Carcharodon megalodon (Agassiz, 1843), the giant "Megatooth" shark, ruled all the warmwater seas during the Neogene Period [Miocene (5-24 million years ago (mya)) and Pliocene (1.67-5 mya)].  This monster is believed to have been over 20m in length and is related to the the current Great White Shark. It appears to have died out at the start of the Pleistocene era, around 1.5 million years ago.

Fossils of C. megalodon are found only in regions that were predominantly warm water environments. It is thought the reduction in ocean temperatures in the mid-Pliocene caused their demise by reducing the number of possible nursery sites on the continental shelf for C. megalodon or their prey may have escaped to colder waters where the Megatooths could not follow.

Reports of giant Great White Sharks up to 10 m long in recent times have led some scientists to suggest that Megatooth still swims the oceans. Gilbert Whitley, the late curator of fishes at the Australian Museum in Sydney, Australia, wrote (1940, p. 125), Large teeth belonging to species of White Pointer have been dredged at great depths in the oceans and indicate that enormous sharks are either still living or only became extinct fairly recently. A man could stand upright with ease in the jaws of such a monster which has been calculated to have been more than 25m in length! Some shark.

However, no well documented "Megatooth" fossils have been found younger than 3 mya. However, the oceans are vast and deep,