Chondrichthyes

Cartilaginous fishes Temporal range: 422–0 Ma[1][2] PreЄ Є O S D C P T J K Pg N Latest Silurian to Present
Great white shark, Carcharodon carcharias
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata
Class:	Chondrichthyes Huxley, 1880
Subclasses
Elasmobranchii Holocephali

Chondrichthyes (; from Greek χονδρ- chondr- 'cartilage', ἰχθύς ichthys 'fish') is a class (biology) that contains the cartilaginous fishes: they are jawed vertebrates with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. 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).

Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates, the extant members of which all fall into Teleostomi.

Anatomy

Skeleton

The skeleton is cartilaginous. The notochord, which is present in the young, is gradually replaced by cartilage. Chondrichthyans also lack ribs, so if they leave water, the larger species' own body weight would crush their internal organs long before they would suffocate.

As they do not have Holocephali, which is a very specialized group, lacks both the Leydig's and epigonal organs.

Appendages

Apart from 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 pubioischiadic bars evolved. In rays, the pectoral fins have 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 toothlike scales called dermal denticles or placoid scales. Denticles provide two functions, 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. Nor is it sure how many times it has happened if it turns out to be the case. It has even been suggested that the original bony plates of all the vertebrates are gone and that the present scales are just modified teeth, even if both teeth and the body armor have a common origin a long time ago. However, there is no evidence of this at the moment.

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.

Immune system

Like all other jawed vertebrates, members of Chondrichthyes have an adaptive immune system.^[4]

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.

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	Sharks	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.[5]	and rays, skates, and sawfish
Holocephali	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
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	6 +33 extinct
	Pristiophoriformes		sawsharks	L. S. Berg, 1958	1	2	6
	Squaliformes		dogfish sharks		1	2	29	1		6
	Squatiniformes		angel sharks	Buen, 1926	1	1	23	3	4	5
Rays	Myliobatiformes		stingrays and relatives	Compagno, 1973	10	29	223	1	16	33
	Pristiformes		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

Evolution

Radiation of cartilaginous fishes, based on Michael Benton, 2005.^[8]

Cartilaginous fish are considered to have evolved from acanthodians. Originally assumed to be closely related to bony fish or a polyphyletic assemblage leading to both groups, 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.^[9][10]

Unequivocal fossils of cartilaginous fishes first appeared in the fossil record by about 395 million years ago, during the middle Devonian. The radiation of elasmobranches in the chart on the right is divided into the taxa: Cladoselache, Eugeneodontiformes, Symmoriida, Xenacanthiformes, Ctenacanthiformes, Hybodontiformes, Galeomorphi, Squaliformes and Batoidea.

By the start of the Early Devonian, 419 mya (million years ago), jawed fishes had divided into four distinct clades: the placoderms and spiny sharks, both of which are now extinct, and the cartilaginous and bony fishes, both of which are still extant. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. Spiny sharks are not classified as true sharks or as cartilaginous fishes, but as a distinct group, class (biology) Acanthodii. However, both the cartilaginous and bony fishes may have arisen from either the placoderms or the spiny sharks. Cartilaginous fishes first appeared about 395 Ma. The first abundant genus of shark, Cladoselache, appeared in the oceans during the Devonian Period.

A Bayesian analysis of molecular data suggests that the Holocephali and Elasmoblanchii diverged in the Silurian () and that the sharks and rays/skates split in the Carboniferous ().

