How plesiosaurs swam underwater

Plesiosaur fossils found in Sahara suggest they weren’t just marine animals: The discovery of plesiosaur fossils in an ancient river bed suggests that some species, traditionally thought to be marine animals, may have lived in fresh water.

Fossils of small plesiosaurs, long-necked marine reptiles from the age of the dinosaurs, have been found in a 100-million-year-old river system in what is now Morocco’s Sahara Desert. This discovery suggests that some species of plesiosaurs, traditionally thought to be marine animals, may have lived in fresh water.

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 Lake Loch Ness Monster, plesiosaurs were marine animals – or were widely believed to be.

Now researchers from the University of Bath and University of Portsmouth in the UK, and Université Hassan II in Morocco, have reported on small plesiosaurs from a Cretaceous river in Africa.

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

These fossils suggest that the plesiosaurs were adapted to tolerate freshwater, and perhaps even spend their lives there, like today’s river dolphins.

The new paper was led by University of Bath Student Georgina Bunker, along with Nick Longrich from the University of Bath’s Milner Center for Evolution, David Martill and Roy Smith from the University of Portsmouth and Samir Zouhri from University Hassan II.

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

“It’s scratchy stuff, but isolated bones actually tell a lot about ancient ecosystems and the animals in them. They’re so much more common than skeletons, they give you more information to work with,” says Dr. Nick Longrich, corresponding author on paper.

“The bones and teeth were found scattered and in different places, not as one skeleton. So each bone and each tooth is a different animal. We have over a dozen animals in this collection.”

Although bones provide information about where animals died, teeth are interesting because they were lost while the animal was alive – so they show where the animals lived.

Also, the teeth show heavy wear, like the fish-eating dinosaurs Spinosaurus are in the same beds.

The researchers say that suggests the plesiosaurs ate the same food – chipping the teeth of the armored fish that lived in the river. This suggests that they spent a lot of time in the river, rather than being occasional visitors.

While marine animals such as whales and dolphins migrate up rivers, either to feed or because they get lost, the number of plesiosaur fossils in the river suggests that this is unlikely.

A more likely possibility is that the plesiosaurs could tolerate fresh and salt water, like some cetaceans, such as the beluga.

It is even possible that the plesiosaurs were permanent river dwellers, like modern river dolphins. Plesiosaurs’ small size would have allowed them to hunt in shallow rivers, and the fossils show an incredibly rich fish fauna.

Dr Longrich said: “We don’t really know why the plesiosaurs are in freshwater.

“It’s a bit controversial, but who’s to say that since we paleontologists have always called them ‘marine reptiles,’ they had to live in the ocean? Lots of marine lineages invaded freshwater.”

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 there are 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, appear in brackish or fresh waters in North America and China.

Plesiosaurs were a diverse and adaptable group and existed for more than 100 million years. Based on what they’ve found in Africa — and what other researchers have found elsewhere — the authors suggest they may have invaded freshwater repeatedly to varying degrees.

“We don’t really know, honestly. That’s how paleontology works. People ask, how can paleontologists know anything for sure about the lives of animals that died out millions of years ago? The reality is we can’t always. All we can do is make educated guesses based on the information we have. We’ll find more fossils. Maybe they’ll confirm those guesses. Maybe not.”

“It’s been really interesting to see the direction this project has gone,” says lead author Georgina Bunker. The study originally began as an undergraduate project involving a single bone, but over time more plesiosaur fossils began to emerge, slowly providing a clearer picture of the animal.

The new discovery also expands the diversity of Morocco’s chalk. Said Dr. Samir Zouhri, “This is another sensational discovery that adds to the many discoveries we have made in Kem Kem during the last fifteen years of work in this region of Morocco. Kem Kem was truly an incredible biodiversity hotspot during the Cretaceous period.”

“What amazes me,” said co-author Dave Martill, “is that the ancient Moroccan river contained so many carnivores all living side by side. This was no place to swim.”

But what does all this mean for the plausibility of something like the Loch Ness Monster? On one level it is reasonable. Plesiosaurs were not limited to the seas, they lived in fresh water. But the fossil record also suggests that after nearly a hundred and fifty million years, the last plesiosaurs finally died out at the same time as the dinosaurs, 66 million years ago.

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