Chondrichthyes

Cartilaginous fishes Temporal range: 439–0  Ma PreꞒ Ꞓ O S D C P T J K Pg N Early Silurian (Aeronian) - Present
Example of cartilaginous fishes: Elasmobranchii at the top of the image and Holocephali at the bottom of the image.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata
Clade:	Eugnathostomata
Class:	Chondrichthyes Huxley, 1880
Living subclasses and orders
Subclass Elasmobranchii Superorder Selachimorpha Order Carcharhiniformes Order Lamniformes Order Orectolobiformes Order Heterodontiformes Order Squaliformes Order Squatiniformes Order Pristiophoriformes Order Hexanchiformes Superorder Batoidea Order Myliobatiformes Order Rajiformes Order Rhinopristiformes Order Torpediniformes Subclass Holocephali Superorder Holocephalimorpha Order Chimaeriformes Incertae sedis †Bandringa[1] †Delphyodontos[2] †Listracanthidae[3][4] † Mcmurdodontidae[5] †Nanocetorhinus[6] †Plesioselachus[7] †Psammodontiformes †Xiphodolamia[8]

Chondrichthyes (

, cartilaginous fishes are distinct from all other jawed vertebrates.

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

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.^[9]

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

(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.^[10]

Body covering

Chondrichthyans have tooth-like scales called

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

, 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

] 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. However, this is only a general rule and many species differ.

A spiracle is a small hole found 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

In chondrichthyans, the nervous system is composed of a small brain, 8–10 pairs of cranial nerves, and a spinal cord with spinal nerves.

Immune system

Like all other jawed vertebrates, members of Chondrichthyes have an adaptive immune system.^[13]

Reproduction

Fertilization is internal. Development is usually live birth (

. 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/rays when fished.^[14] 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).^[14]

Classification

The class Chondrichthyes has two subclasses: the subclass

.

Subclasses of cartilaginous fishes
Elasmobranchii	Sharks and rays, skates, and sawfish	temperate waters.[15]
Holocephali	Chimaeras	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
Galean sharks	Carcharhiniformes		ground sharks	Compagno, 1977	8	51	>270	7	10	21
	Heterodontiformes		bullhead sharks	L. S. Berg, 1940	1	1	9
	Lamniformes		mackerel sharks	L. S. Berg, 1958	7 +2 extinct	10	16			10
	Orectolobiformes		carpet sharks	Applegate, 1972	7	13	43			7
Squalomorph sharks	Hexanchiformes		frilled and cow sharks	de Buen , 1926	2 +3 extinct	4 +11 extinct	7 +33 extinct
	Pristiophoriformes		sawsharks	L. S. Berg, 1958	1	2	6
	Squaliformes		dogfish sharks	Goodrich, 1909	7	23	126	1		6
	Squatiniformes		angel sharks	Buen , 1926	1	1	24	3	4	5
Rays	Myliobatiformes		stingrays and relatives	Compagno , 1973	10	29	223	1	16	33
	Rhinopristiformes		sawfishes		1	2	5–7	5–7
	Rajiformes		skates and guitarfishes	L. S. Berg, 1940	5	36	>270	4	12	26
	Torpediniformes		electric rays	de Buen , 1926	2	12	69	2		9
Holocephali	Chimaeriformes		chimaera	Obruchev, 1953	3 +2 extinct	6 +3 extinct	39 +17 extinct

Taxonomy according to Leonard Compagno, 2005[16] with additions from [17]

