The first ever evidence of a dinosaur which swam, lived and hunted underwater has been discovered in the now-barren wilderness of the Saharan desert.
But 100 million years ago this region would have been a lush oasis interspersed with rivers and waterways teeming with life.
And lurking beneath the surface, sitting atop the entire food chain, was a fearsome aquatic dinosaur.
The river monster — called Spinosaurus aegyptiacus — powered through the water with a fin-like tail and captured slippery prey with six-inch long conical teeth.
A fossilised tail of a juvenile of the species, which belongs to the therepod group, the same as the T-rex, was found in modern-day Morocco.
Adults are known to reach up to 50ft long and weigh up to 20 tonnes but this specimen had yet to reach its full size, measuring 35ft from snout to tail and weighing around four tonnes.
Fully-grown individuals had no natural predators, but researchers say juveniles may have been at risk from giant fish and enormous prehistoric crocodiles.
The researchers who led the project and discovered the fossil deemed this hostile marine world ‘the river of death’.
Around the time of Spinosaurus, several other reptile groups had mastered the water, including ichthyosaurs, but no dinosaurs.
The newly discovered tail of Spinosaurus aegyptiacus shows it was well adapted to an aquatic lifestyle.
Two foot long struts on the main vertebrae broaden the tail into a paddle-like shape.
Dr David Unwin, Reader in Palaeobiology at the University of Leicester, who was involved in the research, 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 study, published today in the journal Nature, comes to the conclusion that this animal was a truly water-dwelling, tail-propelled dinosaur which likely spent most of its life underwater.
National Geographic explorer and University of Detroit Mercy paleontologist Dr Nizar Ibrahim led the research.
His team included Dr Unwin and Professor David Martill from the University of Portsmouth.
Academics began excavating a skeleton of Spinosaurus in southern Morocco in 2015 and made the historic discovery of its almost-complete and well preserved tail in 2018.
Tails of other therapods, which existed on solid land, had a a stiff tapering tail.
But analysis of the vertebrae found in Morocco revealed long spines that supported a large, highly flexible, fin-like tail comparable in shape to that of a crested newt.
The site in the Sahara where the fossil was found is a hotbed for fossil hunters, with other finds including sawfish, crocodiles, flying reptiles and land-dwelling dinosaurs.
Professor Martill said: ‘This fossil site has been incredible. This is the first Spinosaurus skeleton to be found for over a hundred years. It is also one of the few associated dinosaurs skeletons ever to be found in the Kem Kem rocks.
‘Scientists have always puzzled about Spinosaurus, because applying new scientific techniques on this animal have, until now, not been possible because the original material was destroyed in World War Two. Now we have a new baby to play with.
‘Every time we look at this dinosaur we discover something fascinating about it. Discovering its tail was such an amazing gift. We had no idea that its tail was going to be so different from other dinosaur tails.
‘One thing that still puzzles me though, is why only Spinosaurus became aquatic among the dinosaurs. Why are there no aquatic iguanodons, or stegosaurs.’
While it remains a mystery why only one dinosaurs conquered the water, the discovery does put to bed a long-standing belief that dinosaurs never swam.
Dr Ibrahim said: ‘This discovery is the nail in the coffin for the idea that non-avian dinosaurs never invaded the aquatic realm.
‘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.’