with non-prismatic calcified cartilage at the distal ends. [11]

Spine-brush complex

Restoration of a S. altonensis pair Steth pair1.jpg
Restoration of a S. altonensis pair

The spine-brush complex occupies the same site as the first dorsal fin on other ratfish and contains a basal plate extending inside a usually posterior-pointing dorsal spine composed of trabecular dentine. The spines resemble those of modern sharks and rays but curiously lack any enamel-like surface tissue. [12] The trabecular dentine contains patches of fibers suggesting attachments to the epaxial musculature. The way these muscles would have been positioned implies that the spine could have been moved in anterio-posterior direction. The so-called "brush" is not fibrous as was originally believed, but consists of a number of parallel, membranous tubules [6] made of globular calcified cartilage. [13] The brush base and basal plate are covered in a thin, acellular bone layer. [12] [13] Zangerl asserts that these tubules are similar to erectile tissues in humans, and thus the complex may have been inflatable. [6] The complex itself is covered in up to nine rows of large denticles pointing anteriorly. The dorsal side of the head has its own collection of denticles [13] which point posteriorly. [6] The presence of these large denticles has led to theories that the spine-brush complex in combination with the denticles on the head was used to scare away predators by simulating the mouth of a larger fish. [6] The complex has been affirmed only in males, and only in those males that have reached sexual maturity. [14] Whether the complex was present in females of the species is still unknown. [13] Another theory for the spine-brush complex is that it was involved either in attracting a mate [14] or in the mating process itself. [15]

Pectoral fin whip

The pectoral fins of Stethacanthus were composed of the triangular-shaped metapterygium observed in modern-day sharks, but had an additional long, metapterygial structure called a fin whip. These fin whips contain at least 22 axial cartilages and extended past the pelvic fins. The three most anterior axials are shorter than the more posterior axials. [11] The purpose of the fin whips is unknown but it has been suggested that they were used during mating. [15]

Teeth and denticles

The tooth files are whorl-shaped and the palatoquadrate is scalloped with 6-7 recesses to allow for the tooth families. The individual teeth are widely separated from each other in the tooth whorls. [16] The teeth themselves are of the cladodont variety; the bases of the teeth are broadest on the lingual side, and each support a single large cusp and two pairs of smaller accessory cusps for a total of five cusps. [16] The medial and most lateral cusps are the most fluted. The teeth appear to be mostly orthodentine, but when viewed in cross-section, change abruptly to osteodentine. The enameloid is single-layered, overlaying the thick mantle of orthodentine. [14] In addition to the dentition teeth, there are also a number of buccopharyngeal denticles lining the oropharynx. [11] [14] The denticles lining the top of the head and the top of the spine-brush complex are larger than the dentition teeth, and they appear as elongate monocuspid denticles. [13]

Pelvic girdles and claspers

In Stethacanthus, the pelvic girdles consist of sheets of prismatic cartilage, each in the shape of a subtriangular, rounded plate. The anterior edge of each girdle is slightly concave while the posterior is convex. There appears to be no union of the two plates. [11] There are two types of pelvic girdles found in stethacanthids: the primitive condition and the derived condition. In the primitive condition, the pelvic girdles have a metapterygial element supporting only one or two radials and most of the fin radials are attached directly to the pelvic plate. The derived condition differs in that there is a much higher number of radials supported by the pelvic plate. This feature, accompanied with a broadening of the pelvic girdle in order to accommodate the increased number of radials is a characteristic of Stethacanthus and other symmorriids. [14] The males had claspers that were club-shaped at the distal ends and composed of non-prismatic globular calcified cartilage. [12] [14]

Caudal fin

There was some caudal fin variety among Stethacanthus species; while some had low angle heterocercal tails, some had tails approaching homocercal. [14] The broad hypochordal lobe was supported by long, splayed fin radials. [11]

Paleobiology

Restoration of S. productus with prey Stethacanthus1DB.jpg
Restoration of S. productus with prey

It is certain that Stethacanthus was a carnivore, and considering its small size probably fed on small fish, brachiopods, and crinoid ossicles like other sharks of its time. [17] Additionally, as the spine-brush complex is rather a large structure, it seems likely that, in combination with the forward-facing denticles on the structure, it would have produced a drag force during fast locomotion. Therefore, Stethacanthus was probably a slow-moving shark. The fins of Stethacanthus were also smaller than in other sharks of the same size, and their teeth were also on the small side relative to other small Paleozoic sharks, suggesting that Stethacanthus may have been a bottom-dweller. [14] Considering that most of the Stethacanthus specimens were recovered in the Bear Gulch Limestone in Montana, it is possible that this area was not only a breeding ground for other sharks but also for Stethacanthus, suggesting that they were migratory. [18]

