An historic discovery of fossils by an international team, including Professor David Martill at the University of Portsmouth and Dr David Unwin at the University of Leicester has revealed the remains of the first aquatic dinosaur.

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.

In research published in the journal Nature today, Spinosaurus, a 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.”

Professor David Martill, Professor of Palaeobiology at the University of Portsmouth, 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.”

Led by National Geographic Explorer and University of Detroit Mercy paleontologist Dr Nizar Ibrahim, the team including Dr Unwin, began excavating a skeleton of pinosaurus in southern Morocco in 2015 and made the historic discovery in 2018.

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.

The Spinosaurus skeleton was found in the Kem Kem river beds, which preserve the remains of many other Cretaceous creatures including sawfish, coelacanths, crocodiles, flying reptiles and other land-living dinosaurs.

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”.

Dr Nizar Ibrahim, a National Geographic Fellow at University of Detroit Mercy, who led the project, 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.”

The research Tail-propelled aquatic locomotion in a theropod dinosaur is published in the journal Nature.