Paleontologists finally have proof that some dinosaurs were aquatic.

Detailed analysis of the only existing Spinosaurus aegyptiacus remains suggests the world's longest predatory dinosaur lived in a large river system and used its tail to swim.

The discovery -- published this week in the journal Nature -- marks the first time a tail-propelled swimming locomotion has been reported in a dinosaur.

"This discovery really opens our eyes to this whole new world of possibilities for dinosaurs," lead study author Nizar Ibrahim, professor at the University of Detroit Mercy, said in a news release. "It doesn't just add to an existing narrative, it starts a whole new narrative and drastically changes things in terms of what we know dinosaurs could actually do. There's nothing like this animal in over 220 million years of dinosaur evolution, which is pretty remarkable."

The world's only Spinosaurus skeleton was found in 2008 in North Africa. For the new study, researchers returned to the dig site in search of more fossils.

After the initial excavation and fossil analysis, scientists concluded that Spinosaurus aegyptiacus specialized in hunting fish, and was likely adapted to an amphibious lifestyle. The dinosaur had dense bones, short hind limbs, wide feet and long jaws full of conical teeth.

Despite the evidence of its water-friendly physiology, suggestions that the species was fully adapted to life in the water weren't well-received. But when Ibrahim and his colleagues returned to the dig site, they recovered several new fossils, including a fin-like tail characterized by long spines.

Analysis of the tail's unusual structure showed it was capable of extensive lateral movement. In other words, Spinosaurus aegyptiacus could wave its tail the way crocodiles do, powering its long body through the water.

To better understand the tail's functionality, researchers built a 3D model of the tail and attached it to a robot that produces swimming locomotion. Scientists compared the tail's performance to those of dinosaur, crocodile and newt tails.

The results showed the tail of Spinosaurus aegyptiacus was a powerful swimming tool.

"This discovery is the nail in the coffin for the idea that non-avian dinosaurs never invaded the aquatic realm," Ibrahim said. "This dinosaur was actively pursuing prey in the water column, not just standing in shallow waters waiting for fish to swim by. It probably spent most of its life in the water."

Researchers suspect many of the closest relatives of Spinosaurus successfully adapted to a variety of aquatic environs.

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