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Living chondrichthyans can be divided into two groups: the Holocephali, and the Neoselachii (a branch of the Elasmobranchii). This division is based primarily on gill structure, thought the two groups differ in a number of other was as well. The Holocephali (chimaeras, or ratfishes) have a single gill opening on each side, and the gill is covered by a circular operculum (= gill cover). Conversely, the Neoselachii (sharks, skates and rays) have multiple gill openings (typically five to seven on each side) with no opercular cover.
Holocephalans reached their peak of diversity in the Carboniferous. Today, all living Holocephalans (approximately 24 species) belong to the Order Chimaeriformes. Like other chondrichtyans, chimeras have an endoskeleton composed of prismatic calcified cartilage, and the pelvic fins are modified into claspers in all males. They also possess tooth whorls, allowing constant tooth loss and replacement.
Chimaeras are rarely more than one metre in length, and are easily distinguished from their elasmobranch relatives by their possession of an operculum, their unique tooth morphology (teeth shaped like crushing plates), the presence of tentacula (= extensions of unknown function) on their rostrum, and the occasional presence of poison gland associated with a dorsal spine. Holocephalans exhibit autostylic jaw suspension (the palatoquadrate is fused firmly to the neurocranium), and their tooth morphology is probably an adaptation to feeding on hard-shelled marine invertebrates (i.e. gastropods, seas urchins etc.). Holocephalan tail morphology varies, from heterocercal (as in most sharks) to elongate, thin and whip-like.
Chimaeras inhabit deep marine waters (>80 metres) and are therefore rarely seen. However, they are oviparous, and venture into shallow water to lay their eggs.
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The neoselachii encompasses the living sharks and rays (approximately 975 species). They diversified primarily during the Jurassic and Cretaceous, though some existed in the Palaeozoic. Neoselachians are often well adapted for rapid movement and feeding. They have an enlarged brain relative to other fishes, and enhanced senses of vision, smell and electroreception. They are characterized by hyostylic jaw suspension, and thus a wider gape. The jaws are often located below an elongated rostrum (= snout). Neoselachians also possess varying and derived tooth morphologies. These often include serrated and curved forms. In addition, the dermal denticles of neoselachians are often assembled into fused plates, rather than spread individually across the body.
To facilitate faster swimming and more powerful muscle function, the vertebrae surrounding the notochord in neoselachii exhibit a higher degree of mineralization than those in their more primitive relatives. Furthermore, the pectoral and pelvic girdles (= the part of the endoskeleton to which the fins are connected) are fused at the midline to provide more efficient use of muscle (this was not the case in more primitive chondrichthyans). These modifications of the skeleton increase the animal's weight, and this increase is counteracted by a high oil-content in the liver. In the Neoselachii (and in the Holocephali), the liver is used to control buoyancy rather than a gas-filled swim bladder (as is seen in bony fishes).
The Neoselachii can be subdivided into two groups: the Galeomorphi (sharks) and the Squalomorphi (includes the rays and their shark ancestors).
The heterodontiformes (i.e. the bullhead sharks, 8 species) are so named because of their unique dentition consisting of both conical, sharp clutching teeth and flattened grinding teeth. They also possess stout dorsal fin spines. Heterodontiforms first appear in the fossil record during the Jurassic, and are thought to be the most primitive Galeomorphs. They live and feed in the benthos of warm tropical waters, and are oviparous.
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The Orectolobiformes (i.e. the
carpet sharks, 32 species) are a colourful group of sharks that
live in tropical to temperate waters around the world. They possess
a set of oronasal grooves that connect the nostrils to the mouth,
as well as a unique arrangement of muscles in the skull. They
have two dorsal fins with no associated spines, and the upper
lobe of their caudal fin is almost perfectly in line with the
body axis. They can be either viviparous or oviparous. The Orectolobiformes
includes the Whale shark, the largest living fish species (can
reach up to 15 metres in length).
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The Lamniformes (i.e. the mackerel sharks, 15 species) include the well-known predatory great white shark, and the extinct Carcharodon megalodon. C. megalodon had an estimated length of 10 to 20 metres, with a 3 metre jaw gape and triangular teeth that were 168 millimetres long. Lamniform jaws extend back behind the eyes, with an extended rostrum lying overtop. Lamniform sharks are generally known for their distinct tooth pattern (large triangular anterior teeth, slightly smaller intermediate teeth, large lateral teeth and very small posterior teeth). They are viviparous, and adelphophagy (embryos eating eachother in utero) often occurs.
