Biology:Chondrichthyes

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Chondrichthyes (/kɒnˈdrɪkθiiːz/; from grc χόνδρος (khóndros) 'cartilage', and ἰχθύς (ikhthús) 'fish') 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, and conus arteriosus in the heart. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.

The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates and sawfish) which lack opercula, and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class). Extant chondrichthyans range in size from the 10 cm (3.9 in) finless sleeper ray to the over 10 m (33 ft) whale shark.[1]

Anatomy

Skeleton

The skeleton is cartilaginous. The notochord is gradually replaced by a vertebral column during development, except in Holocephali, where the notochord stays intact. In some deepwater sharks, the column is reduced.[2]

As they do not have bone marrow, red blood cells are produced in the spleen and the epigonal organ (special tissue around the gonads, which is also thought to play a role in the immune system). They are also produced in the Leydig's organ, which is only found in certain cartilaginous fishes. The subclass Holocephali, which is a very specialized group, lacks both the Leydig's and epigonal organs.

Appendages

Apart from electric rays, which have a thick and flabby body, with soft, loose skin, chondrichthyans have tough skin covered with dermal teeth (again, Holocephali is an exception, as the teeth are lost in adults, only kept on the clasping organ seen on the caudal ventral surface of the male), also called placoid scales (or dermal denticles), making it feel like sandpaper. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other.

Originally, the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and puboischiadic bars evolved. In rays, the pectoral fins are connected to the head and are very flexible.

One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion.[3]

Body covering

Chondrichthyans have tooth-like scales called dermal denticles or placoid scales. Denticles usually provide protection, and in most cases, streamlining. Mucous glands exist in some species, as well.

It is assumed that their oral teeth evolved from dermal denticles that migrated into the mouth, but it could be the other way around, as the teleost bony fish Denticeps clupeoides has most of its head covered by dermal teeth (as does, probably, Atherion elymus, another bony fish). This is most likely a secondary evolved characteristic, which means there is not necessarily a connection between the teeth and the original dermal scales.

The old placoderms did not have teeth at all, but had sharp bony plates in their mouth. Thus, it is unknown whether the dermal or oral teeth evolved first. It has even been suggested[by whom?] that the original bony plates of all vertebrates are now gone and that the present scales are just modified teeth, even if both the teeth and body armor had a common origin a long time ago. However, there is currently no evidence of this.

Respiratory system

All chondrichthyans breathe through five to seven pairs of gills, depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills, whilst demersal species can actively pump water in through their spiracles and out through their gills.[4] However, this is only a general rule and many species differ.

A spiracle is a small hole behind each eye. These can be tiny and circular, such as found on the nurse shark (Ginglymostoma cirratum), to extended and slit-like, such as found on the wobbegongs (Orectolobidae). Many larger, pelagic species, such as the mackerel sharks (Lamnidae) and the thresher sharks (Alopiidae), no longer possess them.

Nervous system

Regions of a Chondrichthyes brain colored and labeled on dissected skate. The rostral end of the skate is to the right.

In chondrichthyans, the nervous system is composed of a small brain, 8–10 pairs of cranial nerves, and a spinal cord with spinal nerves.[5] They have several sensory organs that provide information to be processed. Ampullae of Lorenzini are a network of small jelly-filled pores called electroreceptors which help the fish sense electric fields in water. This aids in finding prey, navigation, and sensing temperature. The lateral line system has modified epithelial cells located externally that sense motion, vibration, and pressure in the water around them. Most species have large, well-developed eyes. Also, they have very powerful nostrils and olfactory organs. Their inner ears consist of 3 large semicircular canals which aid in balance and orientation. Their sound-detecting apparatus has a limited range and is typically more powerful at lower frequencies. Some species have electric organs which can be used for defense and predation. They have relatively simple brains with the forebrain not greatly enlarged. The structure and formation of myelin in their nervous systems are nearly identical to that of tetrapods, which has led evolutionary biologists to believe that Chondrichthyes were a cornerstone group in the evolutionary timeline of myelin development.[6]

Immune system

Like all other jawed vertebrates, members of Chondrichthyes have an adaptive immune system.[7]

Reproduction

Fertilization is internal. Development is usually live birth (ovoviviparous species) but can be through eggs (oviparous). Some rare species are viviparous. There is no parental care after birth; however, some chondrichthyans do guard their eggs.

