Mosasaurs were enormous reptiles best known for ruling ancient oceans more than 66 million years ago, but new evidence suggests some also lived in rivers. Scientists reached this conclusion after analyzing a mosasaur tooth discovered in North Dakota. The tooth is thought to have come from an animal that may have grown up to 11 meters long. Led by researchers at Uppsala University, the international study shows that mosasaurs adjusted to river environments during the final million years before they disappeared.
A Fossil Discovery That Raised Questions
The tooth was found in 2022 by paleontologists working in North Dakota. It came from a river deposit and was discovered alongside a tooth from a Tyrannosaurus rex and part of a crocodylian jaw. The site is also known for fossils of Edmontosaurus, a duck billed dinosaur. Finding remains from land dinosaurs, river-dwelling crocodiles, and a giant marine reptile in the same place prompted an obvious question. How did a mosasaur tooth end up in a river when these animals were long believed to live only in the sea?
Chemical Clues in Tooth Enamel
A research team from the United States, Sweden, and the Netherlands set out to answer that question by studying the chemical signals preserved in the mosasaur tooth enamel. Their approach relied on isotope analysis, a method that can reveal details about an animal’s habitat and diet.
A newly analyzed mosasaur tooth from North Dakota suggests these legendary marine reptiles also lived in freshwater rivers. Isotope evidence shows they adapted as ancient seas slowly transformed into river systems near the end of the dinosaur era. Some may have hunted close to the surface, feeding on whatever prey was available—including dinosaurs. The finding reshapes how scientists view the final years of these massive predators. Credit: Uppsala University
Isotopes Reveal Habitat and Diet
Because the mosasaur tooth, the T. rex tooth, and the crocodylian jawbone are all about the same age, roughly 66 million years old, the researchers were able to compare their chemical signatures. The analysis was carried out at the Vrije Universiteit (VU) in Amsterdam and focused on isotopes of oxygen, strontium, and carbon. The mosasaur tooth contained higher levels of the lighter oxygen isotope (¹⁶O) than are normally seen in ocean dwelling mosasaurs. This pattern is typical of freshwater environments. Strontium isotope ratios also pointed toward a life spent in freshwater rather than saltwater.
“Carbon isotopes in teeth generally reflect what the animal ate. Many mosasaurs have low ¹³C values because they dive deep. The mosasaur tooth found with the T. rex tooth, on the other hand, has a higher ¹³C value than all known mosasaurs, dinosaurs, and crocodiles, suggesting that it did not dive deep and may sometimes have fed on drowned dinosaurs,” says Melanie During, one of the study’s corresponding authors.
“The isotope signatures indicated that this mosasaur had inhabited this freshwater riverine environment. When we looked at two additional mosasaur teeth found at nearby, slightly older, sites in North Dakota, we saw similar freshwater signatures. These analyses shows that mosasaurs lived in riverine environments in the final million years before going extinct,” says During.
When Ancient Seas Turned Fresh
The findings also offer insight into a major environmental shift in Earth’s past. Over time, increasing amounts of freshwater flowed into the Western Interior Seaway, a vast inland sea that once stretched from north to south across what are now the prairies and split North America in two. As freshwater input grew, the seaway slowly changed from salty to brackish and eventually to mostly freshwater, similar to conditions in the Gulf of Bothnia.
The researchers suggest this process created a ‘halocline’, where a layer of freshwater rested above denser saltwater. Isotope data supports this interpretation.
“For comparison with the mosasaur teeth, we also measured fossils from other marine animals and found a clear difference. All gill-breathing animals had isotope signatures linking them to brackish or salty water, while all lung-breathing animals lacked such signatures. This shows that mosasaurs, which needed to come to the surface to breathe, inhabited the upper freshwater layer and not the lower layer where the water was more saline,” says Per Ahlberg, coauthor of the study and promotor of Dr. During.
Adapted to New Living Conditions
The researchers conclude that the mosasaur teeth they studied came from individuals that had already adjusted to these changing conditions. Large predators shifting between habitats is not unusual in evolutionary history.
“Unlike the complex adaptation required to move from freshwater to marine habitats, the reverse adaptation is generally simpler,” says During.
Modern animals show similar flexibility. River dolphins live entirely in freshwater even though they evolved from marine ancestors. The estuarine crocodile, known in Australia as the saltwater crocodile, also moves easily between rivers and the open ocean, hunting wherever food is available.
A Predator the Size of a Bus
Mosasaur fossils are common in marine deposits across North America, Europe, and Africa that date from 98–66 million years ago. In North Dakota, however, they are rare, which makes this discovery especially important. The size of the tooth suggests an animal that could reach up to 11 meters in length, roughly the size of a bus. Earlier finds of mosasaur bones at a nearby North Dakota site support this estimate.
The tooth is from a prognathodontine mosasaur, although its exact genus cannot be identified with certainty. Close relatives in the genus Prognathodon had large heads, powerful jaws, and thick teeth, and are thought to have been opportunistic predators capable of attacking other large aquatic animals.
“The size means that the animal would rival the largest killer whales, making it an extraordinary predator to encounter in riverine environments not previously associated with such giant marine reptiles,” says Ahlberg.
Reference: ““King of the Riverside”, a multi-proxy approach offers a new perspective on mosasaurs before their extinction” by Melanie A. D. During, Nathan E. Van Vranken, Clint A. Boyd, Per E. Ahlberg, Suzan J. A. Warmerdam-Verdegaal and Jeroen H. J. L. Van der Lubbe, 12 December 2025, BMC Zoology.
DOI: 10.1186/s40850-025-00246-y
The study was conducted by researchers from Uppsala University in collaboration with Eastern West Virginia Community and Technical College, Moorefield, West Virginia, Vrije Universiteit Amsterdam and the North Dakota Geological Survey. The article is based on a chapter of Melanie During’s thesis, which she presented at Uppsala University in November 2024.
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