The megalodon went extinct 3.6 million years ago, and is thought to be the largest shark that ever swam the Earth. But the megalodon may not have been as big as once thought, some researchers suggest.

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MARY LOUISE KELLY, HOST:

The megalodon went extinct 3.6 million years ago. It's thought to be the largest shark that ever swam the Earth, but there is debate over what it looks like. A research team now suggests megalodon may have been slimmer than scientists assumed. Ari Daniel has more.

ARI DANIEL, BYLINE: Phillip Sternes grew up outside of Chicago - not a drop of saltwater in sight. But he remembers he was 5 or 6 when he first encountered megalodon in the movie "Jaws."

PHILLIP STERNES: I watched it over and over on a videotape countless times. There's actually a scene where Chief Brody's flipping through the book, and then he sees a picture of the megalodon.

DANIEL: It was only a matter of time before Sternes became a shark researcher himself. He's currently a grad student at UC Riverside. A while back, he learned that what megalodon looked like was an unresolved question based on incomplete fossil evidence - tons of teeth, a smattering of scales and vertebrae and nothing more.

STERNES: You literally will pick every single detail you can out from that limited fossil evidence to try to figure out what the heck did the shark look like?

DANIEL: For years, researchers assumed that megalodon was just a bigger version of the modern-day great white shark.

STERNES: You know, the large, serrated, triangular teeth, a top-level macro predatory shark, top of its food chain.

DANIEL: Back the '90s, Sternes says there was this research team that calculated the relationship between the largest vertebra of a great white shark and its total length.

STERNES: Basically, if you found a great white shark, its vertebra, and you could plug it into that equation, and you can predict the length of the shark just from the vertebra. So simple.

DANIEL: The researchers then applied the same logic to a vertebra from the most complete megalodon fossil there is. The result was that specimen would have been some 30 feet long. That's double or triple the length of a typical great white. But then a couple years ago, a different group lined those same vertebrae up to make a computer model of megalodon. The spine alone measured about 36 feet, which would make the animal some 52 feet long.

STERNES: So I'm like, OK, there's a huge discrepancy going on right there.

DANIEL: A great white shark is big and bulky, says Sternes. So if megalodon were that long, he's doubtful its spine could have supported a great white's thicker body shape.

STERNES: You got to think about swimming, structural support underwater, just maintaining the actual body shape because everything rides on the spine.

DANIEL: Sternes' conclusion is that megalodon must have been more slender, meaning it would have still been a fearsome predator, but he says it would have cruised more slowly and had a longer digestive canal. The interpretation is published in the journal Palaeontologia Electronica.

Meanwhile, there's that other group that measured the spine and came up with the longer estimate of megalodon's length. It includes Jack Cooper, a graduate student in paleobiology at Swansea University in the U.K. And he says this new publication presents an argument that his group already considered.

JACK COOPER: It does reaffirm that we would need to find a complete skeleton to know for sure what megalodon looked like. But if you're going to present an alternative hypothesis, you should use a lot of statistical work to try to make your case.

DANIEL: Something he says the new paper doesn't do. Cooper does agree this is an important riddle to solve once more fossil evidence becomes available. That's because megalodon's size and body shape would explain how much it likely had to eat and swim to survive.

COOPER: And that can tell us quite a lot about what made it vulnerable to extinction about 3 million years ago.

DANIEL: At the end of megalodon's reign, sea levels were changing, ruining coastal habitats, which would have meant less food, possibly contributing to the giant shark's extinction.

For NPR News, I'm Ari Daniel.

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