Mosasaurs a fearsome group of ancient predators once ruled the seas. Now researchers have turned up a 66-million-year fossil tooth from one. And the big surprise: It came from a site that wasnt part of the ocean. As such, this tooth is rewriting the aquatic reptiles history. Some mosasaurs ruled the rivers, it suggests.
The tooth came from a genus known as Prognathodontini (Prog-NAH-thow-don-TEE-nee). These enormous animals could span up to 11 meters (36 feet) or about the length of a telephone pole. The lizard-like creatures showed up during the Late Cretaceous, some 100 million years ago. Then, like nearly all of their dinosaur cousins, mosasaurs went extinct when a massive asteroid hit Earth 66 million years ago.
Explainer: The age of dinosaurs
Ancient dinos roamed the land. Mosasaurs prowled the water. More closely related to lizards and snakes than dinos, these giants had shark-like tails and paddle-shaped fins. These helped them glide through water to surprise their prey.
With powerful jaws, this lurking predator could bite through big turtles, fishes and reptiles [including dinosaurs]. It was terrifying, says Melanie During. She works at Uppsala University in Sweden. A paleontologist, she uses fossils to learn about ancient life.
In 2022, a team from the North Dakota Geological Society was digging for fossils in a former river floodplain. The North Dakota site is known as the Hell Creek Formation.
In one football-shaped piece of rock, the team found three fossils: a Tyrannosaurus rex tooth, an ancient crocodile jawbone and a mosasaur tooth.
That last one was unexpected. What was a sea reptile doing with a croc and a dinosaur? We were already surprised when a mosasaur tooth was in Hell Creek. We tried everything to prove that the tooth was from a marine reptile, says During. But thats not what the evidence showed.
This fossil tooth from a mosasaur was found in North Dakota.Trissa Shaw
Before giving up, the team compared the new fossils to ones at Vrije University Amsterdam, in the Netherlands. Here, they turned to a chemical technique called isotope analysis. Isotopes are atoms of the same element that have different numbers of neutrons. Isotope patterns can reveal parts of an animals life history, such as where it lived and what it ate.
Because all fossils at Hell Creek were 66 million years old, the researchers could compare them. They focused on isotopes of three elements: oxygen, strontium and carbon.
Oxygen isotopes pointed to what type of water the mosasaur lived in. Living in salt water, a mosasaur would have built up more of a heavier oxygen isotope. Yet the oxygen in the Hell Creek mosasaur tooth had more lighter isotopes than expected. Strontium and carbon isotopes showed a similar pattern.
The results point to the tooths owner having lived and died in freshwater. It was not merely a sea denizen that washed into a river.
Such data suggest scientists will have to reconsider what they know about mosasaur lifestyles, the researchers say. They shared their new findings in BMC Zoology on December 12, 2025.
Members of the North Dakota Geological Survey during a dig where they discovered the mosasaur tooth.Trissa Shaw
Adapting to change
Its a remarkable example of a species apparently adapting to a habitat, says Barry Albright. Hes a paleontologist at the University of North Florida in Jacksonville who didnt work on the study. It was entirely unexpected, he says. The reptiles were long considered to be exclusively marine.
Nicholas Longrich works at the University of Bath in England. In the sea, [mosasaurs] evolve a range of jaw shapes and tooth shapes, body forms and sizes, this paleontologist says. But now, were seeing them occupy other habitats, he says. It indicates they were diverse and thriving before the asteroid struck, killing off much of Earth’s life 66 million years ago.
Diverse predators at the top of the food web imply diverse prey, Longrich points out. So what drew mosasaurs into rivers?
Mosasaurs evolved into a number of species of many sizes. But all were serious predators, as this brief overview shows.
Heres one idea. During the Late Cretaceous, shallow tropical seas covered Earth. One of them the Western Interior Seaway split in half what is now North America. The rich ecosystems of this sea were full of fish and other prey for mosasaurs to eat. Later, as the continent uplifted, the sea underwent major changes.
One change was that its salt levels fell. Seaway mosasaurs might have adapted enough to be able to venture into freshwater. That could have included the river channels at Hell Creek.
Its possible that mosasaurs were following prey upriver, says Femke Holwerda. Shes a mosasaur expert at Utrecht University in the Netherlands.
Mosasaurs had been hardly the only ocean predators. They had rivals for food. Adapting to life in a river may have helped the Hell Creek mosasaurs occupy a new ecological role. Here, they might have competed less for food, Albright thinks.
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It wouldnt be the first time aquatic life left the ocean. Amazon river dolphins adapted to live in murky rivers. Other ancient marine reptiles have been found in riverbeds, too.
There is no reason why mosasaurs would have been constrained to only marine environments, says Kiersten Formoso. A vertebrate paleobiologist, she works at Rutgers University in New Brunswick, N.J.
It would be interesting to see more mosasaur fossils, says Formoso. Was this just a curious mosasaur? she asks. Perhaps a pioneer for its species?
To gather more data, Durings team hopes to return to Hell Creek. Finding the skeleton of this mosasaur would be like winning the jackpot. By finding the entire body, we could see how it adapted, says During.
Indeed, Longrich says, mosasaur bones in the area might have been misidentified before or even ignored. After all, researchers werent looking for them in riverbeds. I cant help but wonder if there are [somewhere, unidentified freshwater mosasaur] teeth and bones sitting in museum drawers.
