The largest predator to walk - and swim - the Earth was a terrifying cross between Tyrannosaurus rex, a crocodile and a whale.
The discovery was made from fossils dug up in the Moroccan Sahara, which show that the Spinosaurus was the first dinosaur to have adapted to an aquatic lifestyle.
Weighing 20 tonnes and measuring almost 50ft (15m), the swimming dinosaur was 9ft (2.7m) longer than the largest T. rex specimen known.
It 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 Dr 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.
A digital model was used to create an anatomically precise 3D replica of the Spinosaurus skeleton, allowing the researchers to confirm its astonishing size.
'What surprised us even more than the dinosaur's size were its unusual proportions,' said co-author Dr Paul Sereno, also from the University of Chicago.
'We see limb proportions like this in early whales, not predatory dinosaurs.'
Spinosaurus's existence has been known for a century since fragmentary remains were found in Egypt by German paleontologist Ernst Stromer.
But those were destroyed in a British bombing raid on Munich in 1944. Other partial remains offered mere glimpses of its anatomy.
Everything changed when a local fossil hunter unearthed a partial skeleton in
southeastern Morocco in 2008 near the Sahara oasis town of Erfoud.
Combined with fossils held in various museums and drawings of Stromer's finds, an accurate reconstruction of Spinosaurus finally evolved.
But nothing was simple. The remains found by the fossil hunter were spirited out of Morocco, depriving scientists of vital information.
They needed to locate him but did not know his identity beyond being 'the mystery man with the mustache.'
He was finally found in 2013 and led the scientists to the excavation site. More fossils were dug up there and the missing partial skeleton turned up in the basement of a Milan museum.
Dr Ibrahim described Spinosaurus's environment as 'the most dangerous place in the history of our planet.'
Its unusual body resembles mammalian whale ancestors that appeared 45 million years later.
The discovery was made from fossils dug up in the Moroccan Sahara, which show that the Spinosaurus was the first dinosaur to have adapted to an aquatic lifestyle.Weighing 20 tonnes and measuring almost 50ft (15m), the swimming dinosaur was 9ft (2.7m) longer than the largest T. rex specimen known.
It 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 Dr 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.
A digital model was used to create an anatomically precise 3D replica of the Spinosaurus skeleton, allowing the researchers to confirm its astonishing size.
'What surprised us even more than the dinosaur's size were its unusual proportions,' said co-author Dr Paul Sereno, also from the University of Chicago.
'We see limb proportions like this in early whales, not predatory dinosaurs.'
Spinosaurus's existence has been known for a century since fragmentary remains were found in Egypt by German paleontologist Ernst Stromer.
Everything changed when a local fossil hunter unearthed a partial skeleton in
southeastern Morocco in 2008 near the Sahara oasis town of Erfoud.
Combined with fossils held in various museums and drawings of Stromer's finds, an accurate reconstruction of Spinosaurus finally evolved.
But nothing was simple. The remains found by the fossil hunter were spirited out of Morocco, depriving scientists of vital information.
They needed to locate him but did not know his identity beyond being 'the mystery man with the mustache.'
He was finally found in 2013 and led the scientists to the excavation site. More fossils were dug up there and the missing partial skeleton turned up in the basement of a Milan museum.
Dr Ibrahim described Spinosaurus's environment as 'the most dangerous place in the history of our planet.'
Its unusual body resembles mammalian whale ancestors that appeared 45 million years later.
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