SHARKS

Sharks are fish that have been around before the dinosaurs existed. They live in waters all over the world, in every ocean, and even in some rivers and lakes.

Unlike bony fish, sharks have no bones; their skeleton is made of cartilage, which is a tough, substance, not as hard as bone. Sharks also have no swim bladder (like bony fish do).

Sharks have a variety of body shapes. Most sharks have streamlined, torpedo-shaped bodies that move easily through the water. Some bottom-dwelling sharks have flattened bodies that allow them to hide in the sand of the ocean floor. Some sharks have an elongated body shape. Sawsharks have elongated snouts, thresher sharks also have a elongated upper tail fin which they use to stun prey, and hammerheads have wide heads. The goblin shark has a large, pointed protuberance on its head; its purpose is unknown.

Sharks have existed for over 350 million years. They evolved over 100 million years before the dinosaurs did. This was long before people evolved. Most fossil evidence of early sharks is from fossilized teeth and a few skin impressions. Cladodonts, primitive sharks, had double-pointed teeth, were up to 3 feet (1 m) long fish-eaters and lived about 400 million years ago.