Whales may have evolved to their current massive sizes only because one of their most fearsome predators, the largest shark species known to science, went extinct, researchers say.

Primitive baleen whales, smaller than their modern-day counterparts, were a likely prey of Megalodon, a prehistoric 50-foot-long predator something like a jumbo-sized Great White, the scientists say.

In a study published in the journal PLOS One, the researchers put the date for Megalodon's extinction at about 2.6 million years ago, after which baleen whales as a group began to evolve and grow to the giant sizes seen today.

The group includes the blue whale, the largest animal on Earth that can reach lengths of 100 feet.

While the researchers acknowledge there is no definite evidence that Megalodon preferred baleen whales as prey, they note that ancient whale fossils are often discovered in company with Megalodon teeth.

If the whales were in fact a feature of Megalodon diets, then the extinction of the ancient mega-shark may have allowed them to flourish and evolve, researchers say.

"When we found out when that [extinction] happened, we noticed it coincided with the [evolution] pattern mentioned in whales," says study co-author Catalina Pimiento from the Florida Museum of Natural History. "Now we need to find out if one event -- Megalodon's extinction -- caused the other -- evolution of gigantism in whales."

The fossil record for Megalodon is spotty with a number of gaps, making an accurate determination of when it went extinct difficult, researchers say.

Pimiento and research colleagues from the University of Florida placed 42 known Megalodon fossils in a database, assigning each one an upper and lower date estimate in an attempt to narrow the gaps.

"Based on the distribution of those gaps and how those gaps change, it will then infer the point in time where that species can be considered to be extinct," researcher Chris Clements said.

That yielded a 99.9 percent certainty the mega-shark species was extinct by 2.6 million years ago, the researchers said.

That's strong evidence refuting the claims made by some people that Megalodon may still be alive and lurking in the world's ocean's today, a claim featuring prominently in some programs aired as part of the Discovery Channel's "Shark Week."

Most shark experts say they've confident the giant species is long gone.

"If a 50-foot-long predator that fed on surface animals and lived in coastal environments were still around, someone would have found evidence of this by now," shark enthusiast David Shiffman says.

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