The Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of cartilage. They consist of several orders:
Hexanchiformes Two families are found within this order. Species of this order are distinguished from other sharks by having additional gill slits (either six or seven). Examples from this group include the cow sharks, frilled shark and even a shark that looks on first inspection to be a marine snake.
Squaliformes Three families and more than 80 species are found within this order. These sharks have two dorsal fins, often with spines, and no anal fin. They have teeth designed for cutting in both the upper and lower jaws. Examples from this group include the bramble sharks, dogfishes and roughsharks.
Pristiophoriformes One family is found within this order. These are the sawsharks, with an elongate, toothed snout that they use for slashing the fishes that they then eat.
Squatiniformes One family is found within this order. These are flattened sharks that can be distinguished from the similar appearing skates and rays by the fact that they have the gill slits along the side of the head like all other sharks. They have a caudal fin (tail) with the lower lobe being much longer in length than the upper, and are commonly referred to as angel sharks.
Heterodontiformes One family is found within this order. They are commonly referred to as the bullhead, or horn sharks. They have a variety of teeth allowing them to grasp and then crush shellfishes.
Orectolobiformes Seven families are found within this order. They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs and the largest of all fishes, the whale sharks. They are distinguished by having barbels at the edge of the nostrils. Most, but not all are nocturnal.
Carcharhiniformes Eight families are found within this order. It is the largest order, containing almost 200 species. They are commonly referred to as the groundsharks, and some of the species include the blue, tiger, bull, reef and oceanic white tipped sharks (collectively called the requiem sharks) along with the houndsharks, catsharks and hammerhead sharks. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
Lamniformes Seven families are found within this order. They are commonly referred to as the mackerel sharks. They include the goblin shark, basking shark, megamouth, the thresher, mako shark and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes contains the extinct Megalodon (Carcharodon megalodon), which like all extinct sharks is only known by the teeth (the only bone found in these cartilaginous fishes, and therefore the only fossils produced). A reproduction of the jaw was based on some of the largest teeth (up to almost 7 inches in length) and suggested a fish that could grow 120 feet in length. The jaw was realized to be inaccurate, and estimates revised downwards to around 50 feet.
Fish were the earliest vertebrates, and presumably evolved from a group of aquatic lower chordates; the terrestrial vertebrates evolved from fishes.
A number of fishes that are born in freshwater spend their adult lives in the ocean, returning to their birthplace to spawn; the reverse of this migration occurs in some fishes born in the ocean.
The cartilaginousfishes (sharks, rays, and chimaeras) are distinguished from the bony fishes by their cartilage skeletons, by the absence of either a swim bladder or lungs, by the construction of their tail fins, and by the absence in most of a gill covering, or operculum.
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