Chondrichthyes (/kɒnˈdrɪkθiiːz/; from Greek χονδρ- chondr- 'cartilage', ἰχθύς ichthys 'fish') is a class 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.
|Great white shark, Carcharodon carcharias|
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.
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.
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 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.
Chondrichthyans have toothlike 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 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.
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.
Chondrichthyes nervous system is composed of a small brain, 8-10 pairs of cranial nerves, and a spinal chord with spinal nerves. They have several sensory organs which 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 which sense motion, vibration, and pressure in the water around them. Most subspecies 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 limited range and is typically more powerful at lower frequencies. Some subspecies 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.
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/rays when fished. 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).
|Subclasses of cartilaginous fishes|
|Elasmobranchii||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.|
|Holocephali||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|
|L. S. Berg, 1940||1||1||9|
|L. S. Berg, 1958||7
|de Buen, 1926||2
|Pristiophoriformes||sawsharks||L. S. Berg, 1958||1||2||6|
|L. S. Berg, 1940||5||36||>270||4||12||26|
|de Buen, 1926||2||12||69||2||9|
Cartilaginous fish are considered to have evolved from acanthodians.By whom? Originally assumedBy whom? 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. In particular, new phylogenetic studies find cartilaginous fish to be well nested among acanthodians, with Doliodus and Tamiobatis being the closest relatives to Chondrichthyes. Recent studies vindicate this, as Doliodus had a mosaic of chondrichthyian and acanthodiian traits.
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.
The earliest unequivocal fossils of cartilaginous fishes first appeared in the fossil record by about 430 million years ago, during the middle Wenlock Epoch of the Silurian period. 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 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.
|Devonian (419–359 mya)|
|Cladoselache||Cladoselache was the first abundant genus of primitive shark, appearing about 370 Ma. 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. 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. 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.|
|Carboniferous (359–299 Ma): Sharks underwent a major evolutionary radiation during the Carboniferous. It is believed that this evolutionary radiation occurred because the decline of the placoderms at the end of the Devonian period caused many environmental niches to become unoccupied and allowed new organisms to evolve and fill these niches.|
|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. 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 Carboniferous seas were inhabited by many fish, mainly Elasmobranchs (sharks and their relatives). These included some, like Psammodus, with crushing pavement-like teeth adapted for grinding the shells of brachiopods, crustaceans, and other marine organisms. Other sharks had piercing teeth, such as 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.
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. Stethacanthus' unusual fin may have been used in mating rituals. 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 (9.8–11.8 in) long. 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 (6.6 ft) in length.|
|Permian||Permian (298–252 Ma): The Permian ended with the most extensive extinction event recorded in paleontology: the Permian-Triassic extinction event. 90% to 95% of marine species became extinct, as well as 70% of all land organisms. Recovery from the Permian-Triassic extinction event was protracted; land ecosystems took 30M years to recover, and marine ecosystems took even longer.|
|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 became extinct.|
|Jurassic||Jurassic (201–145 Ma):|
|Cretaceous||Cretaceous (145–66 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, and more than 90% of teleost fish (bony fish) families survived.|
|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. Ptychodus mortoni (pictured) was about 32 feet (9.8 meters) long and was unearthed in Kansas, United States.|
|Cenozoic Era (65 Ma to present): The current era has seen great diversification of bony fishes.|
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). Found in all oceans it was one of the largest and most powerful predators in vertebrate history, and probably had a profound impact on marine life.
|Extinct orders of cartilaginous fishes|
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.
Apristurus is a genus of catshark, the family Scyliorhinidae, commonly known as the ghost or demon catsharks.Barremian
The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 125.0 ± 1.0 Ma). It is a subdivision of the Early Cretaceous epoch (or Lower Cretaceous series). It is preceded by the Hauterivian and followed by the Aptian stage.Chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, rat fish, spookfish or rabbit fish; the latter three names are not to be confused with rattails, Opisthoproctidae or Siganidae, respectively.
At one time a "diverse and abundant" group (based on the fossil record), their closest living relatives are sharks, though their last common ancestor with sharks lived nearly 400 million years ago. Today, they are largely confined to deep water.Clouded angelshark
The clouded angelshark (Squatina nebulosa) is an angelshark of the family Squatinidae found in the northwest Pacific from the southeastern Sea of Japan to Taiwan between latitudes 47° N and 22° N. Its length is up to 1.63 m.
Reproduction is ovoviviparous.Egg case (Chondrichthyes)
An egg case or egg capsule is the casing that surrounds the eggs of oviparous sharks, skates, and chimaeras. Egg cases typically contain one embryo, except for big skate and mottled skate egg cases, which contain up to 7 embryos. Oviparity is completely absent in the superorder Squalomorphii.Elasmobranchii
Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including the sharks (superorder Selachii) and the rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins and small placoid scales on the skin. 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 details of this jaw anatomy vary between species, and help distinguish the different elasmobranch clades. The pelvic fins in males are modified to create claspers for the transfer of sperm. There is no swim bladder; instead, these fish maintain buoyancy with large livers rich in oil.
The earliest elasmobranch fossils came from the Devonian and many surviving orders date back to the Cretaceous, or even earlier. Many species became extinct during the Permian and there was a burst of adaptive radiation during the Jurassic.Gnathostomata
Gnathostomata are the jawed vertebrates. The term derives from Greek: γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing jaws, living gnathostomes have teeth, paired appendages, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheathes of neurons. Another is an adaptive immune system that uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.It is now assumed that Gnathostomata evolved from ancestors that already possessed a pair of both pectoral and pelvic fins. These ancestors, known as antiarchs, were previously thought to not possess pectoral or pelvic fins until recently. In addition to this, some placoderms were shown to have a third pair of paired appendages, that had been modified to claspers in males and basal plates in females--a pattern not seen in any other vertebrate group.The Osteostraci are generally considered the sister taxon of Gnathostomata.It is believed that the jaws evolved from anterior gill support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more effectively – the buccal pump mechanism. The mouth could then grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would, with time, become stronger and tougher, being transformed into real jaws.
