The newly-discovered tail fossil of Spinosaurus aegyptiacus, which was unearthed in southern Morooco, shows the giant predator was a powerful swimmer and the first known to have lived in the water.
The six-tonne predator 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, published in the journal Nature, was carried out by an international team including from the universities of Portsmouth and Leicester.
A University of Portsmouth spokeswoman 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, such as the high position of the nostrils, heavy bones, short legs, and paddle-like feet point to a life spent in the water rather than on land.
“Not only did dinosaurs dominate the land and take to the air as birds, they even went back into the water and became the top predators there as well.”
The Portsmouth spokeswoman said: “The team found that in place of a stiff tapering tail, typical of other theropod dinosaurs such as the Tyrannosaurus Rex, the tail vertebrae of Spinosaurus had extraordinarily long spines that supported a large, highly flexible, fin-like tail comparable in shape to that of a crested newt.
“After preparing all the fossils, the team used photogrammetry to digitally capture the anatomy of the tail.
“To quantitatively assess the performance of the tail, a team of Harvard researchers made a flexible model of the tail and attached it to a robotic system that mimics swimming movements.
“They then compared the swimming performance of the Spinosaurus tail to model tails from other animals, including dinosaurs, crocodiles and newts.
“The results are fully consistent with the idea of a truly water-dwelling, tail-propelled, ‘river monster’.”