Capture-induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks and rays when fished.[8] Capture-induced parturition is often mistaken for natural birth by recreational fishers and is rarely considered in commercial fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date).[8]

Classification

The class Chondrichthyes has two subclasses: the subclass Elasmobranchii (sharks, rays, skates, and sawfish) and the subclass Holocephali (chimaeras). To see the full list of the species, click here.

Subclasses of cartilaginous fishes
Elasmobranchii 140px
Sharks
and rays, skates, and sawfish
 Elasmobranchii is a subclass that includes the sharks and the rays and skates. Members of the elasmobranchii have no swim bladders, five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins, and small placoid scales. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The eyes have a tapetum lucidum. The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm. These fish are widely distributed in tropical and temperate waters.[9]
Holocephali 140px
Chimaeras
 Holocephali (complete-heads) is a subclass of which the order Chimaeriformes is the only surviving group. This group includes the rat fishes (e.g., Chimaera), rabbit-fishes (e.g., Hydrolagus) and elephant-fishes (Callorhynchus). Today, they preserve some features of elasmobranch life in Paleaozoic times, though in other respects they are aberrant. They live close to the bottom and feed on molluscs and other invertebrates. The tail is long and thin and they move by sweeping movements of the large pectoral fins. There is an erectile spine in front of the dorsal fin, sometimes poisonous. There is no stomach (that is, the gut is simplified and the 'stomach' is merged with the intestine), and the mouth is a small aperture surrounded by lips, giving the head a parrot-like appearance.

The fossil record of the Holocephali starts in the Devonian period. The record is extensive, but most fossils are teeth, and the body forms of numerous species are not known, or at best poorly understood.

Extant orders of cartilaginous fishes
Group Order Image Common name Authority Families Genera Species Note
Total File:CR IUCN 3 1.svg File:EN IUCN 3 1.svg File:VU IUCN 3 1.svg
Galean
sharks 		Carcharhiniformes 140px ground
sharks 		Compagno, 1977 8 51 >270 7 10 21
Heterodontiformes 140px bullhead
sharks 		L. S. Berg, 1940 1 1 9
Lamniformes 140px mackerel
sharks 		L. S. Berg, 1958 7
+2 extinct 		10 16 10
Orectolobiformes 140px carpet
sharks 		Applegate, 1972 7 13 43 7
Squalomorph
sharks 		Hexanchiformes 140px frilled
and
cow sharks 		de Buen, 1926 2
+3 extinct 		4
+11 extinct 		7
+33 extinct
Pristiophoriformes 140px sawsharks L. S. Berg, 1958 1 2 6
Squaliformes 140px dogfish
sharks 		Goodrich, 1909 7 23 126 1 6
Squatiniformes 140px angel
sharks 		Buen, 1926 1 1 24 3 4 5
Rays Myliobatiformes 140px stingrays
and
relatives 		Compagno, 1973 10 29 223 1 16 33
Rhinopristiformes 140px sawfishes 1 2 5–7 5–7
Rajiformes 140px skates
and
guitarfishes 		L. S. Berg, 1940 5 36 >270 4 12 26
Torpediniformes 140px electric
rays 		de Buen, 1926 2 12 69 2 9
Holocephali Chimaeriformes 140px chimaera Obruchev, 1953 3
+2 extinct 		6
+3 extinct 		39
+17 extinct

Evolution

Cartilaginous fish are considered to have evolved from acanthodians. The discovery of Entelognathus and several examinations of acanthodian characteristics indicate that bony fish evolved directly from placoderm like ancestors, while acanthodians represent a paraphyletic assemblage leading to Chondrichthyes. Some characteristics previously thought to be exclusive to acanthodians are also present in basal cartilaginous fish.[12] In particular, new phylogenetic studies find cartilaginous fish to be well nested among acanthodians, with Doliodus and Tamiobatis being the closest relatives to Chondrichthyes.[13] Recent studies vindicate this, as Doliodus had a mosaic of chondrichthyan and acanthodian traits.[14] Dating back to the Middle and Late Ordovician Period, many isolated scales, made of dentine and bone, have a structure and growth form that is chondrichthyan-like. They may be the remains of stem-chondrichthyans, but their classification remains uncertain.[15][16][17]

