A mosasaur tooth from North Dakota reveals that these giant marine reptiles could thrive in freshwater rivers as well as the ocean. Dated to roughly 66 million years ago, the tooth likely belonged to an animal that reached about 11 metres in length. An international team led by researchers at Uppsala University used isotope analysis of the tooth enamel to show that mosasaurs adapted to riverine environments in the last millions of years before their extinction.
In 2022, paleontologists uncovered a large mosasaur tooth in a fluvial deposit in North Dakota. The find occurred alongside a Tyrannosaurus rex tooth and a crocodylian jawbone, in a site also associated with Edmontosaurus remains. The presence of land-dwelling dinosaurs, river-dwelling crocodylians, and giant marine reptiles in the same place raised a key question: how did a creature tied to the sea end up in a river?
To answer this, researchers from the United States, Sweden, and the Netherlands compared the chemical signatures of related fossils of similar ages (about 66 million years old) using isotope analyses. The work, performed at Vrije Universiteit Amsterdam, examined oxygen, strontium, and carbon isotopes in the enamel. The mosasaur tooth contained more of the lighter oxygen isotope (¹⁶O) than is typical for marine mosasaurs, indicating a freshwater habitat. Strontium isotope ratios likewise pointed to freshwater influence.
Carbon isotopes in teeth generally reflect diet. Many mosasaurs show low ¹³C values due to deep diving, but the mosasaur tooth found with the T. rex tooth exhibited the highest ¹³C value among known mosasaurs, dinosaurs, and crocodilians, suggesting it did not dive deeply and may have occasionally fed on drowned dinosaurs. As During notes, these isotope signatures indicate the individual inhabited a freshwater riverine setting. Additional, slightly older mosasaur teeth from nearby sites showed similar freshwater signatures, supporting the idea that mosasaurs consumed riverine habitats in the final life stages before their extinction.
This finding aligns with a broader history of shifting oceans. The Western Interior Seaway—an inland sea that once split North America—gradually lost salinity as freshwater flowed in, transforming seawater into brackish and eventually predominantly freshwater. The researchers propose the formation of a halocline: a surface freshwater layer over a deeper, saltier layer. Isotope data from various fossils support this scenario. Per Ahlberg explains that gill-breathing marine animals show signatures tied to brackish or salty water, while lung-breathing animals do not, implying mosasaurs—air-breathers that must surface—lived in the upper freshwater zone rather than the lower saline layer.
The study concludes that the mosasaur teeth analyzed came from individuals adapted to these changing conditions. Such transitions are not unusual among top predators. As During points out, reversing a marine-to-freshwater shift is often simpler than moving from freshwater to the sea. Modern analogues include river dolphins, descended from marine ancestors, and the estuarine crocodile, which roams between rivers and the ocean depending on prey availability.
The North Dakota tooth suggests a mosasaur of considerable size, roughly bus-sized at 11 metres, consistent with other nearby finds. The tooth originates from a prognathodontine mosasaur, though the exact genus remains uncertain; Prognathodon relatives had robust jaws and teeth and were effective predators in aquatic ecosystems.
Lead author Melanie During and colleagues from Uppsala University, Eastern West Virginia Community and Technical College, Vrije Universiteit Amsterdam, and the North Dakota Geological Survey contributed to these findings. The paper appears in BMC Zoology and draws from During’s 2024 Uppsala University thesis.
This discovery invites reevaluation of mosasaur ecology, suggesting a more versatile lifestyle than previously thought and highlighting how ancient marine reptiles could exploit freshwater environments late in their history.
