A lone tooth from a massive, extinct lizard-like reptile unearthed in North Dakota’s Hell Creek Formation is rewriting the story of mosasaurs, long seen as rulers of ancient seas but now revealed to have prowled freshwater rivers too.
The well-preserved tooth crown, discovered in 2022 amid a jumble of dinosaur and crocodile fossils in a muddy floodplain deposit, belongs to a mosasaur from the Prognathodontini tribe — a group known for robust, cutting teeth suited to crunching prey. Unlike typical marine finds, this site lacks any sea creatures, and the tooth shows no signs of being washed in from afar. Instead, detailed analyses indicate that the animal lived and died in a freshwater environment about 66 million years ago, during the Late Maastrichtian stage of the Cretaceous Period.
Researchers used a battery of tests on the tooth’s enamel, including stable carbon and oxygen isotopes from carbonate and phosphate parts, plus strontium ratios, to trace its environment. These chemical signatures match those of nearby land-dwelling dinosaurs like Tyrannosaurus and freshwater crocs, not ocean dwellers like sharks or ammonites. The tooth’s owner, possibly up to 11 meters long based on similar nearby fossils, likely hunted in rivers flowing into the shrinking Western Interior Seaway, a vast inland ocean that once split North America.
“This study employs a multi-proxy approach to compare the tooth’s isotopic signatures with other taxa from the same and nearby formations (including teeth of other mosasaurines), providing insights into the environmental transitions of the Western Interior Seaway (WIS) towards the end of the Cretaceous,” the researchers explain in their report.
Such flexibility echoes hints from smaller mosasaurs elsewhere, like those in Hungary or Canada, which ventured into estuaries or shallow waters. But this North Dakota specimen suggests even giant apex predators adapted as the seaway freshened due to rising temperatures and more rain. Warmer conditions may have eased the shift, much like how modern marine animals sometimes invade rivers today.
The find adds to growing evidence that mosasaurs weren’t picky about salinity. As the Cretaceous waned, these reptiles — cousins to modern snakes and lizards — exploited new niches amid changing climates. “These findings challenge the notion of mosasaurs as exclusively marine predators, highlighting their potential ecological flexibility during the Late Maastrichtian,” the team notes.
Citations: BMC Zoology. DOI: 10.1186/s40850-025-00246-y
