Fearsome Marine Predators Prowled Ancient Rivers, Too

Explore

A The discovery of a tooth tooth in an ancient riverbed in North Dakota sheds light on one of the most fearsome reptiles to ever swim the seven seas: the mosasaur.

You may remember the huge mosasaur from the movie Jurassic world. The giant was kept in a vast sea enclosure and forced to perform in front of park visitors before slaughtering a genetically modified animal. Tyrannosaurus rex (called Indominus rex) in the film’s climactic battle.

In reality, an ocean mosasaur would probably not encounter a land mosasaur. T. rexThat’s why it was so curious that paleontologists found a tooth from each prehistoric giant in the same riverbed. They published their findings today in Zoology BMC.

Read more: »Conjuration of imaginary creatures»

The mosasaur tooth and the T. rex tooth (as well as a nearby jaw from an ancient crocodilian) were the same age, about 66 million years old. The scientists were thus able to compare the chemical compositions of the remains, by measuring the ratios of the different isotopes of oxygen, strontium and carbon. Their results led to a surprising conclusion: the ancient owner of the mosasaur tooth lived in a freshwater habitat.

“Carbon isotopes in teeth generally reflect what the animal has eaten. Many mosasaurs have low ¹³C values ​​because they dive deep,” study co-author Melanie Durant, a paleontologist at Vrije Universiteit Amsterdam, said in a statement. “The mosasaur tooth found with the T. rex the tooth, on the other hand, has a higher ¹³C value than that of all known mosasaurs, dinosaurs and crocodiles, suggesting that it did not dive deeply and may have sometimes fed on drowned dinosaurs.

Rather than these 36-foot-long marine predators suddenly deciding to swim upriver to eat a snack, scientists believe their environment changed around them. During the Cretaceous period, when mosasaurs lived, North America was divided in two by a vast sea of ​​salt water covering the grasslands. As the Cretaceous came to an end 66 million years ago, this sea of ​​salt water was gradually diluted by an influx of fresh water that eventually settled on top of the heavier salt water, forming a clear separation between the two. As the waters around the mosasaurs changed, they adapted to their new river home.

This transition from salt water to fresh water is not unknown in the animal kingdom; it has been observed in river dolphins, saltwater crocodiles, and some shark species that transition between the two environments. Still, imagining a bus-sized predator lazily swimming up an inland river is a mental image that gives you chills.

Enjoy Nautilus? Subscribe for free to our newsletter.

Main image: Christopher DiPiazza

• Jake Currie

  Published on December 12, 2025

  Jake Currie is a writer based in Brooklyn, New York.