Mosasaurs Could Inhabit Freshwater Environments, New Fossil Discovery Suggests

Paleontologists have discovered a 66-million-year-old mosasaur tooth in the Hell Creek Formation in North Dakota, USA. The discovery adds to growing evidence that mosasaurs, traditionally considered marine reptiles, hunted in rivers.

“Mosasaurs were apex marine predators that diversified during the Late Cretaceous, dominating the seas and occupying a variety of marine niches,” said Dr Melanie Dühring, a paleontologist at Uppsala University and Vrije Universiteit Amsterdam, and her colleagues.

“Within Mosasaurinae there are three subfamilies: Mosasaurinae, Plioplatecarpinae and Tylosaurinae, each characterized by different morphological adaptations that allowed them to exploit different ecological opportunities.”

“Although mosasaurs are primarily associated with shallow seas, mosasaurian remains are found in estuarine and freshwater environments, challenging their traditional strictly marine classification.”

In 2022, researchers discovered a large mosasaurus tooth at a multispecies fossil site in the Hell Creek Formation.

The area, formerly associated with an ancient sea known as the Western Interior Seaway, is notable for its lack of marine species and the fossil record is dominated by terrestrial and freshwater species.

The fossil was found in river sediments along with a tooth from Tyrannosaurus rexand a crocodile jaw in an area known to contain the remains of a duck-billed dinosaur. Edmontosaurus.

The specimen belonged to a representative of the mosasaurian group. Prognatodontini due to the similarity of the textured patterns on its surface and on the teeth of other members of this group.

The authors also analyzed isotopes in tooth enamel to infer the conditions in which the mosasaurus lived, and found signatures of oxygen and strontium isotopes associated with freshwater environments.

They speculate that this may have been caused by the mosasaurus hunting freshwater animals, indicating that it may have lived and hunted far from the sea.

“Carbon isotopes in teeth typically reflect what the animal ate,” Dr. Duhring said.

“Many mosasaurs have low ¹³C values ​​because they dive deep.”

“A mosasaurus tooth found with Tyrannosaurus rex the tooth, on the contrary, has a higher ¹³The C value is higher than all known mosasaurs, dinosaurs and crocodiles, suggesting it did not dive deeply and may have occasionally fed on drowned dinosaurs.”

“Isotopic signatures indicated that this mosasaurus lived in a freshwater riverine environment.”

“When we looked at two additional mosasaurus teeth found at nearby, slightly older sites in North Dakota, we saw similar tracks in fresh water.”

“This analysis shows that mosasaurs lived in fluvial environments for the last million years before going extinct.”

Additional analyzes of the teeth of old mosasaurs and other animals of the Western Interior Seaway revealed isotope concentrations more consistent with their freshwater lifestyle than their marine habitat.

This indicates that salt levels in the region have gradually decreased over time.

The authors suggest that representatives of Prognathodontini could have been opportunistic predators, occupying the same niche as modern ones. Saltwater crocodiles (Crocodile porous) and that they may have adapted to a freshwater environment in response to falling salt levels in the Western Interior Seaway, gradually entering Hell Creek channels as the seaway receded.

“For comparison with mosasaurus teeth, we also measured fossils of other marine animals and found clear differences,” said Dr. Per Ahlberg, a paleontologist at Uppsala University.

“All gill-breathing animals had isotopic signatures that associated them with brackish or salt water, while all lung-breathing animals did not.”

“This shows that mosasaurs, which needed to come to the surface to breathe, inhabited the upper layer of fresh water rather than the lower layer where the water was saltier.”

teams paper published December 12, 2025 in the magazine BMC Zoology.

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crazy during etc.. 2025. “King of Riverside,” a multiproxy approach offers new insight into mosasaurs before their extinction. BMK Zool 10, 25; doi: 10.1186/s40850-025-00246-y