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The leviathans of the Late Cretaceous ocean were swift-swimming lizards, large as sperm whales and finned like sharks. New evidence shows how similar the flippers of these top predators from 90 million years ago were to the limbs of everyone's favorite predatory fish today.

The first mosasaur fossil was discovered in the 1700s. From their run-on spines, researchers first guessed the animals were related to snakes, and later proposed that the ocean-swimming reptiles swam like fish. Rare soft-tissue preserved on the new prognathodon fossil, one member of the family of mosasaurs, shows a well-defined body plan and the trademark shark-like forked tail, supporting that theory.



This fossil, collected from the Maastrichtian of Harrana site in Jordan in 2008, contains the first soft tissue evidence of the tail shape of the mosasaur. Johan Lindgren

"For more than 200 years there hasn't been a single specimen showing the outlines of the fins and most important, the tail fin," Johan Lindgren, a paleontologist at Lund University in Sweden, told NBC News. Lindgren is a member of the team that describes the fossil in the Sept. 10 issue of Nature Communications.

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Mosasaurs could grow up to 50 feet in length, and were the top predators of the Late Cretaceous oceans. Stefan Sølberg

The new fossil, only about 6 feet long and therefore a young 'un, adds to evidence that the mosasaurs, which started out as land-living reptiles, entered the water and changed their body plan over tens of millions of years.

"The proportions of its body are amazingly similar to those that we see in pelagic sharks," Lindgren said. He expects that other, later mosasaurs may have been "even more fish-like than this guy."

Remarkably, the mosasaurs, the Cambrian-age ichthyosaurs before them, and today's toothy sharks — all top ocean predators in their time — independently arrived at roughly the same, "drop-shaped" stream-lined body plan and a two pronged tail.

Unlike sharks, the spine of the mosasaurs curved downward into the lower lobe of the tail. This may have been designed to assist the reptilian swimmers come up to the surface for air, Lindgren said.

Johan Lindgren, Hani Kaddumi and Michael Polcyn are authors of "Soft tissue preservation in a fossil marine lizard with a bilobed tail fin" published in Nature Communications.

Nidhi Subbaraman writes about science and technology. You can follow her on Facebook, Twitter and Google+.

