Size and Weight:
The giant prehistoric shark Megalodon is one of the few animals to be identified by its species, rather than its genus, name: technically, this predator is known as Carcharodon megalodon, which places it in the same family as the modern Great White Shark. However, not everyone is convinced that this fearsome shark was a direct ancestor of the Great White, hence its popular name Megalodon. (Read 10 Facts About Megalodon and see a gallery of Megalodon pictures.).
Until a better candidate comes along--which doesn't seem likely--Megalodon stands as the biggest shark in earth's history, a true apex predator that counted everything in the ocean as part of its ongoing dinner buffet, including whales, squids, fish and dolphins (there's some speculation that Megalodon may even have preyed on Leviathan, a giant, prehistoric sperm whale announced to the world in 2010; see Megalodon vs. Leviathan - Who Wins? for a blow-by-blow analysis of this epic battle.)
Why would such a successful predator have vanished off the face of the earth about a million years ago? No one knows for sure, but global climatic changes or a diminution of its food supply (after all, an adult Megalodon would have had to eat hundreds of pounds a day to stay in shape) probably had something to do with it. In any event, Megalodon's legacy has been carried on to the present day by the Great White Shark, which strikes as much fear in the average beachgoer as Megalodon must have inspired in the average prehistoric fish.