Paleontologists are challenging long-held beliefs about mosasaurs, the massive marine reptiles that dominated the oceans during the Cretaceous period. New research, published this week, reveals evidence that these apex predators weren’t strictly ocean-dwelling, but also thrived in freshwater environments-a surprising discovery based on chemical analysis of a mosasaur tooth unearthed in North Dakota alongside remains of a Tyrannosaurus rex and a freshwater crocodile. This finding sheds new light on the adaptability of mosasaurs in the final million years of their existence, just before the mass extinction event 66 million years ago.
December 15, 2025•
Mosasauruses, massive marine reptiles from prehistoric times, also inhabited rivers. An international team of researchers from the Netherlands, the United States, and Sweden discovered this while analyzing a mosasaur tooth.
The mosasaur was a prehistoric reptile characterized by a long tail, small fins, and a large mouth filled with sharp teeth. These creatures could grow up to 36 feet long. Previously, it was believed that this species was exclusively found in marine environments.
However, the research team, including Dutch paleontologist Melanie During, demonstrated that mosasaurs adapted to freshwater environments in the last million years before their extinction. This finding expands our understanding of the adaptability of these ancient reptiles.
The mosasaur – also known as the “Meuse lizard” – existed in many parts of the world, but is named after the Latin name for the Meuse River. Dozens of remains have been found in the vicinity of Maastricht, Netherlands. Between 70 and 66 million years ago, this area was home to a subtropical Cretaceous Sea in South Limburg, where mosasaurs also swam.
Chemical Composition Reveals Freshwater Habitat
The tooth was discovered three years ago in a river deposit in North Dakota. It was found alongside a Tyrannosaurus rex tooth and a piece of jaw from a freshwater crocodile.
“That was right in the middle of the continent at the time,” explained During in a radio interview on Vroege Vogels. “There were some rivers, but no sea. So we thought, what was that tooth doing there?”
The chemical composition of the tooth enamel was analyzed at the Vrije Universiteit Amsterdam, as detailed in the published research. “I measured the tooth geochemically in all sorts of ways,” said During. “Everything points to freshwater.” Two other mosasaur teeth found in nearby, slightly older locations in North Dakota showed similar traces of freshwater.
Last year, a giant skeleton of a mosasaur was put on display at the Natural History Museum Maastricht. The skeleton is 66 million years old.
The assembled skeleton is the first and only life-size one in the world:
Mosasaurus Bèr in full size on display in Maastricht
