Plesiosaur fossils found in a 100-million-year old river

Plesiosaur fossils occur commonly in deposits from the late Triassic Period, through the Jurassic and right to the end of the Late Cretaceous Period. These iconic reptiles, with their small heads, long necks, rounded bodies and four long flippers, reigned for over 100 million years and had a wide geographical distribution. They are considered to be highly successful marine reptiles and so many of their fossils have now been found that over a hundred different extinct species are recognized today. 

The majority of these fossil remains have been recovered from ancient marine rocks, although a few have also been found in coastal environments, such as shallow waters, bays or estuaries, where salinity would have been lower than in the ocean. Now, in a new study led by the University of Bath, the fossil remains of small plesiosaurs have been reported from a Cretaceous-aged river system in Africa.

The fossils have been accumulated from several different regions of the Kem Kem deposits in eastern Morocco and include vertebrae from the neck, back and tail regions, shed teeth and a small flipper bone thought to be from a juvenile individual. Initially, Dr. Nick Longrich from the University of Bath’s Milner Centre for Evolution identified the flipper bone mixed in amongst various fossils in a rock shop in Erfoud, on the edge of the Sahara. He was surprised to find it there, as he thought of plesiosaurs as marine reptiles and the Kem Kem is in an ancient river system. 

Once his interest had been piqued, he began to search around for other plesiosaur fossils from the area. His student, Georgina Bunker, identified the flipper bone as a humerus and his colleague Dave Martill, from the University of Portsmouth, recognized the bone as most likely belonging to a member of the Leptocleididae, a family of small plesiosaurs that typically only grows to around 3 m in length. Over next few years, the scientists gathered a small collection of teeth and vertebrae that had been found in the Kem Kem deposits; their analysis of these fossils appears in a publication in the journal Cretaceous Research.

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

“The 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.”

The researchers were interested in the fact that the numerous shed teeth showed significant signs of wear, including damage to the tips. This pattern was also found on the teeth of the fish-eating dinosaurs, Spinosaurus, which are found in the same deposits. Damage to the teeth is thought to have resulted from preying upon the heavily armored fish species that are also represented in the Kem Kem fossil record, and that attained enormous sizes. Furthermore, the authors feel that this implies the newly discovered plesiosaurs fed routinely on the same resources as Spinosaurus and were therefore not simply occasional visitors to the freshwater environment. 

Although plesiosaur fossils have occasionally been found in estuarine or brackish water environments, they are not thought of as fresh water inhabitants. However, at Kem Kem they lived amongst numerous other freshwater species, such as frogs, salamanders, turtles, Spinosaurus, lungfish and bivalves. This has led the researchers to propose that these plesiosaurs were fully adapted to living in fresh water habitats and spent most of their time, if not all of it, in this environment.

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 because we paleontologists have always called them ‘marine reptiles’, they had to live in the sea? Lots of marine lineages invaded freshwater.”

Marine mammals sometimes enter fresh water environments to seek refuge, while river dolphins and the Baikal seal only inhabit fresh water habitats. In fact, fossils from the family Leptocleididae – to which the Kem Kem plesiosaurs are thought to belong – have been found in brackish or freshwater deposits elsewhere, in England, Africa, and Australia. And other fossil plesiosaurs, including the long-necked elasmosaurs, turn up in brackish or fresh waters in North America and China.

Based on these various lines of evidence, the researchers hypothesize that the new plesiosaurs were adapted to life in fresh water. 

“We don’t really know, honestly,” said Dr. Longrich. “That’s how paleontology works. People ask, how can paleontologists know anything for certain about the lives of animals that went extinct 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 in,” 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’s Cretaceous fauna. “This 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,” noted Dr. Samir Zouhri. “Kem Kem was truly an incredible biodiversity hotspot in the Cretaceous.”

“What amazes me,” said co-author Dave Martill, “is that the ancient Moroccan river contained so many carnivores all living alongside each other. This was no place to go for a swim.”

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By Alison Bosman, Earth.com Staff Writer