Newer research suggests that a branch of Placoderms was most likely the ancestor of present-day gnathostomes. A 419-million-year-old fossil of a placoderm named Entelognathus had a bony skeleton and anatomical details associated with cartilaginous and bony fish, demonstrating that the absence of a bony skeleton in Chondrichthyes is a derived trait. The fossil findings of primitive bony fishes such as Guiyu oneiros and Psarolepis, which lived contemporaneously with Entelognathus and had pelvic girdles more in common with placoderms than with other bony fish, show that it was a relative rather than a direct ancestor of the extant gnathostomes. It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes. Fossils findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making it impossible to replace or regrow as they broke or wore down as they grew older, proves the common ancestor of all gnathostomes had teeth and place the origin of teeth along with, or soon after, the evolution of jaws.Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians or "shark-like fishes", may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by the early Silurian, and become abundant by the start of the Devonian.Holocephali
The subclass Holocephali ("complete heads") is a taxon of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primitive forms, and the only surviving group in the subclass is the order Chimaeriformes. This group includes the rat fishes in the genus Chimaera, and the elephant fishes in the genus Callorhynchus. These fishes move by using sweeping movements of their large pectoral fins. They have long slender tails and live close to the seabed feeding on benthic invertebrates. They lack a stomach, food moving directly into the intestine.Ichthyology
Ichthyology (from Greek: ἰχθύς, ikhthys, "fish"; and λόγος, logos, "study"), also known as fish science, is the branch of zoology devoted to the study of fish. This includes bony fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). While a large number of species have been discovered, around 250 new species are officially described each year. According to FishBase, 33,400 species of fish had been described as of October 2016.Isurus
Isurus is a genus of mackerel sharks in the family Lamnidae, commonly known as the mako sharks.Lamna
Lamna is a genus of mackerel sharks in the family Lamnidae, containing two extant species: the porbeagle (L. nasus) of the North Atlantic and Southern Hemisphere, and the salmon shark (L. ditropis) of the North Pacific.List of chordate orders
This page contains a list of all of the classes and orders that are located in the Phylum Chordata.Potamotrygonidae
River stingrays or freshwater stingrays are Neotropical freshwater fishes of the Potamotrygonidae family in the order Myliobatiformes, one of the four orders of batoids, cartilaginous fishes related to sharks. They are found in rivers in tropical and subtropical South America (freshwater stingrays in Africa, Asia and Australia are in another family, Dasyatidae). A single marine genus, Styracura, of the tropical West Atlantic and East Pacific are also part of Potamotrygonidae. They are generally brownish, greyish or black, often with a mottled, speckled or spotted pattern, have disc widths ranging from 31 to 200 centimetres (1.0–6.6 ft) and venomous tail stingers. River stingrays feed on a wide range of smaller animals and the females give birth to live young. There are more than 35 species in five genera.Pristiophorus
Pristiophorus is a genus of sawsharks found in the Pacific, Atlantic and Indian oceans. Members of this genus differ from the Sixgill Sawshark (Pliotrema warreni) in having five gill slits. Their rostral sawteeth lack prominent transverse ridges on the basal ledges, and the large teeth are not posteriorly serrated.Scyliorhinus
Scyliorhinus is a genus of catsharks in the family Scyliorhinidae. This genus is known in the fossil records from the Cretaceous period, late Albian age to the Pliocene epoch.Skate (fish)
Skates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays. More than 150 species have been described, in 17 genera. Softnose skates and pygmy skates were previously treated as subfamilies of Rajidae (Arhynchobatinae and Gurgesiellinae), but are now considered as distinct families. Alternatively, the name "skate" is used to refer to the entire order of Rajiformes (families Anacanthobatidae, Arhynchobatidae, Gurgesiellidae and Rajidae).Members of Rajidae are distinguished by their stiff snout and a rostrum that is not reduced.Symmoriida
Symmoriida is an extinct order of ratfish that contains three families. It was synonymized subjectively with Cladodontida by Lund (1986); it was corrected as Symmoriiformes by Maisey (2008). It was assigned to Cladoselachii by Goto et al. (1988); to Elasmobranchii by Williams (1998), and to Chondrichthyes by Sepkoski in 2002 and by Maisey in 2008. In the fossil record, they appear at the beginning of the Carboniferous. Most of them died out at the start of the Permian; however, a single tooth described by Guinot et al. (2013) from the Valanginian of France indicates that members of the family Falcatidae might have survived until the Early Cretaceous.Teleostomi
Teleostomi is an obsolete clade of jawed vertebrates that supposedly includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives. The teleostomes include all jawed vertebrates except the chondrichthyans and the extinct class Placodermi.
Recent studies indicate that Osteichthyes evolved from placoderms like Entelognathus, while acanthodians are more closely related to modern chondrichthyes. Teleostomi, therefore, is not a valid, natural clade, but a polyphyletic group of species.The clade Teleostomi should not be confused with the similar-sounding fish clade Teleostei.Triakis
Triakis is a genus of houndsharks in the family Triakidae. The name comes from the Greek word τρι, tri meaning "three", and the Latin word acis meaning "sharp" or "pointed", in reference to the three-pointed teeth of these sharks.
|Taxonomy according to Leonard Compagno, 2005 with additions from |
* position uncertain
Extant chordate classes
Extant cartilaginous fish orders