History

The several species of Stethacanthus discovered in the late 1800s were established based solely upon isolated spines, which initially confused paleontologist John Strong Newberry into thinking the spines constituted a new kind of fin. He originally believed that the spines were part of the pectoral fins and that they were not bilaterally symmetrical. [6] Meanwhile, the first associated skeletal remains found in the Mississippian of Montana and the Devonian and Mississippian of Ohio remained undescribed for nearly a century. [11] Since complete skeletons were extremely rare, Stethacanthus classification was vague and based on few characteristics. It was not until 1974 that the family Stethacanthidae was defined by Richard Lund because Stethacanthus differed so greatly from other elasmobranchs of the time. [11] Relative classifications of symmoriids compared to stethacanthids are still debated. More Stethacanthus specimens have been discovered, expanding their range from the Midwestern United States to the Lower Carboniferous of Bearsden, Scotland [11] and the Lower Tournaisian of the Tula Region of Central Russia [19] and China. [20] Stethacanthus teeth have been recovered from the Frasnian-Famennian Napier Formation and the Tournaisian Laurel Formation and Moogooree Limestone in Western Australia. [21] A partial palate and jawbone referred to a Stethacanthus sp. has also been recovered from the Bonaparte Basin, Western Australia. [22]

Classification

Restoration of the possible synonym Symmorium reniforme Symmorium1DB.jpg
Restoration of the possible synonym Symmorium reniforme

The presence of globular calcified cartilage in both the spine-brush complex base plate and brush and in the claspers is interesting because it is the first record of such a large mass of globular calcified cartilage in chondrichthyan. The high presence of globular calcified cartilage raises several questions about the evolution of sharks. It is possible that prismatic cartilage, a defining feature of chondrichthyans, is an evolutionary derivative of globular calcified cartilage. If this were the case, primitive chondrichthyans would have appeared with shark-like scales based instead on globular calcified cartilage. Another feature of note is the thin, acellular bone layer coating the brush and baseplate of the spine-brush complex. It is possible that the coating on the spine-brush complex is the first record of endoskeletal bone in primitive chondrichthyans, and that these endoskeletal features were lost in extant chondrichthyans. It is also possible that the fin spine could be a unique distribution of dermal skeleton and thus derived from neural crest. Following this assumption, the brush would be a fin-baseplate extension. The endoskeletal location and absence of fin radials supports the latter hypothesis. [12] [13]

Taxonomic relationships are hard to define for Stethacanthus as there is much variability in the characteristics of the discovered specimens. [14] Chondrichthyes is a monophyletic group characterized by the development of endoskeletal tesserae (mineralized blocks of cartilage) and internal fertilization. [23] Chondrichthyes is further divided into two subclasses: Elasmobranchii and Holocephali. Stethacanthids have been classified as a member of the group Paleoselachii, which is a subdivision of Elasmobranchii. Stethacanthus has been further classified as part of the order Symmoriida, a classification that has sparked a controversy. There are two main hypotheses regarding this classification. One hypothesis states that the order Symmoriida consists of the families Symmoriidae, Stethacanthidae and Falcatidae and thus are a monophyletic group. Another is that symmoriids are actually the females of stethacanthids [10] [13] or are derived from stethacanthids. [10] This hypothesis is due to the fact that stethacanthids and symmoriids are poorly defined; symmoriids are thought to lack a spine-brush complex but are otherwise identical to Stethacanthidae. Stethacanthids are identified by the presence of a spine-brush complex, which is in some cases non-existent (e.g. juvenile males), making the certain classification of stethacanthids and symmoriids difficult. [10]

More recently, Symmoriiformes as a whole has been reclassified as part of Holocephali, meaning that Stethacanthus may have been more closely related to modern chimaeras than to sharks. [24]

See also

Related Research Articles

<span class="mw-page-title-main">Chondrichthyes</span> Class of jawed cartilaginous fishes

Chondrichthyes is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fish, which have skeletons primarily composed of bone tissue. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opercula and swim bladders. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.

<span class="mw-page-title-main">Chimaera</span> Cartilaginous fish in the order Chimaeriformes

Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively.

