Mosasaurs are best known as fearsome marine reptiles that ruled the oceans at the end of the dinosaur era. But new research suggests these giant predators were not limited to the sea.
Evidence from a single fossil tooth now shows that some mosasaurs also lived in freshwater rivers during the final chapter of their existence, just before they went extinct 66 million years ago.
The discovery comes from an international research team led by scientists at Uppsala University and is published in the journal BMC Zoology.
By analyzing the chemical makeup of a mosasaur tooth found in North Dakota, the researchers uncovered strong evidence that this enormous animal lived in a river environment rather than the open ocean.
The tooth was discovered in 2022 in a river deposit alongside fossils from very different creatures: a tooth from Tyrannosaurus rex and a jawbone from a crocodile-like reptile.
This unusual mix raised an intriguing question. How did a giant marine reptile end up in the same freshwater setting as land dinosaurs and river-dwelling crocodilians?
To answer this, scientists turned to isotope analysis, a technique that examines tiny chemical signatures preserved in fossil teeth.
Teeth are especially useful because they lock in information about the water an animal drank and the food it ate while the tooth was forming.
The researchers compared the mosasaur tooth with fossils of similar age, around 66 million years old. They focused on isotopes of oxygen, strontium, and carbon.
Oxygen isotopes revealed a key clue. The mosasaur tooth contained a higher proportion of a lighter oxygen isotope than is typical for marine mosasaurs. This pattern strongly points to freshwater rather than seawater. Strontium isotopes supported the same conclusion.
Carbon isotopes added another layer to the story. Many mosasaurs show low carbon values because they hunted deep in the ocean.
The North Dakota tooth, however, had unusually high carbon values, higher than those of known mosasaurs, dinosaurs, or crocodiles. This suggests the animal stayed in shallow water and may even have fed on animals that drowned in rivers, possibly including dinosaurs.
To be sure this was not an isolated case, the team examined two additional mosasaur teeth found at nearby sites in North Dakota that were slightly older. These teeth showed similar freshwater isotope signatures.
Together, the findings suggest that mosasaurs were living in river systems during the final million years before their extinction.
The study also sheds light on major environmental changes happening at the time. Much of central North America was once covered by the Western Interior Seaway, a vast inland sea that split the continent in two.
Toward the end of the Cretaceous period, increasing freshwater input gradually diluted this sea, turning salty water into brackish water and eventually into mostly freshwater. A layered system likely formed, with freshwater sitting on top of denser saltwater.
Because mosasaurs breathed air, they would have spent much of their time near the surface. Isotope comparisons with other animals support this idea. Marine animals that breathed through gills showed salty or brackish signatures, while lung-breathing animals, including mosasaurs, showed freshwater signatures, indicating they lived in the upper freshwater layer.
The tooth itself is massive, suggesting an animal up to 11 meters long, roughly the size of a bus. This predator would have rivaled modern killer whales and would have been an astonishing presence in river environments not previously associated with such giants.
The researchers argue that adapting from saltwater to freshwater is relatively straightforward for large predators, and modern animals like river dolphins and saltwater crocodiles show similar flexibility. This discovery reveals mosasaurs as more adaptable than once thought, rewriting part of their story just before the age of dinosaurs came to an end.