The earliest unequivocal fossils of acanthodian-grade cartilaginous fishes are Qianodus and Fanjingshania from the early Silurian (Aeronian) of Guizhou, China around 439 million years ago, which are also the oldest unambiguous remains of any jawed vertebrates.[18][19] Shenacanthus vermiformis, which lived 436 million years ago, had thoracic armour plates resembling those of placoderms.[20]

By the start of the Early Devonian, 419 million years ago, jawed fishes had divided into three distinct groups: the now extinct placoderms (a paraphyletic assemblage of ancient armoured fishes), the bony fishes, and the clade that includes spiny sharks and early cartilaginous fish. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. The first abundant genus of shark, Cladoselache, appeared in the oceans during the Devonian Period. The first cartilaginous fishes evolved from Doliodus-like spiny shark ancestors.

Extinct orders of cartilaginous fishes
Group Order Image Common name Authority Families Genera Species Note
Holocephali †Orodontiformes 140px Orodonts Zangerl, 1981[21] 2[22] Early members of Chondrichthyes based mostly on teeth. Includes many species that are unrelated to one another.[21]
†Petalodontiformes 147x147px Petalodonts Patterson, 1965[23][24][25] (equiv. to Petalodontida Zangerl, 1981)[21][26][27] 5 or 6[22][24] Members of holocephali. Some resembled skates or rays,[28] while others were compressed and had beak-like jaws.[27]
†Helodontiformes 140x140px Helodonts Patterson, 1965[25][29] 1[22] 1[28] 1?[29] Members of Holocephali based mostly on teeth.[28] A single species is known from skeletal fossils.[29]
†Iniopterygiformes 140x140px Iniopterygians Zangerl & Case, 1973 (as Iniopterygia)[30] 2[22] Early chondrichthyans that resembled flying fish.[30] May be members of Holocephali.[28]
†Debeeriiformes 140x140px Grogan & Lund, 2000[31] 1[22] 2[31] 2[31] Members of Holocephali with a primitive form of jaw suspension called autodiastyly.[31]
†Symmoriiformes 140x140px Symmoriids Zangerl, 1981 (sensu Maisey, 2007)[32] 2 or 3[22][28] Potential members of Holocephali that were heavily sexually dimorphic. Males had an organ called a "spine-brush complex" instead of a first dorsal fin.[33]
†Eugeneodontiformes 140x140px Eugeneodonts Zangerl, 1981 (as Eugeneodontida)[21] 4 or 5[22][34] Members of holocephali, they are characterized by large tooth whorls in their jaws.[35]
†Psammodonti-
formes 		Obruchev, 1953 1[22] Members of Holocephali, almost exclusively known from teeth.[28] A single skull is known, which had armor plates.[36]
†Copodontiformes Obruchev, 1953 1[22] Members of Holocephali. Relationships and appearance are uncertain as this order is only known from teeth.[28]
†Squalorajiformes 140x140px 1[22] 2?[37] 2?[37] Members of Holocephali, sometimes included in Chimaeriformes.[28][37]
†Chondrenchelyi-
formes 		140x140px Patterson, 1965[25] 2[22] Members of Holocephali with eel-like bodies and teeth similar to modern chimaeras. Members are sexually dimorphic.[38]
†Menaspiformes 140px Obruchev, 1953 (equiv. to Menaspoidei Patterson, 1965)[25] 3[22] Members of Holocephali with heavy armor plating. Historically misinterpreted as placoderms or ostracoderms.[25]
†Cochliodontiformes Cochliodonts Obruchev, 1953 2[22] Members of Holocephali known almost exclusively from teeth; some body fossils are known but have not been described.[39]
Squalomorph
sharks 		†Protospinaci-
formes 		1[22]
Other †Squatinactiformes 140px Cappetta et al., 1993 1[22] 1 1 Early chondrichthyans with ray-like bodies.[28]
†Protacrodonti-
formes 		Zangerl, 1981 1[22] Early members of Elasmobranchii.
†Cladoselachi-
formes 		140x140px Dean, 1894 1[22] 2[40] Potentially members of Holocephali and of the Symmoriiformes.[40]
†Xenacanthiformes 147x147px Xenacanths Glikman, 1964 2 or 3[22][28] Eel-like chondrichthyans that typically lived in freshwater. May be members of Elasmobranchii.
†Ctenacanthi-
formes 		147x147px Ctenacanths Glikman, 1964 1 or 2[22][28] Shark-like chondrichthyans characterized by their robust heads and large dorsal fin spines. May be members of Elasmobranchii
†Hybodontiformes 147x147px Hybodonts Patterson, 1966 10 or 11[22][28] Shark-like elasmobranchs distinguished by their conical tooth shape, and the presence of a spine on each of their two dorsal fins.

