The biggest and the baddest among meat-eating dinosaurs, Spinosaurus may have also been the first dinosaur to take to the water, swimming in North Africa's rivers some 97 million years ago, researchers reported on Thursday.

Floating like a crocodile to stalk prey, the 50-foot-long (15.2 meters) predator bore a massive sail on its back that would have risen from the water like a shark's fin. The carnivore probably ate fish, ancient crocodiles, and anything else afloat.

"It was the biggest carnivorous dinosaur, but Spinosaurus wasn't a land animal," says University of Chicago paleontologist Nizar Ibrahim, a National Geographic Society Emerging Explorer and lead author of the new report. "This was a creature adapted to life in the water."

Up to then, dinosaurs had ruled only the land. After 150 million years of dinosaur evolution, "suddenly we see these adaptations in Spinosaurus where it is able to swim," says University of Chicago paleontologist Paul Sereno, a co-author of Thursday's report in the journal Science describing new fossils that reveal how the fearsome ancient predator lived. (Read "Mr. Big" in National Geographic magazine.)

The fossil bones of Spinosaurus aegyptiacus described in the study turned up in sandstone beds in the Moroccan Sahara. Complete with skull, claws, and bones that formed the sail on its back, the fossils reveal a crocodilian snout, paddle-like feet, and dense bones that aided buoyancy, adding up to a life aquatic for the giant predator.

"All in all, the discoveries by this team show that Spinosaurus is an extremely unusual and specialized carnivorous dinosaur," says dinosaur expert Thomas Holtz of the University of Maryland in College Park, who was not part of the discovery team.

The fossil finds are also featured in the October National Geographic magazine, and will appear in a National Geographic/NOVA special on PBS in November.

"Spinosaurus has almost no 'junk in the trunk,'" Holtz says, noting its narrow hips and short thighs. "This doesn't make much sense for a land animal that makes a living chasing other land animals. But if it is an animal that doesn't spend most of its time on land, but instead in the water, it doesn't need strong leg muscles."

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