Mosasaurs, giant marine reptiles that existed more than 66 million years ago, lived not only in the sea but also in rivers. This is shown by new research based on analyses of a mosasaur tooth found in North Dakota and believed to belong to an animal that could reach a length of 11 metres. The study, conducted by an international team of researchers led from Uppsala University, shows that mosasaurs adapted to riverine environments in the final million years before they became extinct.
In 2022, palaeontologists found a large tooth from a mosasaur in North Dakota. It was discovered in a fluvial deposit, together with a tooth from a Tyrannosaurus rex, and a crocodylian jawbone in an area known for remains of the duck-billed dinosaur Edmontosaurus. The fact that land-dwelling dinosaurs, river-dwelling crocodylians and giant marine reptiles were found together raised the question: how did a mosasaur tooth end up in a river, when this reptile was assumed to live in the sea?
An international team of researchers from the United States, Sweden and the Netherlands has now answered the question using isotope analyses of the mosasaur's tooth enamel.
Isotopes show how they lived and what they ate
Since the mosasaur tooth, the T. rex tooth and the crocodylian jawbone are of similar age, around 66 million years old, the researchers were able to compare their chemical composition by means of isotope analyses. The analysis, carried out at the Vrije Universiteit (VU) in Amsterdam, studied the ratio between different isotopes of the elements oxygen, strontium and carbon. The mosasaur teeth contained more of the lighter oxygen isotope (¹⁶O) than is usually seen in marine mosasaurs, indicating that they lived in freshwater. The ratio between different strontium isotopes also suggests a freshwater habitat.
"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 seas became rivers
The discovery sheds light on an interesting chapter in the Earth's history. The influx of freshwater into the Western Interior Seaway, an inland sea that then stretched from north to south across today's prairies and divided North America in two, increased over time. This gradually transformed the sea water from saltwater to brackish water and eventually to mostly freshwater, rather like in the Gulf of Bothnia. The authors believe that this led to the formation of a 'halocline', in which a layer of freshwater sat on top of heavier saltwater. This theory is borne out by isotope analyses.
"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 argue that the mosasaur teeth analysed clearly came from individuals that were adapted to these changing environments. Such a transition is not an unknown phenomenon among large predators.
"Unlike the complex adaptation required to move from freshwater to marine habitats, the reverse adaptation is generally simpler," says During.
Modern examples of such adaptation can be seen in river dolphins, which live in freshwater rivers despite being descended from marine ancestors. The estuarine crocodile, known in Australia as the saltwater crocodile, is another example. It moves freely between freshwater rivers and the open sea, hunting in both environments depending on what prey is available.
Could be as big as a bus
Mosasaur fossils are abundant in North American, European and African marine deposits that are 98–66 million years old. However, they are only rarely found in North Dakota, which makes the new discovery particularly noteworthy. The size of the tooth testifies to an impressive creature that could grow up to 11 metres long, about the size of a bus. This estimate is corroborated by a handful of mosasaur bones found earlier at a nearby site in North Dakota. The tooth comes from a prognathodontine mosasaur, though the genus cannot be determined with certainty. Mosasaurs of the genus Prognathodon, closely related to the animal that lost this tooth, had bulky heads with sturdy jaws and teeth. They are widely regarded as opportunistic predators that posed a significant threat to 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.
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.
