Bold claim: giant sea monsters once roamed freshwater rivers too, not just the ocean. That’s the core idea behind a new study on mosasaurs, colossal marine reptiles that lived over 66 million years ago. Researchers examined a mosasaur tooth unearthed in North Dakota and found compelling evidence that some of these predators inhabited freshwater rivers late in their lineage. The tooth likely came from an individual reaching about 11 meters in length, suggesting these creatures could thrive in river systems during the final million years before their extinction. The international team, led by scientists at Uppsala University, concluded that mosasaurs adapted to riverine environments as seas shifted and freshwater became more dominant in western North America.
The fossil site, excavated in 2022 from a North Dakota river deposit, yielded a surprising fossil assemblage: a mosasaur tooth alongside a Tyrannosaurus rex tooth and a crocodylian jawbone. This mix is unusual because it combines land-dwelling dinosaurs with river-dwelling crocodilians and a giant marine reptile. The question was clear: if mosasaurs were ocean-dwellers, how did one of their teeth end up in a river setting?
Isotopes Speak Volumes
To unravel this mystery, researchers from the United States, Sweden, and the Netherlands analyzed the tooth enamel’s chemical fingerprints using isotope analysis. Since the mosasaur tooth, the T. rex tooth, and the crocodylian jawbone date to roughly 66 million years ago, the team could compare their chemical signals directly. The study, conducted at Vrije Universiteit Amsterdam, focused on oxygen, strontium, and carbon isotopes. The mosasaur tooth showed elevated levels of the lighter oxygen isotope, 16O, which is characteristic of freshwater environments rather than marine settings. Strontium isotope ratios also pointed to freshwater influence.
According to Melanie During, a corresponding author, carbon isotopes in teeth reflect diet. Many mosasaurs exhibit low 13C values due to deep-water feeding, while the mosasaur tooth found with the T. rex tooth displayed a comparatively high 13C value. This suggests the individual did not feed exclusively in deep waters and may have sometimes consumed drowned dinosaurs, indicating more varied feeding behavior than previously thought.
The team concluded that the isotope signatures indicate a freshwater riverine lifestyle for this mosasaur. They also analyzed two additional mosasaur teeth from nearby, slightly older North Dakota sites and found similar freshwater signals. Taken together, the data show that mosasaurs occupied freshwater river environments in the final million years before their extinction.
From Sea to Stream: How Change Happened
The researchers also proposed a mechanism for this ecological shift. Over time, more freshwater flowed into the Western Interior Seaway, a massive inland sea that once split North America. As freshwater input increased, the seaway transformed from salty to brackish and eventually to largely freshwater—an evolution reminiscent of modern Gulf of Bothnia conditions. This process likely created a halocline: a light freshwater layer at the surface over denser, saltier water below. Isotope data supported this idea.
For comparison, the team measured fossils from other marine animals and found a clear pattern: gill-breathing species tended to show signatures tied to brackish or saltwater, while lung-breathing animals did not. This aligns with mosasaurs, which needed to surface to breathe, inhabiting the upper freshwater layer rather than the deeper, more saline zone.
Adapting to a Shifting World
The findings suggest the mosasaurs whose teeth were analyzed had adjusted to these freshwater conditions. Though moving between freshwater and marine habitats typically demands substantial adaptation, the reverse—transitioning from marine back to freshwater—can be comparatively simpler.
The study also draws parallels with modern wildlife. River dolphins are descendants of marine ancestors and now reside entirely in freshwater, while the estuarine crocodile routinely traverses between rivers and the open ocean in search of prey.
A Bus-Sized Predator in Unlikely Waters
Mosasaur fossils are common in marine sediments across continents dating from about 98 to 66 million years ago, but finding one in North Dakota is rare and striking. The size estimate—about 11 meters—puts the individual on par with the length of a bus, corroborated by earlier nearby bone discoveries. While the tooth is likely from a prognathodontine mosasaur, the exact genus remains uncertain. Related Prognathodon species are known for their massive heads, strong jaws, and robust teeth and are considered opportunistic predators capable of taking large prey.
The authors emphasize that a giant predator moving into riverine environments showcases remarkable ecological flexibility. “The size alone would make this animal a formidable riverine hunter, comparable to the largest predators seen in modern ecosystems,” notes Per Ahlberg, coauthor of the study.
Collaborative effort and context
This research was conducted by scientists from Uppsala University in cooperation with Eastern West Virginia Community and Technical College, Moorefield, West Virginia; Vrije Universiteit Amsterdam; and the North Dakota Geological Survey. The work builds on Melanie During’s doctoral chapter completed at Uppsala University in November 2024.
Would you like a brief explanation of how isotope analysis can distinguish freshwater from marine environments, or should I add a few discussion questions to spark conversation in the comments about this surprising biome shift?
