For over a hundred years, mosasaurs have captured the scientific imagination. They were the dominating reptiles of the Late Cretaceous seas. These sleek, muscular creatures swam through warm oceans while dinosaurs roamed on land. But a discovery from the heart of North America is rewriting that story, revealing that these marine giants were not confined to saltwater at all.
A single tooth, buried in a river deposit in North Dakota, has opened a window into a forgotten ecosystem where land, river, and sea predators collided.
In 2022, palaeontologists excavating a fluvial deposit stumbled upon an unlikely trio: a mosasaur tooth, a Tyrannosaurus rex tooth, and a crocodylian jawbone, all preserved alongside the remains of the duck-billed dinosaur Edmontosaurus.
The mix raised an irresistible question: What was a marine reptile doing in a river system?
An international team of researchers from the United States, Sweden, and the Netherlands set out to solve the mystery. Their answer, published now, suggests that mosasaurs were far more adaptable and far more widespread than previously believed.
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The team analyzed the chemical signatures locked inside the tooth enamel. Conducted at the Vrije Universiteit in Amsterdam, the isotope study compared oxygen, strontium, and carbon ratios in the mosasaur tooth with those of the T. rex and crocodylian fossils found nearby.
The results were startling. The mosasaur tooth contained elevated levels of the lighter oxygen isotope (16O), a hallmark of freshwater environments. Strontium isotopes told the same story.
And then came the carbon data.
“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.
Two additional mosasaur teeth from slightly older North Dakota sites showed the same freshwater signature.
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The isotope signatures indicated that this mosasaur had inhabited this freshwater riverine environment. When we looked at two additional mosasaur teeth found nearby, slightly older sites in North Dakota, we saw similar freshwater signatures. These analyses show that mosasaurs lived in riverine environments in the final million years before going extinct,” says During.
The findings show a significant environmental change in the Western Interior Seaway, an inland sea that once divided North America. Over time, freshwater from rivers flowed into the seaway. This gradually changed its salinity from salty to brackish and eventually to something similar to today’s Gulf of Bothnia.
This stratified water column likely formed a halocline: a freshwater layer floating atop denser saltwater.
“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 promoter of Dr During.
The team argues that these mosasaurs were not accidental visitors; they were adapted to this shifting environment.
“Unlike the complex adaptation required to move from freshwater to marine habitats, the reverse adaptation is generally simpler,” says During.
Modern analogues support this idea: river dolphins evolved from marine ancestors, and Australia’s estuarine crocodile moves effortlessly between rivers and the open sea.
Mosasaur fossils are common in ancient marine deposits across three continents, but rare in North Dakota. That rarity makes the new find especially striking.
The tooth belonged to a prognathodontine mosasaur, likely a close relative of Prognathodon, an animal with a bulky skull and crushing jaws. Based on the tooth’s size, the creature may have stretched up to 11 metres, rivaling today’s largest killer whales.
“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.
This discovery changes our understanding of mosasaurs. They were not just ocean specialists; they were flexible predators that could take advantage of new habitats as their environment changed. In their last million years, when the seaway became less salty and ecosystems altered, these giants moved up rivers. They probably even hunted dinosaurs that got swept into the water.
A single tooth, worn down by time and river currents, has uncovered a creature much more versatile than previously thought. The seas may have been their domain, but the rivers were accessible too.
Journal Reference:
- During, M.A.D., Van Vranken, N.E., Boyd, C.A. et al. “King of the Riverside”, a multi-proxy approach offers a new perspective on mosasaurs before their extinction. BMC Zool 10, 25 (2025). DOI: 10.1186/s40850-025-00246-y
