Large prehistoric marine reptiles known as mosasaurs dominated the oceans until their extinction 66 million years ago. Now, according to new chemical evidence, some mosasaurs also lived in freshwater rivers near the end of the Cretaceous period, indicating their ability to adapt as environments changed before the mass extinction.
A recent study published in Springer Nature reports that researchers from Uppsala University analyzed the chemical makeup of a large mosasaur tooth found in North Dakota. Their findings show this animal lived in freshwater, not the ocean, giving direct evidence that some mosasaurs entered rivers during the last million years of the Cretaceous.
A Marine Predator in an Unexpected Place
The tooth was found in 2022 within river sediments of North Dakota’s Hell Creek Formation, a site well known for yielding dinosaur fossils such as Tyrannosaurus rex and Edmontosaurus. The discovery of a mosasaur tooth alongside a T. rex tooth and a crocodylian jawbone was unexpected, prompting researchers to examine how these animals may have shared the same environment.
Mosasaurs are usually found in marine deposits in North America, Europe, and Africa, but riverine fossils are rare. The tooth’s size shows it came from a large animal, possibly up to 36 feet long, about as long as a bus.
Isotopes Tell the Story
The international team analyzed oxygen, strontium, and carbon isotopes preserved in the mosasaur’s tooth enamel at Vrije Universiteit Amsterdam. Tooth enamel forms early in life and resists chemical alteration; therefore, it preserves a record of the environment an animal inhabited.
The results showed elevated levels of oxygen-16, an isotope more common in freshwater environments. Strontium isotope values further supported the idea that this mosasaur lived in a river system rather than the open ocean.
Carbon isotope analysis offered further insight. Unlike most mosasaur fossils, which show low carbon-13 values, this specimen had higher carbon-13 levels, suggesting a different way of life for this river-dwelling predator.
“Carbon isotopes in teeth generally reflect what the animal ate,” said Melanie During, one of the study’s corresponding authors. “The mosasaur tooth found with the T. rex tooth has a higher carbon-13 value than all known mosasaurs, dinosaurs, and crocodiles, suggesting that it did not dive deep and may sometimes have fed on drowned dinosaurs.”
A Freshwater Phase Before Extinction
The team also analyzed two additional mosasaur teeth from nearby, slightly older sites in North Dakota. These samples displayed similar freshwater isotope signatures, suggesting that the presence of mosasaurs in river systems was not an isolated occurrence.
“The isotope signatures indicated that this mosasaur had inhabited this freshwater riverine environment,” During said. “These analyses show that mosasaurs lived in riverine environments in the final million years before going extinct.”
When Seas Turned Into Rivers
In the late Cretaceous, the Western Interior Seaway was receding as more freshwater flowed in, gradually turning the vast inland sea into a mostly freshwater environment. This shift likely created a halocline, with a layer of freshwater sitting above denser saltwater. The isotope data from the fossils fit this scenario.
“For comparison with the mosasaur teeth, we also measured fossils from other marine animals and found a clear difference,” said Per Ahlberg, coauthor of the study. “All gill-breathing animals had isotope signatures linking them to brackish or salty water, while all lung-breathing animals lacked such signatures.”
Since mosasaurs were air-breathing reptiles, they likely remained in the freshwater surface layer, hunting near river mouths and floodplains instead of diving into deeper waters.
Ecological Flexibility at the End of an Era
Although it is uncommon for large predators to move between habitats, there are modern examples of this. For example, river dolphins inhabit freshwater despite their marine origins, while saltwater crocodiles move between rivers and the ocean.
“Unlike the complex adaptation required to move from freshwater to marine habitats, the reverse adaptation is generally simpler,” During said.
“The size means that the animal would rival the largest killer whales,” Ahlberg added, “making it an extraordinary predator to encounter in riverine environments not previously associated with such giant marine reptiles.”
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds a Master of Business Administration and a Bachelor of Science in Business Administration, as well as a certification in Data Analytics. His work combines analytical training with a focus on emerging science, aerospace, and astronomical research.