†Order Mongolepidiformes Karatajüte-Talimaa & Novitskaya, 1990 †Order Omalodontiformes Turner, 1997 †Order Coronodontiformes Zangerl, 1981 †Order Symmoriiformes Zangerl, 1981 Subclass Holocephali †Superorder Paraselachimorpha †Order Desmiodontiformes Zangerl, 1981 †Order Polysentoriformes Cappetta, 1993 †Order Orodontiformes Zangerl, 1981 †Order Petalodontiformes Zangerl, 1981 †Order Helodontiformes Patterson, 1965 †Order Iniopterygiformes Zanger, 1973 †Order Debeeriiformes Grogan & Lund, 2000 †Order Eugeneodontiformes Zangerl, 1981 Superorder Holocephalimorpha †Order Psammodontiformes* Obruchev, 1953 †Order Copodontiformes Obručhev, 1953 †Order Squalorajiformes †Order Moy-Thomas , 1939 †Order Menaspiformes †Order Cochliodontiformes Obručhev, 1953 Order Chimaeriformes Berg, 1940 sensu Obručhev, 1953 (chimaeras) Subclass Elasmobranchii †Plesioselachus †Order Antarctilamniformes Ginter, Liao & Valenzuela-Rios, 2008 †Order Elegestolepidiformes Andreev et al., 2016 †Order Lugalepidida Karatajute-Talimaa, 1997 †Order Squatinactiformes Zangerl, 1981 †Order Protacrodontiformes Zangerl, 1981 †Infraclass Cladoselachimorpha †Order Cladoselachiformes Dean, 1909 †Infraclass Xenacanthimorpha Berg, 1940 †Order Bransonelliformes Hampe & Ivanov, 2007 †Order Xenacanthiformes Berg, 1940 Infraclass Euselachii (sharks and rays) †Order Altholepidiformes Andreev et al., 2015 †Order Polymerolepidiformes †Order Ptychodontiformes †Order Ctenacanthiformes Zangerl, 1981 †Division Hybodonta †Order Hybodontiformes Owen, 1846 Division Neoselachii Compagno, 1977 Subdivision Selachii (modern sharks) Superorder Galeomorphi Compagno, 1977 Order Heterodontiformes (bullhead sharks) Order Orectolobiformes (carpet sharks) Order Lamniformes (mackerel sharks) Order Carcharhiniformes (ground sharks) Superorder Squalomorphi Order Chlamydoselachiformes Order Hexanchiformes (frilled and cow sharks) Order Squaliformes (dogfish sharks) †Order Protospinaciformes †Order Synechodontiformes Order Squatiniformes (angel sharks) Order Pristiophoriformes (sawsharks) Subdivision Batoidea Order Torpediniformes (electric rays) Order Pristiformes (sawfishes) Order Rajiformes (skates and guitarfishes) Order Myliobatiformes (stingrays and relatives) * position uncertain

Evolution

Cartilaginous fish are considered to have evolved from

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.^[24][25] Shenacanthus vermiformis, which lived 436 million years ago, had thoracic armour plates resembling those of placoderms.^[26]

By the start of the Early Devonian, 419 million years ago,

ancestors.

Extinct orders of cartilaginous fishes
Group	Order	Image	Common name	Authority	Families	Genera	Species	Note
Holocephali	†Orodontiformes
	†Petalodontiformes		Petalodonts	Zangerl, 1981	4			Members of the holocephali, some genera resembled parrot fish, but some members of the Janassidae resembled skates.
	†Helodontiformes
	†Iniopterygiformes							Members of the holocephali that resembled flying fish, are often characterized by large eyes, large upturned pectoral fins, and club-like tails.
	†Debeeriiformes
	†Symmoriida		Symmoriids	Zangerl, 1981 (sensu Maisey, 2007)	4			Members of the holocephali, they were heavily sexually dimorphic.[27]
	†Eugeneodontida		Eugeneodonts	Eugeneodontida Zangerl, 1981	4			Members of the holocephali, they are characterized by large tooth whorls in their jaws.[28]
	†Psammodonti- formes							Position uncertain
	†Copodontiformes
	†Squalorajiformes
	†Chondrenchelyi- formes
	†Menaspiformes
	†Cochliodontiformes
Squalomorph sharks	†Protospinaci- formes
Other	†Squatinactiformes			Cappetta et al., 1993	1	1
	†Protacrodonti- formes
	†Cladoselachi- formes			Dean, 1894	1	2		Holocephalans, and potential members of the symmoriida.
	†Xenacanthiformes		Xenacanths	Glikman, 1964	4			Eel-like elasmobranchs that were some of the top freshwater predators of the late Paleozoic.
	†Ctenacanthi- formes		Ctenacanths	Glikman, 1964	2			Shark-like elasmobranchs characterized by their robust heads and large dorsal fin spines.
	†Hybodontiformes		Hybodonts	Patterson, 1966	5			Shark-like elasmobranchs distinguished by their conical tooth shape, and the presence of a spine on each of their two dorsal fins.

Taxonomy

Subphylum 
Vertebrata
└─Infraphylum Gnathostomata
      ├─
Placodermi
 — extinct (armored gnathostomes)
      └
Eugnathostomata (true jawed vertebrates)
         ├─Acanthodii (stem cartilaginous fish)
         └─Chondrichthyes (true cartilaginous fish)
             ├─Holocephali (chimaeras + several extinct clades)
             └Elasmobranchii (shark and rays)
                ├─
Selachii
 (true sharks)
                └─Batoidea (rays and relatives)

 

• Note: Lines show evolutionary relationships.

See also

• List of cartilaginous fish
• Cartilaginous versus bony fishes
• Largest cartilaginous fishes
• Threatened rays
• Threatened sharks
• Placodermi

References

Further reading

Wikispecies has information related to Chondrichthyes.

The Wikibook Dichotomous Key has a page on the topic of: Chondrichthyes

• Taxonomy of Chondrichthyes
• Images of many sharks, skates and rays on Morphbank