The newly discovered tail fossil of the first "river monster" dinosaur shows the giant predator was a powerful swimmer and the first known to have lived in the water.
The six-tonne Spinosaurus aegyptiacus prowled the rivers that flowed through the Sahara desert 100 million years ago, living and catching its prey in the water, according to the new research.
The study on the tail, which was unearthed in southern Morocco, was carried out by an international team including from the universities of Portsmouth and Leicester in England.
The University of Portsmouth said: "Until now it was believed that dinosaurs lived exclusively on land, but the newly discovered tail of Spinosaurus aegyptiacus, a giant predator, shows that it was actually well adapted to an aquatic lifestyle.
"The 15-metre-long, six-tonne predator was in fact a powerful swimmer propelled by a huge fin-like tail, which hunted down its prey in vast river systems that flowed through the Sahara desert 100 million years ago.
"It is the first time that such an adaptation has been reported in a dinosaur."
Dr David Unwin, reader in palaeobiology at the University of Leicester, said: "The Spinosaurus' fin-like tail is a game-changing discovery for us that fundamentally alters our understanding of how this dinosaur lived and hunted - it was actually a river monster.
"As well as its tail, many other features of this dinosaur point to a life spent in the water rather than on land."
Paleontologist Dr Nizar Ibrahim, of the University of Detroit Mercy, said: "This discovery is the nail in the coffin for the idea that non-avian dinosaurs never invaded the aquatic realm."