Taxonomy

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See also

  • List of cartilaginous fish
  • Cartilaginous versus bony fishes
  • Largest cartilaginous fishes
  • Important Shark and Ray Areas (ISRA)
  • Threatened rays
  • Threatened sharks
  • Placodermi
  • Acanthodii

References

  1. ↑ "Chondrichthyan | Definition, Species, & Facts | Britannica". https://www.britannica.com/animal/chondrichthian. 
  2. ↑ Compagno, Leonard J. V.; Nations, Food and Agriculture Organization of the United (20 November 2001). Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date. Food & Agriculture Org.. ISBN 978-92-5-104543-5. https://books.google.com/books?id=cxxSN4YA2i8C&dq=Notochord:+Chimaeroids+%22Some+deepwater+squaloid,+hexanchoid,+and+lamnoid+sharks+%22&pg=PA23. 
  3. ↑ Wilga, C. D.; Lauder, G. V. (2002). "Function of the heterocercal tail in sharks: quantitative wake dynamics during steady horizontal swimming and vertical maneuvering". Journal of Experimental Biology 205 (16): 2365–2374. doi:10.1242/jeb.205.16.2365. PMID 12124362. Bibcode: 2002JExpB.205.2365W. http://jeb.biologists.org/content/205/16/2365.short. 
  4. ↑ Long, Douglas (September 8, 1998). "Chondrichthyan/Form and function". https://www.britannica.com/animal/chondrichthian/Form-and-function. 
  5. ↑ Collin, Shaun P. (2012). "The Neuroecology of Cartilaginous Fishes: Sensory Strategies for Survival". Brain, Behavior and Evolution 80 (2): 80–96. doi:10.1159/000339870. PMID 22986825. 
  6. ↑ de Bellard, Maria Elena (2016-06-15). "Myelin in cartilaginous fish". Brain Research 1641 (Pt A): 34–42. doi:10.1016/j.brainres.2016.01.013. PMID 26776480. 
  7. ↑ Flajnik, M. F.; Kasahara, M. (2009). "Origin and evolution of the adaptive immune system: genetic events and selective pressures". Nature Reviews Genetics 11 (1): 47–59. doi:10.1038/nrg2703. PMID 19997068. 
  8. ↑ 8.0 8.1 Adams, Kye R.; Fetterplace, Lachlan C.; Davis, Andrew R.; Taylor, Matthew D.; Knott, Nathan A. (January 2018). "Sharks, rays and abortion: The prevalence of capture-induced parturition in elasmobranchs". Biological Conservation 217: 11–27. doi:10.1016/j.biocon.2017.10.010. Bibcode: 2018BCons.217...11A. http://marxiv.org/k2qvy/. Retrieved 18 January 2019. 
  9. ↑ Bigelow, Henry B.; Schroeder, William C. (1948). Fishes of the Western North Atlantic. Sears Foundation for Marine Research, Yale University. pp. 64–65. 
  10. ↑ Compagno, Leonard; Dando, Marc; Fowler, Sarah L. (2005). Sharks of the World. Princeton University Press. ISBN 978-0-691-12072-0. 
  11. ↑ Haaramo, Mikko. Chondrichthyes – Sharks, Rays and Chimaeras. http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/chondrichthyes/chondrichthyes.html#Elasmobranchii. Retrieved 22 October 2013. 
  12. ↑ Min Zhu; Xiaobo Yu; Per Erik Ahlberg; Brian Choo; Jing Lu; Tuo Qiao; Qingming Qu; Wenjin Zhao et al. (2013). "A Silurian placoderm with osteichthyan-like marginal jaw bones". Nature 502 (7470): 188–193. doi:10.1038/nature12617. PMID 24067611. Bibcode: 2013Natur.502..188Z. 