<span class="mw-page-title-main">Acanthodii</span> Paraphyletic group of cartilaginous fishes

Acanthodii or acanthodians is an extinct class of gnathostomes. They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras. Acanthodians possess a mosaic of features shared with both osteichthyans and chondrichthyans. In general body shape, they were similar to modern sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteians.

<i>Cladoselache</i> Extinct genus of chondrichthyans

Cladoselache is an extinct genus of shark-like chondrichthyan from the Late Devonian (Famennian) of North America. It was similar in body shape to modern lamnid sharks, but was not closely related to lamnids or to any other modern (selachian) shark. As an early chondrichthyan, it had yet to evolve traits of modern sharks such as accelerated tooth replacement, a loose jaw suspension, enameloid teeth, and possibly claspers.

<span class="mw-page-title-main">Xenacanthida</span> Extinct order of cartilaginous fishes

Xenacanthida is an order or superorder of extinct shark-like chondrichthyans known from the Carboniferous to Triassic. They were native to freshwater, marginal marine and shallow marine habitats. Some xenacanths may have grown to lengths of 5 m (16 ft). Most xenacanths died out at the end of the Permian in the End-Permian Mass Extinction, with only a few forms surviving into the Triassic.

<span class="mw-page-title-main">Symmoriiformes</span> Extinct order of cartilaginous fishes

Symmoriiformes is an extinct order of stem-group holocephalians. Originally named Symmoriida by Zangerl (1981), it has subsequently been known by several other names. Lund (1986) synonymized the group with Cladodontida, while Maisey (2008) corrected the name to Symmoriiformes in order to prevent it from being mistaken for a family. The symmoriiform fossils record begins during the late Devonian. Most of them died out at the start of the Permian, but Dwykaselachus is known from the Artinskian-Kungurian of South Africa. Teeth described from the Valanginian of France and Austria indicate that members of the family Falcatidae might have survived until the Early Cretaceous; however, these teeth were also argued to be more likely neoselachian teeth.

<span class="mw-page-title-main">Stethacanthidae</span> Extinct family of cartilaginous fishes

Stethacanthidae is an extinct family of prehistoric holocephalians. It is estimated to have existed approximately between 380 and 300 million years ago. Members of this family are noted for their peculiar dorsal fin.

<i>Akmonistion</i> Extinct genus of cartilaginous fishes

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<i>Symmorium</i> Extinct genus of cartilaginous fishes

Symmorium is an extinct symmoriiform cartilaginous fish from the Devonian and Carboniferous of the United States (Illinois) and Russia. The type species, Symmorium reniforme, was named by Edward Drinker Cope in 1893, with other species assigned to the genus having since been reclassified into other genera such as Petalodus. Symmorium bears close similarity in size and appearance to Stethacanthus but lacks the "spine-brush complex" in place of the first dorsal fin. Some paleontologists think that the two forms represented the males and females of related species, while other scientists think they were distinct genera.

<span class="mw-page-title-main">Falcatidae</span> Extinct family of cartilaginous fishes

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<span class="mw-page-title-main">Hybodontiformes</span> Extinct order of chondrichthyans

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<i>Ornithoprion</i> Extinct genus of cartilaginous fishes

Ornithoprion is a genus of extinct cartilaginous fish in the family Caseodontidae. The only species, O. hertwigi, lived during the Moscovian stage of the Pennsylvanian, between 315.2 to 307 million years ago, and is known from black shale deposits in what is now the Midwestern United States. The discovery and description of Ornithoprion, performed primarily via radiography, helped clarify the skull anatomy of eugeneodonts; a group which includes O. hertwigi and which were previously known primarily from isolated teeth. The genus derives its name from its pointed, bill-like armored skull and large eyes, which vaguely resemble the features of a bird, while the species name honors Oscar Hertwig. It is known from multiple specimens preserving the skull and frontmost portion of the body.

Dwykaselachus is an extinct genus of symmoriiform, a cartilaginous fish that lived in what is now South Africa during the Permian period around 280 million years ago. It was first discovered in the 1980s, in a nodule of sediments from the Karoo Supergroup. Dwykaselachus was named based on Dwyka Group, the group of sedimentary geological formation in the southeastern part of Africa. It represents the place where the type species Dwykaselachus oosthuizeni was found.

<i>Plesioselachus</i> Extinct genus of cartilaginous fishes

Plesioselachus is an extinct genus of Late Devonian (Famennian) cartilaginous fish with uncertain classification, which contains only one species, P. macracanthus from the Waterloo Farm lagerstätte in South Africa. Known from a single incomplete articulated skeleton and some isolated remains, it is characterized by having a long dorsal spine with length about one third of body length.

