A terrifying cross between Tyrannosaurus rex, a crocodile and a whale was the largest predator ever to walk the earth – or swim in its rivers; in fact it is the first dinosaur known to have adapted to an aquatic lifestyle.

Weighing 20 tons and measuring almost 15 metres, it was 2.7 metres longer than the largest T. rex specimen known. It also had jaws filled with vicious slanted teeth and a snout like that of a massive crocodile, while its short muscular hind legs were equipped for paddling and resembled those seen in early limbed whales.

The creature also had powerful forelimbs with curved, blade-like claws ideal for hooking into or slicing prey.

While other dinosaurs such as T. rex hunted on land, Spinosaurus pursued sharks and other large fish in the deep river system it inhabited. Plesiosaurs, a family of giant marine reptiles, lived alongside Spinosaurus 97 million years ago, but were not dinosaurs. Until now, all dinosaurs were thought to have been confined to the land.

Lead researcher Nizar Ibrahim, from the University of Chicago, said: “Working on this animal was like studying an alien from outer space; it’s unlike any other dinosaur I have ever seen.”

The scientists based their findings on new fossils from the Moroccan Sahara as well as specimens in museum collections.

Unlike other predators, Spinosaurus had feet similar to some shorebirds that stand on or move across soft surfaces rather than perch. Its feet may even have been webbed.

Its most distinctive feature was a row of spines that created an enormous sail down the middle of its back. Scientists think it was used for display purposes rather than to trap heat or store fat.

The research is reported in the journal Science.