  13. ↑ Burrow, CJ; Den Blaauwen, J.; Newman, MJ; Davidson, RG (2016). "The diplacanthid fishes (Acanthodii, Diplacanthiformes, Diplacanthidae) from the Middle Devonian of Scotland". Palaeontologia Electronica. doi:10.26879/601. 
  14. ↑ Maisey, John G.; Miller, Randall; Pradel, Alan; Denton, John S.S.; Bronson, Allison; Janvier, Philippe (2017). "Pectoral Morphology in Doliodus: Bridging the 'Acanthodian'-Chondrichthyan Divide". American Museum Novitates (3875): 1–15. doi:10.1206/3875.1. https://www.archive.org/download/pectoralmorphol00mais/pectoralmorphol00mais.pdf. 
  15. ↑ Andreev, Plamen S.; Coates, Michael I.; Shelton, Richard M.; Cooper, Paul R.; Smith, M. Paul; Sansom, Ivan J. (2015). "Ordovician chondrichthyan-like scales from North America". Palaeontology 58 (4): 691–704. doi:10.1111/pala.12167. 
  16. ↑ Sansom, Ivan J.; Davies, Neil S.; Coates, Michael I.; Nicoll, Robert S.; Ritchie, Alex (2012). "Chondrichthyan-like scales from the Middle Ordovician of Australia". Palaeontology 55 (2): 243–247. doi:10.1111/j.1475-4983.2012.01127.x. Bibcode: 2012Palgy..55..243S. 
  17. ↑ Andreev, Plamen; Coates, Michael I.; Karatajūtė-Talimaa, Valentina; Shelton, Richard M.; Cooper, Paul R.; Wang, Nian-Zhong; Sansom, Ivan J. (2016). "The systematics of the Mongolepidida (Chondrichthyes) and the Ordovician origins of the clade". PeerJ 4. doi:10.7717/peerj.1850. PMID 27350896. 
  18. ↑ Andreev, Plamen S.; Sansom, Ivan J.; Li, Qiang; Zhao, Wenjin; Wang, Jianhua; Wang, Chun-Chieh; Peng, Lijian; Jia, Liantao et al. (September 2022). "Spiny chondrichthyan from the lower Silurian of South China". Nature 609 (7929): 969–974. doi:10.1038/s41586-022-05233-8. PMID 36171377. Bibcode: 2022Natur.609..969A. https://www.nature.com/articles/s41586-022-05233-8. 
  19. ↑ Andreev, Plamen S.; Sansom, Ivan J.; Li, Qiang; Zhao, Wenjin; Wang, Jianhua et al. (2022-09-28). "The oldest gnathostome teeth". Nature 609 (7929): 964–968. doi:10.1038/s41586-022-05166-2. PMID 36171375. Bibcode: 2022Natur.609..964A. https://research.birmingham.ac.uk/en/publications/1478b876-dc9b-431a-88ea-1f0f30614aea. 
  20. ↑ Zhu, Y. A.; Li, Q.; Lu, J.; Chen, Y.; Wang, J.; Gai, Z.; Zhao, W.; Wei, G. et al. (2022). "The oldest complete jawed vertebrates from the early Silurian of China". Nature 609 (7929): 954–958. doi:10.1038/s41586-022-05136-8. PMID 36171378. Bibcode: 2022Natur.609..954Z. 
  21. ↑ 21.0 21.1 21.2 21.3 Ginter, Michał; Hampe, Oliver; Duffin, Christopher J. (2010). Handbook of paleoichthyology: teeth. München: F. Pfeil. ISBN 978-3-89937-116-1. 
  22. ↑ 22.00 22.01 22.02 22.03 22.04 22.05 22.06 22.07 22.08 22.09 22.10 22.11 22.12 22.13 22.14 22.15 22.16 22.17 22.18 22.19 Van der Laan, Richard (2018-10-11). "Family-group names of fossil fishes". European Journal of Taxonomy (466). doi:10.5852/ejt.2018.466. ISSN 2118-9773. http://www.europeanjournaloftaxonomy.eu/index.php/ejt/article/view/597. 
  23. ↑ Egli, H. Chase; Hodnett, John-Paul M.; Hodge, Cody M.; Ward, Gabriel V. (2025-12-02). "Obruchevodid petalodonts (Chondrichthyes, Holocephali) from the Upper Mississippian (Serpukhovian) Bangor Limestone of northern Alabama, U.S.A." (in en). Historical Biology 37 (12): 2642–2651. doi:10.1080/08912963.2024.2412139. https://www.tandfonline.com/doi/full/10.1080/08912963.2024.2412139. 
