Paleontologists have found several fossilized bones of plesiosaurs, traditionally thought to be sea creatures, in the beds of the Kem Kem, a 100-million-year-old river system in what is now Morocco.
Artist’s impression of Spinosaurus and Plesiosaurus in a river. Image credit: Nick Longrich / University of Bath.
Plesiosaurs (‘close to lizard’ in Greek) are a group of extinct marine reptiles that roamed the vast seas of the Triassic, Jurassic and Cretaceous periods, from 235 to 66 million years ago.
First found in 1823 by fossil hunter Mary Anning, these creatures had small heads, long necks and four long flippers.
Their fossils have been found on every continent on Earth, with key discoveries made in Australia, Europe and North America.
The Kem Kem plesiosaurs belong to the Leptocleididae, a group of small-bodied plesiosaurs that were widely distributed in near-shore, non-marine environments in the Early Cretaceous.
“These fossils are the first freshwater plesiosaurs from Morocco and are among the youngest representatives of Leptocleididae,” said University of Bath palaeontologist Nick Longrich and colleagues.
Plesiosaur fossils from the Kem Kem Beds include 3 m long adult bones and teeth and a 1.5 m long infant arm bone.
“These are discoveries, but isolated bones tell us a lot about the ancient ecosystems and the animals that were there,” Dr Longrich said.
“They’re much more common than skeletons, they give you more information to work with.”
“The bones and teeth were found scattered and in different localities, not like a skeleton. Thus, each bone and each tooth is a different animal. We have over a dozen animals in this collection.”
Fossils from Kem Kem indicate that these creatures commonly lived and fed in fresh water, along with frogs, crocodiles, turtles, fish and the huge aquatic dinosaur Spinosaurus.
These fossils suggest that plesiosaurs were adapted to tolerate fresh water, possibly even living in it, like today’s river dolphins.
“We don’t really know why plesiosaurs are in freshwater,” Dr Longrich said.
“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 sea? Many marine lineages invaded freshwater.”
The findings appear in the journal Cretaceous Research.
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Georgina Bunker et al. Plesiosaurs from the Kem Kem Fluvial Group (Middle Cretaceous) of eastern Morocco and a review of non-marine plesiosaurs. Cretaceous Research, published online July 21, 2022; doi: 10.1016/j.cretres.2022.105310