<span class="mw-page-title-main">Ctenacanthiformes</span> Extinct order of cartilaginous fishes

Ctenacanthiformes is an extinct order of cartilaginous fish. They possessed ornamented fin spines at the front of their dorsal fins and cladodont-type dentition, that is typically of a grasping morphology, though some taxa developed cutting and gouging tooth morphologies. Some ctenacanths are thought to have reached sizes comparable to the great white shark, with body lengths of up to 7 metres (23 ft) and weights of 1,500–2,500 kilograms (3,300–5,500 lb), while others reached lengths of only 30 centimetres (12 in). The earliest ctenacanths appeared during the Frasnian stage of the Late Devonian, with the group reaching their greatest diversity during the Early Carboniferous (Mississippian), and continued to exist into at least the Middle Permian (Guadalupian). Some authors have suggested members of the family Ctenacanthidae may have survived into the Cretaceous based on teeth found in deep water deposits of Valanginian age in France and Austria, however, other authors contend that the similarity of these teeth to Paleozoic ctenacanths is only superficial, and they likely belong to neoselachians instead.

<i>Dracopristis</i> Extinct genus of cartilaginous fishes

Dracopristis is an extinct genus of ctenacanth that lived around 307 million years ago during the Pennsylvanian subperiod of the Carboniferous period. A single species is known, Dracopristis hoffmanorum, which is named in honor of Ralph and Jeanette Hoffman.

<i>Romerodus</i>

Romerodus is an extinct genus of cartilaginous fish in the family Caseodontidae. While it and the rest of its family were historically considered elasmobranchs related to sharks and rays, they are now regarded as holocephalans, a diverse subclass which is today only represented by chimaeras. Romerodus is known from the Carboniferous and possibly Permian periods of North America, and the only named species, R. orodontus, was discovered in organic shale deposits in the U.S. state of Nebraska. It is one of few members of its order, the Eugeneodontida, that is known from multiple complete, well preserved body fossils, and is thus an important taxon for understanding the anatomy and ecology of less well preserved eugeneodonts such as Helicoprion. Unlike its larger relatives, the largest known Romerodus were approximately 50 cm (20 in) in total length.

<i>Squatinactis</i> Extinct genus of cartilaginous fishes

Squatinactis is a genus of extinct elasmobranch chondrichthyan known from the Carboniferous aged Bear Gulch Limestone in Montana. This fish was discovered in 1974 by Richard Lund. The type specimen, named CMNH 46133, consists of a brain case, poorly preserved jaws and gills, a pectoral fin, and a partial vertebral axis. This creatures most startling feature were its broad pectoral fins which resembled those of stingrays and angel sharks (Squatina). The holotype specimen has about 15 teeth in its jaw. This creature is named after the angel shark. Remains found in the South Urals of Russia and the Eyam Limestone of Derbyshire, England, have been tentatively identified as those belonging to S. caudispinatus.

<i>Maghriboselache</i> Extinct genus of cartilaginous fish

Maghriboselache is an extinct genus of cladoselachid symmoriiform fish that lived 369 million years ago during the Late Devonian of Morocco. The genus contains a single species, M. mohmezanei. Maghriboselache represents the first cladoselachid to be discovered with significant details of the jaws, braincase and even endocranium. Along with Cladoselache, it shares a unique and distinctive tooth and upper jaw morphology.

Cosmoselachus is an extinct genus of symmoriiform chondrichthyan from the Upper Carboniferous aged Fayetteville Shale of Arkansas, United States. The genus contains a single species, C. mehlingi, which is known from a partial specimen that includes the cranium, jaws, gill arches, pectoral fins, and teeth.