  24. ↑ 24.0 24.1 Hodnett, John-Paul M.; Toomey, Rickard; Olson, Rickard; Tweet, Justin S.; Santucci, Vincent L. (2024). "Janassid petalodonts (Chondrichthyes, Petalodontiformes, Janassidae) from the middle Mississippian (Viséan) Ste. Genevieve Formation, Mammoth Cave National Park, Kentucky, USA" (in en). Historical Biology 36 (9): 1783–1792. doi:10.1080/08912963.2023.2231955. https://www.tandfonline.com/doi/full/10.1080/08912963.2023.2231955. 
  25. ↑ 25.0 25.1 25.2 25.3 25.4 Patterson, Colin (1965-06-10). "The phylogeny of the chimaeroids" (in en). Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 249 (757): 101–219. doi:10.1098/rstb.1965.0010. https://royalsocietypublishing.org/rstb/article/249/757/101/113044/The-phylogeny-of-the-chimaeroids. 
  26. ↑ Gai, Zhikun; Bai, Zhijun; Lin, Xianghong; Meng, Xinyuan; Zhang, Junwen (2021). "First Record of Petalodus Owen, 1840 (Chondrichthyes, Petalodontidae) in the Lower Permian (Cisuralian) of China" (in en). Acta Geologica Sinica - English Edition 95 (4): 1057–1064. doi:10.1111/1755-6724.14784. https://onlinelibrary.wiley.com/doi/10.1111/1755-6724.14784. 
  27. ↑ 27.0 27.1 Robb, Albert J. (2003). "Notes on the occurrence of some petalodont shark fossils from the Upper Pennsylvanian rocks of northeastern Kansas" (in en). Transactions of the Kansas Academy of Science 106 (1 & 2): 71–80. doi:10.1660/0022-8443(2003)106[0071:NOTOOS2.0.CO;2]. http://www.bioone.org/doi/abs/10.1660/0022-8443%282003%29106%5B0071%3ANOTOOS%5D2.0.CO%3B2. 
  28. ↑ 28.00 28.01 28.02 28.03 28.04 28.05 28.06 28.07 28.08 28.09 28.10 28.11 Nelson, Joseph S.; Grande, Terry C.; Wilson, Mark V. H. (2016-02-22) (in en). Fishes of the World (1 ed.). Wiley. doi:10.1002/9781119174844. ISBN 978-1-118-34233-6. https://onlinelibrary.wiley.com/doi/book/10.1002/9781119174844. 
  29. ↑ 29.0 29.1 29.2 Coates, Michael; Tietjen, Kristen; Johanson, Zerina; Friedman, Matt; Sang, Stephanie (2021). Pradel, Alan. ed. The cranium of Helodus simplex (Agassiz, 1838) revised. Ancient fishes and their living relatives: a tribute to John G. Maisey. München: Verlag Dr. Friedrich Pfeil. pp. 193–204. ISBN 978-3-89937-269-4. https://www.researchgate.net/publication/396168558_Coates_et_al2021_Helodus. 
  30. ↑ 30.0 30.1 Zangerl, Rainer; Case, Gerard R. (1973). Iniopterygia : a new order of Chondrichthyan fishes from the Pennsylvanian of North America. Fieldiana. 6. Chicago: Field Museum of Natural History. doi:10.5962/bhl.title.5158. https://www.biodiversitylibrary.org/bibliography/5158. 
  31. ↑ 31.0 31.1 31.2 31.3 Grogan, Eileen D.; Lund, Richard (2000). "Debeerius ellefseni (Fam. Nov., Gen. Nov., Spec. Nov.), an autodiastylic chondrichthyan from the Mississippian Bear Gulch Limestone of Montana (USA), the relationships of the chondrichthyes, and comments on gnathostome evolution". Journal of Morphology 243 (3): 219–245. doi:10.1002/(SICI)1097-4687(200003)243:3<219::AID-JMOR1>3.0.CO;2-1. https://onlinelibrary.wiley.com/doi/10.1002/(SICI)1097-4687(200003)243:33.0.CO;2-1. 
