Paleontologists have discovered a 66-million-year-old Mosasaurus tooth within the Hell Creek Formation in North Dakota, USA. This find reinforces the idea that mosasaurs, often viewed as marine reptiles, also hunted in freshwater rivers.

“Mosasaurs were apex predators in marine environments, diversifying during the Late Cretaceous and dominating a range of ecological niches,” stated paleontologist Melanie Dooling from Uppsala University and Vrije Universiteit Amsterdam alongside her team.

“The mosasauridae family is divided into three subfamilies: mosasauridae, plioplatecarpinidae, and tylosauridae, each showcasing unique adaptations that allow them to exploit various ecological opportunities.”

“Although mosasaurid fossils are often linked to shallow marine environments, findings of fossils in estuarine and freshwater settings challenge the notion that they were solely marine creatures.”

In 2022, a significant mosasaur tooth was unearthed from a multi-species fossil site in the Hell Creek Formation.

This area, once part of the ancient Western Interior Seaway, is well-known for a scarcity of marine species, primarily housing fossils of terrestrial and freshwater organisms.

The fossil was found in river sediments alongside an ancient tooth from a tyrannosaurus rex and a crocodile jawbone, indicating a rich ecosystem that included fossilized dinosaurs like edmontosaurus.

This specimen is related to the mosasaurid family prognatodontini, as its surface texture resembles that of other members in this group.

Researchers also studied isotopes within the tooth enamel to deduce the habitat of mosasaurs, discovering oxygen and strontium isotopic signatures indicative of freshwater conditions.

This may imply that mosasaurs preyed on freshwater animals, suggesting they could thrive and hunt away from oceanic environments.

“The carbon isotopes in teeth generally reflect the diet of the organism,” Dr. Dühring explained.

“Many mosasaurs exhibit low ¹³C values, allowing for deeper dives.”

“The teeth of Mosasaurus, in contrast, have higher ¹³C values compared to those of all known mosasaurs, dinosaurs, and crocodiles, indicating they likely did not dive deeply and may have occasionally consumed drowned dinosaurs.”

“Isotopic data suggest this mosasaur inhabited freshwater river environments.”

“When we examined two additional mosasaurus teeth from a nearby, slightly older site, we noted similar freshwater isotopic signatures.”

“These analyses indicate that mosasaurs inhabited fluvial environments for about the last million years before their extinction.”

Further examination of older mosasaurus teeth and other fauna from the Western Interior Seaway suggested isotopic concentrations more aligned with freshwater rather than saltwater habitats.

This points to a gradual decrease in salinity in the region over time.

The authors propose that members of Prognathodontini might have been opportunistic predators inhabiting niches similar to modern species, akin to the saltwater crocodile (Crocodylus porosus), believed to have adapted to freshwater systems in response to the receding salinity of the Western Interior Seaway, gradually moving into the Hell Creek channel.

“We also analyzed fossils from other marine species and found distinct differences,” stated Dr. Per Ahlberg, a paleontologist at Uppsala University.

“Gill-breathing animals possessed isotopic signatures linked to brackish or saltwater, while all lung-breathing organisms did not.”

“This indicates that mosasaurs, which required surface access to breathe, lived in the upper freshwater layer rather than the saltier deeper layers.”

The team’s paper was published in the Journal on December 12, 2025, in BMC Zoology.

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in the midst of chaos others. 2025. King of the Riverside offers a fresh perspective on pre-extinct mosasaurs through a multi-proxy approach. BMC Zuhl 10, 25; doi: 10.1186/s40850-025-00246-y