Devonian	Devonian (419–359 mya)
		Cladoselache	Cladoselache was the first abundant genus of primitive shark, appearing about 370 Ma.[11] It grew to 6 feet (1.8 m) long, with anatomical features similar to modern mackerel sharks. It had a streamlined body almost entirely devoid of scales, with five to seven gill slits and a short, rounded snout that had a terminal mouth opening at the front of the skull.[11] It had a very weak jaw joint compared with modern-day sharks, but it compensated for that with very strong jaw-closing muscles. Its teeth were multi-cusped and smooth-edged, making them suitable for grasping, but not tearing or chewing. Cladoselache therefore probably seized prey by the tail and swallowed it whole.[11] It had powerful keels that extended onto the side of the tail stalk and a semi-lunate tail fin, with the superior lobe about the same size as the inferior. This combination helped with its speed and agility which was useful when trying to outswim its probable predator, the heavily armoured 10 metres (33 ft) long placoderm fish Dunkleosteus.[11]
Carbon- iferous	[12]
		Orthacanthus senckenbergianus	The first 15 million years of the Carboniferous has very few terrestrial fossils. This gap in the fossil record, is called Romer's gap after the American palaentologist Alfred Romer. While it has long been debated whether the gap is a result of fossilisation or relates to an actual event, recent work indicates that the gap period saw a drop in atmospheric oxygen levels, indicating some sort of ecological collapse.[13] The gap saw the demise of the Devonian fish-like ichthyostegalian labyrinthodonts, and the rise of the more advanced temnospondyl and reptiliomorphan amphibians that so typify the Carboniferous terrestrial vertebrate fauna. The Symmoriida; some, the petalodonts, had peculiar cycloid cutting teeth. Most of the sharks were marine, but the Xenacanthida invaded fresh waters of the coal swamps. Among the bony fish, the Palaeonisciformes found in coastal waters also appear to have migrated to rivers. Sarcopterygian fish were also prominent, and one group, the Rhizodonts, reached very large size. Most species of Carboniferous marine fish have been described largely from teeth, fin spines and dermal ossicles, with smaller freshwater fish preserved whole. Freshwater fish were abundant, and include the genera Ctenodus, Uronemus, Acanthodes, Cheirodus, and Gyracanthus.
		Stethacanthidae	As a result of the evolutionary radiation, carboniferous sharks assumed a wide variety of bizarre shapes; e.g., sharks belonging to the family Stethacanthidae possessed a flat brush-like dorsal fin with a patch of denticles on its top.[12] Stethacanthus' unusual fin may have been used in mating rituals.[12] Apart from the fins, Stethacanthidae resembled Falcatus (below).
		Falcatus	Falcatus is a genus of small cladodont-toothed sharks which lived 335–318 Ma. They were about 25-30 cm (10-12 inches) long.[14] They are characterised by the prominent fin spines that curved anteriorly over their heads.
		Orodus	Orodus is another shark of the Carboniferous, a genus from the family Orodontidae that lived into the early Permian from 303 to 295 Ma. It grew to 2 m (7 ft) in length.
Permian	[15] and marine ecosystems took even longer.[16]
Triassic	Triassic (252–201 Ma): The fish fauna of the Triassic was remarkably uniform, reflecting the fact that very few families survived the Permian extinction. In turn, the Triassic ended with the Triassic–Jurassic extinction event. About 23% of all families, 48% of all genera (20% of marine families and 55% of marine genera) and 70% to 75% of all species went extinct.[17]
Jurassic	Jurassic (201–145 Ma): The end of the Cretaceous was marked by the Cretaceous–Paleogene extinction event (K-Pg extinction). There are substantial fossil records of jawed fishes across the K–T boundary, which provides good evidence of extinction patterns of these classes of marine vertebrates. Within cartilaginous fish, approximately 80% of the sharks, rays, and skates families survived the extinction event,[18] and more than 90% of teleost fish (bony fish) families survived.[19]
Cretaceous	Cretaceous (145–66 Ma):
		Squalicorax falcatus	Squalicorax falcatus is a lamnoid shark from the Cretaceous
		Ptychodus	Ptychodus is a genus of extinct hybodontiform shark which lived from the late Cretaceous to the Paleogene.[20][21] Ptychodus mortoni (pictured) was about 32 feet long (10 meters) and was unearthed in Kansas, United States.[22]
Cenozoic Era	Cenozoic Era (65 Ma to present): The current era has seen great diversification of bony fishes.
		Megalodon	External video Megalodon battle History Channel The Nightmarish Megalodon Discovery Megalodon is an extinct species of shark that lived about 28 to 1.5 Ma. It looked much like a stocky version of the great white shark, but was much larger with fossil lengths reaching 20.3 metres (67 ft).[23] Found in all oceans[24] it was one of the largest and most powerful predators in vertebrate history,[23] and probably had a profound impact on marine life.[25]

Extinct orders of cartilaginous fishes
Group	Order	Image	Common name	Authority	Families	Genera	Species	Note
Holocephali	†Orodontiformes
	†Petalodontiformes
	†Helodontiformes
	†Iniopterygiformes
	†Debeeriiformes
	†Eugeneodonti formes							[26]
	†Psammodonti formes							Position uncertain
	†Copodontiformes
	†Squalorajiformes
	†Chondrenchelyi formes
	†Menaspiformes
	†Coliodontiformes
Squalomorph sharks	†Protospinaci- formes
Other	†Squatinactiformes
	†Protacrodonti- formes
	†Cladoselachi- formes
	†Xenacanthiformes
	†Ctenacanthi- formes
	†Hybodontiformes

Phylogeny

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

• Cartilaginous versus bony fishes
• Largest cartilaginous fishes
• Threatened rays
• Threatened sharks

References

Citations

Bibliography

Further reading

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