For twenty million years, the world's oceans were home to a monstrous shark, named the 'megalodon'. Then suddenly, without explanation, the 18-metre-long (50 foot) super predator disappeared.

It's a juicy bit of ancient history that has inspired a host of books, documentaries and blockbuster films, some of which like to imagine that this bloody thirsty monster is still alive today, lurking somewhere out there in the mysterious deep. 

A new study has once again confirmed that this is not the case.

Taking a fresh look at the fossil record, researchers are now proposing that this mega marine creature may have been killed off by none other than the modern great white shark (Carcharodon carcharias).

The timing is everything. Past research suggests that the megalodon (Otodus megalodon) went missing 2.6 million years ago alongside a wave of marine extinction, potentially caused by a supernova that triggered severe climate and biodiversity changes during this time.

Going through every megalodon fossil collected on the west coast of North America, however, the researchers noticed that some of the samples had been misidentified and misdated.

After making extensive adjustments, they calculated that the shark's disappearance must have happened at least one million years earlier than once thought.

"The extinction of O. megalodon was previously thought to be related to this marine mass extinction - but in reality, we now know the two are not immediately related," explains lead author Robert Boessenecker, a vertebrate paleontologist at the College of Charleston in South Carolina.

Given the new timeline, a new explanation is needed, and Boessenecker and his team think they've caught a big one. 

Together, they argue that the fossil evidence aligns better with the rise of the megalodon's smaller but still fierce relative.

Great whites first showed up on Earth about six million years ago, and at first, they were confined to the Pacific. But flash forward just two million years later, and these predators had successfully spread themselves across the entire globe.

As they ventured from ocean to ocean, the researchers think the great whites might have infringed on the megalodon's territory, outcompeting the younger ones for food.

When competition gets really fierce, the fight for survival turns into a zero sum game, and as the available prey began to dwindle in number, the great white appears to have come out on top.

"We propose that this short overlap (3.6-4 million years ago) was sufficient time for great white sharks to spread worldwide and outcompete O. megalodon throughout its range, driving it to extinction-rather than radiation from outer space," says Boessenecker.

The hypothesis is intriguing, but the debate over the megalodon's extinction isn't likely to end here. An expert on ancient sharks told National Geographic that he is not convinced that just one species can cause such a decline.

Instead, he suggests the authors may have overlooked other culprits, like the modern tiger shark, which also shared similar territory to the juvenile megalodon.

Even Boessenecker says we need more research before we can solve this mystery.

This study has been published in PeerJ.