  32. ↑ Maisey, John G. (2007). "THE BRAINCASE IN PALEOZOIC SYMMORIIFORM AND CLADOSELACHIAN SHARKS" (in en). Bulletin of the American Museum of Natural History 307: 1–122. doi:10.1206/0003-0090(2007)307[1:TBIPSA2.0.CO;2]. http://www.bioone.org/doi/abs/10.1206/0003-0090%282007%29307%5B1%3ATBIPSA%5D2.0.CO%3B2. 
  33. ↑ Coates, M.; Gess, R.; Finarelli, J.; Criswell, K.; Tietjen, K. (2016). "A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes". Nature 541 (7636): 208–211. doi:10.1038/nature20806. PMID 28052054. Bibcode: 2017Natur.541..208C. 
  34. ↑ Lebedev, O. A.; Itano, W. M.; Johanson, Z.; Alekseev, A. S.; Smith, M. M.; Ivanov, A. V.; Novikov, I. V. (2022). "Tooth whorl structure, growth and function in a helicoprionid chondrichthyan Karpinskiprion (nom. nov.) (Eugeneodontiformes) with a revision of the family composition". Earth and Environmental Science Transactions of the Royal Society of Edinburgh 113 (4): 337–360. doi:10.1017/S1755691022000251. Bibcode: 2022EESTR.113..337L. 
  35. ↑ Tapanila, L; Pruitt, J; Pradel, A; Wilga, C; Ramsay, J; Schlader, R; Didier, D (2013). "Jaws for a spiral-tooth whorl: CT images reveal novel adaptation and phylogeny in fossil Helicoprion". Biology Letters 9 (2). doi:10.1098/rsbl.2013.0057. PMID 23445952. 
  36. ↑ Ahlberg, Per Erik; Coates, Michael I. (1997). "There's a ratfish in our cellar!". Geology Today 13 (1): 20–23. doi:10.1046/j.1365-2451.1997.00013.x. Bibcode: 1997GeolT..13...20A. https://onlinelibrary.wiley.com/doi/10.1046/j.1365-2451.1997.00013.x. 
  37. ↑ 37.0 37.1 37.2 Itano, Wayne M.; Duffin, Christopher J. (2023-06-20). "An enigmatic chondrichthyan spine from the Visean of Indiana, USA that resembles a median rostral cartilage of Squaloraja (Holocephali, Chimaeriformes)". Spanish Journal of Palaeontology 38 (1): 57–68. doi:10.7203/sjp.26305. https://turia.uv.es/index.php/sjpalaeontology/article/view/26305. 
  38. ↑ Finarelli, John A.; Coates, Michael I. (2014). "Chondrenchelys problematica (Traquair, 1888) redescribed: a Lower Carboniferous, eel-like holocephalan from Scotland" (in en). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 105 (1): 35–59. doi:10.1017/S1755691014000139. https://www.cambridge.org/core/product/identifier/S1755691014000139/type/journal_article. 
  39. ↑ Lund, Richard (1977). New information on the evolution of the Bradyodont Chondrichthyes. Chicago: Field Museum of Natural History. doi:10.5962/bhl.title.5311. https://www.biodiversitylibrary.org/bibliography/5311. 
  40. ↑ 40.0 40.1 Klug, Christian; Coates, Michael; Frey, Linda; Greif, Merle; Jobbins, Melina; Pohle, Alexander; Lagnaoui, Abdelouahed; Haouz, Wahiba Bel et al. (2023). "Broad snouted cladoselachian with sensory specialization at the base of modern chondrichthyans" (in en). Swiss Journal of Palaeontology 142 (1). doi:10.1186/s13358-023-00266-6. PMID 37009301. Bibcode: 2023SwJP..142....2K. 

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Further reading

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