A artist's impression of a plesiosaur dinosaur about to be eaten by a Spinosaurus, the largest predatory dinosaur known.

Fossils of small plesiosaurs have been found in a 100-million year old river system that is now Morocco鈥檚 Sahara Desert

26 July 2022

6 min read

Fossils of small plesiosaurs, long-necked marine reptiles from the age of dinosaurs, have been found in a 100-million year old river system that is now Morocco鈥檚 Sahara Desert. This discovery suggests some species of plesiosaur, traditionally thought to be sea creatures, may have lived in freshwater.

Plesiosaurs, first found in 1823 by fossil hunter Mary Anning, were prehistoric reptiles with small heads, long necks, and four long flippers. They inspired reconstructions of the Loch Ness Monster, but unlike the monster of Lake Loch Ness, plesiosaurs were marine animals - or were widely thought to be. 

Now, scientists from the universities of 兔子先生 and Bath in the UK, and Universit茅 Hassan II in Morocco, have reported small plesiosaurs from a Cretaceous-aged river in Africa. 

The fossils include bones and teeth from three-metre long adults and an arm bone from a 1.5 metre long baby. They hint that these creatures routinely lived and fed in freshwater, alongside frogs, crocodiles, turtles, fish, and the huge aquatic dinosaur Spinosaurus. 

These fossils suggest the plesiosaurs were adapted to tolerate freshwater, possibly even spending their lives there, like today鈥檚 river dolphins. 

The new paper was headed by University of Bath student Georgina Bunker, along with Nick Longrich from the University of Bath鈥檚 Milner Centre for Evolution, David Martill and Roy Smith from the 兔子先生, and Samir Zouhri from the Universite Hassan II. 

Professor of Palaeobiology, David Martill, from the 兔子先生鈥檚 School of the Environment, Geography and Geosciences, said: 鈥淚 am always amazed at how many spectacular animals we discover in this ancient river system. Almost everything in it is a predator. Everything is also really huge. It must have been the most dangerous place on the planet.鈥

 

An artist's impression of a plesiosaur and Spinosaurus, the largest predatory dinosaur known. Image credit: Nick Longrich, University of Bath

An artist's impression of a plesiosaur and Spinosaurus, the largest predatory dinosaur known. Image credit: Nick Longrich, University of Bath.

The fossils include vertebrae from the neck, back, and tail, shed teeth, and an arm bone from a young juvenile. 

鈥淚t鈥檚 scrappy stuff, but isolated bones actually tell us a lot about ancient ecosystems and animals in them. They鈥檙e so much more common than skeletons, they give you more information to work with鈥 said Dr Nick Longrich, corresponding author on the paper.

鈥淭he bones and teeth were found scattered and in different localities, not as a skeleton. So each bone and each tooth is a different animal. We have over a dozen animals in this collection.鈥 

Whilst bones provide information on where animals died, the teeth are interesting because they were lost while the animal was alive - so they show where the animals lived. 

What鈥檚 more, the teeth show heavy wear, like those fish-eating dinosaur Spinosaurus found in the same beds. 

The scientists say that implies the plesiosaurs were eating the same food - chipping their teeth on the armored fish that lived in the river. This hints they spent a lot of time in the river, rather than being occasional visitors.

While marine animals like whales and dolphins wander up rivers, either to feed or because they鈥檙e lost, the number of plesiosaur fossils in the river suggest that鈥檚 unlikely. 

A more likely possibility is that the plesiosaurs were able to tolerate fresh and salt water, like some whales, such as the beluga whale. 

It鈥檚 even possible that the plesiosaurs were permanent residents of the river, like modern river dolphins. The plesiosaurs鈥 small size would have let them hunt in shallow rivers, and the fossils show an incredibly rich fish fauna.

 

Freshwater fauna from the Kem Kem, Morocco

Freshwater fauna from the Kem Kem, Morocco. Image credit: Nick Longrich, University of Bath.

Dr Longrich said: 鈥淲e don鈥檛 really know why the plesiosaurs are in freshwater.

鈥淚t鈥檚 a bit controversial, but who鈥檚 to say that because we palaeontologists have always called them 鈥榤arine reptiles鈥, they had to live in the sea? Lots of marine lineages invaded freshwater.鈥

Freshwater dolphins evolved at least four times - in the Ganges River, the Yangtze River, and twice in the Amazon. A species of freshwater seal inhabits Lake Baikal, in Siberia, so it's possible plesiosaurs adapted to freshwater as well. 

The plesiosaurs belong to the family Leptocleididae- a family of small plesiosaurs often found in brackish or freshwater elsewhere in England, Africa, and Australia. And other plesiosaurs, including the long-necked elasmosaurs, turn up in brackish or fresh waters in North America and China. 

Plesiosaurs were a diverse and adaptable group, and were around for more than 100 million years. Based on what they鈥檝e found in Africa - and what other scientists have found elsewhere 鈥 the authors suggest they might have repeatedly invaded freshwater to different degrees.

"It's been really interesting to see the direction this project has gone in,鈥 said lead author Georgina Bunker. The study initially began as an undergraduate project involving a single bone, but over time, more plesiosaur fossils started turning up, slowly providing a clearer picture of the animal.

The new discovery also expands the diversity of Morocco鈥檚 Cretaceous.  Said Dr Samir Zouhri, 鈥淭his is another sensational discovery that adds to the many discoveries we have made in the Kem Kem over the past fifteen years of work in this region of Morocco. Kem Kem was truly an incredible biodiversity hotspot in the Cretaceous.鈥

Professor Martill added: 鈥淭his ancient Moroccan river contained so many carnivores all living alongside each other 鈥 it was certainly no place to go for a swim.鈥

But what does this all mean for the Loch Ness Monster? On one level, it鈥檚 plausible. Plesiosaurs weren鈥檛 confined to the seas, they did inhabit freshwater. But the fossil record also suggests that after almost a hundred and fifty million years, the last plesiosaurs finally died out at the same time as the dinosaurs, 66 million years ago.