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Mosasaurs were formidable prehistoric predators that dominated ancient oceans. New evidence, however, indicates that some of these colossal reptiles also inhabited freshwater river systems. This conclusion stems from the discovery of a 66-million-year-old mosasaur tooth found far from any ancient sea, compelling scientists to reconsider the established narrative of this aquatic reptile's lifestyle.
The fossil tooth belongs to the genus Prognathodontini, a group of mosasaurs that could grow up to 11 meters long—approximately the length of a standard telephone pole. These lizard-like creatures first appeared roughly 100 million years ago during the Late Cretaceous period. Their reign, along with that of most dinosaurs, ended abruptly 66 million years ago when a massive asteroid collided with Earth.
While dinosaurs ruled the land, mosasaurs were the apex predators of the water. More closely related to modern lizards and snakes than to dinosaurs, mosasaurs possessed streamlined bodies, shark-like tails, and paddle-shaped limbs that enabled efficient movement through water. These adaptations allowed them to ambush prey with surprising speed.
"With their powerful jaws, this lurking predator could bite through large turtles, fish, and other reptiles, including dinosaurs," explains paleontologist Melanie During of Uppsala University in Sweden. "It was terrifying."
The pivotal discovery occurred in 2022. A team from the North Dakota Geological Survey was excavating fossils in a region known as the Hell Creek Formation, an ancient river floodplain. Within a single, football-shaped rock, they uncovered three distinct fossils: a tooth from a Tyrannosaurus rex, a jawbone from an ancient crocodile, and a single mosasaur tooth.
The mosasaur find was immediately puzzling. Why would a marine reptile be found alongside terrestrial and freshwater animals? "We were already surprised to find a mosasaur tooth in Hell Creek," says During. "We tried everything to prove the tooth was from a marine reptile, but the evidence consistently pointed in a different direction."
To accurately interpret their discovery, the research team employed a sophisticated chemical technique called stable isotope analysis. This method examines the ratios of different isotopes—atoms of the same element with varying numbers of neutrons—within a fossil. These isotopic signatures can reveal details about an animal's life history, including its diet and the environment in which it lived.
The scientists analyzed isotopes of three key elements: oxygen, strontium, and carbon. All fossils from the Hell Creek site were the same age, allowing for direct comparison. Oxygen isotopes are particularly indicative of water type. An animal living in saltwater accumulates a distinct ratio, with more of a heavier oxygen isotope. The mosasaur tooth, however, showed a higher proportion of lighter oxygen isotopes, a pattern consistent with a freshwater habitat. The strontium and carbon isotope data supported this same conclusion.
The collective evidence strongly suggests the mosasaur did not simply wash into a river from the sea. Instead, it likely lived and died in a freshwater environment. This finding, published in the journal BMC Zoology on December 12, 2025, forces a significant reconsideration of mosasaur ecology.
"This is a remarkable example of a species apparently adapting to a new habitat," comments Barry Albright, a paleontologist at the University of North Florida who was not involved in the study. "It was entirely unexpected. These reptiles were long considered to be exclusively marine."
Nicholas Longrich, a paleontologist at the University of Bath in England, notes that mosasaurs exhibited great diversity in the oceans, evolving various jaw shapes, tooth types, and body sizes. "But now, we're seeing them occupy other habitats," he observes. "It indicates they were a diverse and thriving group right up until the asteroid impact."
The presence of such large predators at the top of a food web implies a rich ecosystem with diverse prey. This raises the question: what attracted mosasaurs into freshwater rivers? One compelling hypothesis involves major geological changes during the Late Cretaceous.
At that time, a shallow tropical sea called the Western Interior Seaway bisected the North American continent. This sea was teeming with life, providing abundant food for mosasaurs. As the continent gradually uplifted, this seaway began to shrink and undergo significant changes, including a decrease in salinity. Some populations of mosasaurs may have gradually adapted to these less salty waters, eventually venturing fully into the freshwater channels of places like Hell Creek.
"It's possible that mosasaurs were following prey upriver," suggests Femke Holwerda, a mosasaur expert at Utrecht University in the Netherlands.
Barry Albright offers another perspective: competition. Mosasaurs shared their oceanic realm with other large predators. Adapting to river life may have allowed this group to occupy a new ecological niche with less direct competition for food resources.
This migration from saltwater to freshwater is not without precedent in natural history. For instance, Amazon river dolphins evolved from marine ancestors to live in turbid rivers. Fossils of other ancient marine reptiles have also been discovered in freshwater deposits.
"There is no reason why mosasaurs would have been constrained to only marine environments," states Kiersten Formoso, a vertebrate paleobiologist at Rutgers University. "It would be interesting to find more mosasaur fossils from similar settings. Was this just one curious individual, or was it a pioneer for its species?"
To answer these questions, During's team plans further excavations at the Hell Creek Formation. Discovering a complete skeleton of this freshwater mosasaur would be an extraordinary scientific achievement. "By finding the entire body, we could see precisely how it was adapted for river life," says During.
Longrich speculates that other mosasaur remains from freshwater environments may already reside in museum collections, misidentified or overlooked because researchers did not expect to find them there. "I can't help but wonder if there are teeth and bones sitting in museum drawers somewhere, waiting to be identified as freshwater mosasaurs," he says.
The single tooth from Hell Creek, therefore, represents more than an isolated fossil. It is a key that unlocks a new chapter in our understanding of mosasaur adaptability and the complex ecosystems that existed just before one of Earth's most catastrophic extinction events.