Sharks are a superorder of fish called the Selachimorpha. They, like other Chondrichthyes, have skeletons made of cartilage instead of bone. Cartilage is tough, rubbery material which is less rigid than bone. Cartilaginous fish also include skates and rays.
There are more than 350 different kinds of sharks, such as the great white and whale sharks. Fossils show that sharks have been around for 420 million years, since the early Silurian.
Most sharks are predators, meaning they hunt and eat fish, marine mammals, and other sea creatures. However, the largest shark eats krill, like whales. This is the whale shark, the largest fish in the world. Some common kinds of shark include the hammerhead shark, the great white shark, the tiger shark, and the mako shark.