Plesiosaurs are among the most familiar dinosaur-era marine creatures, discovered nearly 200 years ago. However, we're only now learning that some species could have adapted to freshwater ecosystems – specifically, in what is now the Sahara Desert.
When fossil hunter Mary Anning discovered giant long-necked reptiles with four flippers in the rocks of Dorset's Jurassic Coast, it created a storm that alerted the world to the existence of extinct giants. Reconstructions of Anning's discovery shaped portrayals of the Loch Ness Monster (Nessie) and other mythical creatures worldwide.
However, Anning's discoveries lived in salty environments. The same has been true for most subsequent plesiosaur discoveries – until the new announcement in the journal Cretaceous Research of vertebrae and teeth from river-dwelling species from Morocco's Kem Kem deposits.
The three adult river-plesiosaurs described were around three meters long (10 feet). That's large enough you wouldn't want to meet one paddling, but small compared to their marine cousins, which reached 15 meters (50 feet). The paper also includes the arm bone of a juvenile whose total length was probably around 1.5 meters (5 feet).
The authors consider it more likely they lived permanently in freshwater, like modern river dolphins, rather than switching between the environments, but acknowledge the question is unsettled. However, with pieces of at least a dozen animals, it appears plesiosaurs were common in freshwater.
"It's scrappy stuff, but isolated bones actually tell us a lot about ancient ecosystems and animals in them,” co-author Bath's Dr Nick Longrich said in a statement. Freshwater plesiosaur teeth were highly worn, suggesting armored fish were a big part of the diet.
Longrich described in a blog post encountering a plesiosaur vertebrate in a rock shop in Erfoud “On the edge of the Sahara”, rather than its original location. This sparked a search for more.
"It's been really interesting to see the direction this project has gone in," lead author Georgina Bunker said.
The giant theropod Spinosaurus also hunted in these waters. Nevertheless, freshwater environments may have been something of a relief for them compared to the alternative.
The waters off Morocco in the late Cretaceous housed an ecosystem unlike any other we know. Where most modern and ancient environments have 1-2 large predatory species, a dozen are known to have lived off Africa near the same time, and there may be more to find. This occurred somewhat after the finds described here, but no wonder some enterprising plesiosaurs decided to investigate river mouths.
If plesiosaurs could adapt to freshwater in one place, shrinking a little in the process, they may have inhabited other freshwater environments in the Cretaceous. After all, dolphins have adapted to river environments at least four times, and small plesiosaurs have been found from what would have been brackish, or possibly freshwater, environments elsewhere.
However, numerous media outlets have taken this news and drawn a bow so long it would have been useful at Agincourt. These reports try to make the claim that Nessie might be both real and a freshwater plesiosaur. They hinge on the media release including the words; “On one level, it's plausible,” ignoring “The last plesiosaurs finally died out at the same time as the dinosaurs, 66 million years ago,” in the following sentence.
Plesiosaurs may indeed have played in an ancient Loch, but there is no evidence whatsoever they survived the asteroid-induced end-Cretaceous extinction. Even if they had, survivors would almost certainly have favored equatorial environments.
Loch Ness' bitter cold and low primary productivity could hardly be further from the warm, rich waters of the Moroccan River plesiosaurs enjoyed in the Cretaceous.