Megalodon is a terrifying huge shark that lived in shallow continental seas from about 18 to 1.5 million years ago. Like other fossil sharks, megalodon, whose name means "big tooth" in Greek, is primarily known from fossilized teeth. These teeth are so hard that they barely deteriorated during the life of the shark, leaving behind fossil teeth so sharp that some amateurs have taken them as evidence that they were left behind relatively recently. Only a few non-dental megalodon fossils have been found, in the form of several vertebra. The megalodon is one of the most interesting and popular fossil fishes, both among the public and the paleontology community.

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A megalodon tooth is about 5 in (13 cm) long, around the size of a human hand. This is several times larger than a tooth from a great white shark, which usually only measures about an inch (2.5 cm). The fossilized teeth of megalodon are black, though they would have been white during life. Some of these fossils are prized collectors items, and considered among the best investment fossils.

Although early reconstructions of the megalodon based on its teeth led to size estimates in the range of 25 m (82 ft), these were subsequently found to be based on an inaccurate reconstruction of the jaw. Modern size estimates are in the range of 12 m (39 ft) to 18.2 m (60 ft), about two to three times the size of a great white shark. This is more than enough to qualify it as the largest predatory fish of all time, by extension the largest shark, and one of the largest fishes ever to have lived. Some might say it is the most terrifying predator to exist since Tyrannosaurus rex.

Initially, it was thought that the megalodon and the great white shark were closely related, but today most scientists believe that it parted evolutionary ways with the great white longer ago than previously thought, deserving classification under an extinct genus, Carcharocles, rather than the great white's genus Carcharodon. The "Carcharocles vs. Carcharodon" debate is one of the most contentious in marine paleontology.

Megalodon is thought to have preyed on the early whales, which tended to be smaller and slower than today's whales. This has been supported by fossil evidence of whale bones with gigantic teeth marks. With a gaping maw 7 ft (2.1 m) in size, megalodon could have swallowed small whales whole, and dealt fatal bites to even the largest whales of the time. It would have needed to consume a huge amount of food to sustain its tremendous bulk, estimated at approximately 50 tons.

One and a half million years ago, for an unknown reason, this sea monster went extinct. A variety of hypotheses have been proposed, including climate change and the extinction of key prey animals.