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The Carchirhiniformes (i.e. ground sharks) are the largest group of sharks, with approximately 216 species. They possess a unique lower eyelid, one or two dorsal fins and an unusually elongated clasper skeleton. They can be oviparous or viviparous, and exploit a variety of ecological niches. The Carcharhiniforms include bottom dwellers (i.e. the catsharks) as well as streamlined swimmers (i.e. the blue and tiger sharks). The morphologically distinct hammerhead shark is also a Carcharhiniform. Many of these sharks are marine, but some (i.e. the bull shark) are freshwater dwellers.
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The Hexanchiformes (six- and sevengill sharks, 5 species) are deep-water ovoviviparous sharks (young are born alive from a thin-walled egg stored within the mother). They possess one or two extra gill arches (for a total of 6 or 7, rather than the typical 5), and a propterygium (one of the basal fin cartilages that connects the pectoral/pelvic fin to the girdle) that is disconnected from its fin radials. Hexanchiforms have a single dorsal fin that is located far back on the body, just anterior to the tail. The Hexanchiform Chlamydoselachus anguineus has an anguiliform (= eel shaped) body with three-cusped teeth, and was once thought to be a remnant of the Palaeozoic Stem Elasmobranch radiation.
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The Echinorhiniformes (bramble sharks, 2 species) are another small group of deep-water viviparous sharks that inhabit temperate to tropical zones. They have enlarged dermal denticles scattered over their skin, and two spineless dorsal fins lying far back on the body (almost in line with the pelvic fins). They lack an anal fin.
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The Squaliformes (dogfishes, 121 species) are deep-water dwelling sharks, though some species undertake a nightly migration toward the surface. Many Squaliforms are bioluminescent (= they can generate light from their bodies). They vary greatly in size, from 0.25 to 6 metres. Squaliform sharks have the longest gestation time of any vertebrates (up to 24 months) and are viviparous. They possess two dorsal fins, each with an associated fin spine, and they have no anal fin.
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The Squatiniformes (angelsharks, 15 species) are a group of ovoviviparous bottom-dwelling sharks that are very ray-like in their appearance. They are dorsoventrally flattened, with broad pectoral fins and eyes located on the top of their head. The gills are located ventrally, and they have two spineless dorsal fins. No anal fin is present. They have a heterocercal tail, with the lower lobe of the caudal fin longer than the upper.
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The Pristiophoriformes (sawsharks, 5 species) are so named because they have an elongated rostrum that takes the shape of a saw, with lateral teeth (alternating long and short) running along both sides. This "saw" is used to kill prey, and the teeth are replaced throughout life. They have two spineless dorsal fins, and no anal fin. They live in primarily in benthic marine environments along the continental shelf, but occasionally migrate into estuaries (= zones of mixed fresh and salt water - where a river runs into the sea). Pristiophoriformes are ovoviviparous.
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The Batoids comprise the sawfishes,
guitarfishes, electric rays, skates and sting rays (555 species).
They are dorsoventrally flattened, with eyes located on the top
of their heads, and mouths and gills located on the ventral side.
Once again, like all other Chondrichthyans, rays have a cartilaginous
endoskeleton with prismatic calcification. The pectoral fins are
broad and enlarged, and are attached to the side of the head (unlike
those of the angelsharks, which are morphologically similar, but
not attached). They swim, often along the ocean bottom, by undulating
their modified pectoral fins. To improve fin flexibility, rays
exhibit a reduction in the number of dermal scales. Their tails
can have reduced caudal fins, or can be thin and whip-like.
Many rays are have dentitions that are sexually dimorphic (= teeth
are shaped differently in males and females). Both, however, feed
on hard-shelled benthic invertebrates. Furthermore, females are
typically much larger than males. They can be oviparous or ovoviviparous,
and most are marine (though some can live in estuarine environments,
and a few are restricted to freshwater). Electric rays and Torpedo
rays have the ability to shock by generating electricity from
their pectoral fin muscles.
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Author: Andrew Gillis
Last updated: 15 November 2004
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