Megalodon tooth. "Shark tooth", 12 million years old, from Beaufort County, South Carolina, U.S.A. (M/L) in perfect condition.

The megalodon (C. megalodon, Greek: mega(lo-) = large, odon = tooth, or large tooth) is an extinct species of shark that could reach enormous dimensions. The megalodon appeared around 18 million years ago (in the Miocene) and became extinct around 1.5 million years ago. It dominated the temperate seas of the world with virtually no predators; it could reach a length of 18 metres. It is closely related to the modern white shark. Megalodon could open its mouth up to 2 metres. Megaselachus megalodon is a synonym of Carcharodon megalodon or Carcharocles megalodon. Even today, researchers regularly find the fossilised teeth of Megalodon; however, the rest of its cartilaginous skeleton typically exhibits decay. Its fairly large teeth are very characteristic (pyramidal).