References

  1. ↑ "Fossilworks: Stethacanthus altonensis".
  2. ↑ Michał Ginter (2018). "Symmoriiform sharks from the Pennsylvanian of Nebraska". Acta Geologica Polonica. 68 (3): 391–401. doi:10.1515/agp-2018-0009 (inactive 1 November 2024).{{cite journal}}: CS1 maint: DOI inactive as of November 2024 (link)
  3. ↑ "Fossilworks: Stethacanthus gansuensis".
  4. ↑ "Fossilworks: Stethacanthus praecursor".
  5. ↑ "Fossilworks: Stethacanthus productus".
  6. 1 2 3 4 5 6 Zangerl, Rainer (1984). "On the microscopic anatomy and possible function of the spine-"brush" complex of Stethacanthus (Elasmobranchii: Symmoriida)". Journal of Vertebrate Paleontology. 4 (3): 372–378. Bibcode:1984JVPal...4..372Z. doi:10.1080/02724634.1984.10012016.
  7. ↑ Lund, Richard; Greenfest-Allen, Emily; Grogan, Eileen D. (2015-02-01). "Ecomorphology of the Mississippian fishes of the Bear Gulch Limestone (Heath formation, Montana, USA)". Environmental Biology of Fishes. 98 (2): 739–754. Bibcode:2015EnvBF..98..739L. doi:10.1007/s10641-014-0308-x. ISSN   1573-5133. S2CID   14850973.
  8. ↑ Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 27. ISBN   978-1-84028-152-1.
  9. ↑ Elasmo-research
  10. 1 2 3 4 Maisey, J.G. (2007). "The braincase in Paleozoic symmoriiform and cladoselachian sharks". Bulletin of the American Museum of Natural History. 307: 1–122. doi:10.1206/0003-0090(2007)307[1:tbipsa]2.0.co;2. hdl: 2246/5883 . S2CID   85643864.
  11. 1 2 3 4 5 6 7 8 Coates S.E.K., M.I.; Sequeira, S.E.K. (2001). "A new stethacanthid chondrichthyan from the lower Carboniferous of Bearsden, Scotland". Journal of Vertebrate Paleontology. 21 (3): 438–459. doi:10.1671/0272-4634(2001)021[0438:anscft]2.0.co;2. S2CID   29522435.
  12. 1 2 3 4 Coates, M. I.; Sequeira, S.E.K.; Sansom, I.J.; Smith, M.M. (December 1998). "Spines and tissues of ancient sharks". Nature. 396 (6713): 729–730. Bibcode:1998Natur.396..729C. doi:10.1038/25467. S2CID   4383421.
  13. 1 2 3 4 5 6 7 Maisey, John G. (2009). "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. S2CID   86408226.
  14. 1 2 3 4 5 6 7 8 9 Lund, Richard (1985). "Stethacanthid elasmobranch remains from the Bear Gulch Limestone (Namurian E2b) of Montana". American Museum Novitates (2828): 1–24.
  15. 1 2 Wood, S.P. (1982). "New basal Namurian (Upper Carboniferous) fishes and crustaceans found near Glasgow". Nature. 297 (5867): 574–7. Bibcode:1982Natur.297..574W. doi:10.1038/297574a0. S2CID   4306826.
  16. 1 2 Smith, M.M.; Coates, M.I. (2001). The evolution of vertebrate dentitions: phylogenetic pattern and developmental models. pp. 223–240.
  17. ↑ Walker, S.E.; Brett, C.E. (2002). "Predators and predation in Paleozoic marine environments". Paleontological Society Papers. 8: 93–118. doi:10.1017/S1089332600001078.
  18. ↑ Grogan, E.D.; Lund, R. (2002). "The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition". Geodiversitas. 24: 295–315.
  19. ↑ Lebedev, O.A. (1996). "Fish assemblages in the Tournaisian-Visean environments of the East European Platform". Geological Society, London, Special Publications. 107 (1): 387–415. Bibcode:1996GSLSP.107..387L. doi:10.1144/gsl.sp.1996.107.01.28. S2CID   129309437.
  20. ↑ N. Wang, J. Fan & W. Wang - 2004. Early Carboniferous Fishes (Acanthodian, Actinopterygians, and Chondrichthyes) from the East Sector of North Qilian Mountain, China - Vertebrata PalAsiatica 42(2):89-110
  21. ↑ Trinajstic, Katherine (2014). "Devonian vertebrates from the Canning and Carnarvon Basins with an overview of Paleozoic vertebrates of Western Australia". Journal of the Royal Society of Western Australia. 97: 133–151.
  22. ↑ Burrow, Carole (2010). "Middle Palaeozoic microvertebrate assemblages and biogeography of East Gondwana (Australasia, Antarctica)". Palaeoworld. 19 (1–2): 37–54. doi:10.1016/j.palwor.2009.11.001.
  23. ↑ Grogan, E.D.; Lund, Richard; Greenfest-Allen, E. (2012). "The Origin and Relationships of Early Chondrichthyans". Biology of Sharks and Their Relatives. CRC press, USA. pp. 3–29.
  24. ↑ Coates, M., Gess, R., Finarelli, J., Criswell, K., Tietjen, K. 2016. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature. doi: 10.1038/nature20806
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