[Animals] Sharks

[S.S.I]

Sharks are a superorder of fish, also know by its scientific name 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.[1] Most sharks are predators: 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. It is widely believed that sharks are "silent killers". However, a recent study shows that sharks emit a low growl from their throats which resonates through their scales. Some common kinds of shark are the hammerhead shark, the great white shark, the tiger shark, and the mako shark. Most sharks are cold-blooded but some, like the great white shark and the mako shark are partially warm-blooded. Just a few million years ago, a giant shark called Megalodon swam in the seas. It was 18 meters long, twice as long as the closely-related great white shark, and it ate whales. Megalodon died out 1.6 million years ago. Much of what we understand about prehistoric sharks comes from the study of their fossils. While sharks have skeletons made of soft cartilage that can fall apart before fossilizing, their teeth are harder and easily fossilized. Prehistoric sharks, like their modern descendants, would grow and shed many thousands of teeth over their lifetime. For this reason shark teeth are one of the most common fossils.