Shark

Hexanchiformes

Classification

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It is currently thought that the sharks form a polyphyletic group: in particular, some sharks are more closely related to rays than they are to some other sharks.

Related Topics:
Elasmobranchii - Chondrichthyes - Rays - Skate - Chimaera - Polyphyletic

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There are more than 360 described species of sharks.

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The first sharks appeared in the oceans 400 to 350 million years ago. Most of the species we know today are as old as the Jurassic period. There are eight orders of sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:

Related Topics:
Ocean - Jurassic - Orders

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• Hexanchiformes: 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: Examples from this group include the bramble sharks, dogfish sharks and roughsharks.
• Pristiophoriformes: These are the sawsharks, with an elongated, toothed snout that they use for slashing the fishes that they subsequently eat.
• Squatiniformes: Angel sharks.
• Heterodontiformes: They are commonly referred to as the bullhead or horn sharks.
• Orectolobiformes: They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs and the largest of all fishes, the whale shark.
• Carcharhiniformes: They are commonly referred to as the groundsharks, and some of the species include the blue, tiger, bull, reef and oceanic whitetip 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: They are commonly referred to as the mackerel sharks. They include the goblin shark, basking shark, megamouth shark, the thresher sharks, mako shark and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction.

The Lamniformes include the extinct Megalodon (Carcharodon megalodon), which like all extinct sharks is only known from its 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 17 cm (7 inches) in length) and suggested a fish that could grow 15 m (50 feet) long. The jaw was realized to be inaccurate, and estimates revised downwards to around 6 m (20 feet).

Related Topics:
Megalodon - Fossils

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