A Marine Predator the Size of a Bus Patrolled Ancient Rivers 66 Million Years Ago

For decades, mosasaurs were considered one of the dominant predators of the Cretaceous seas – marine reptiles growing up to 11 meters long and patrolling the waters of the Western Interior Seaway. But a new analysis of a tooth from North Dakota suggests that at least one lineage did not remain confined to the sea.

Reported in Zoology BMCThe results indicate that this predator ventured upstream, navigating the river channels that flowed through the Hell Creek region more than 66 million years ago. The tooth, discovered in 2022 in an ancient river deposit, was found with a T. rex tooth and part of crocodilian jaw in a region where Edmontosaurus bones were also common – an unlikely combination that hinted at a much more complex ecosystem than scientists expected.

“Isotopic signatures indicated that this mosasaur had inhabited this freshwater river environment. When we examined two additional mosasaur teeth found at nearby, slightly older sites in North Dakota, we saw similar freshwater signatures. These analyzes show that mosasaurs lived in riverine environments for the last million years before going extinct,” study co-author Melanie Durant said in a statement from press.

Freshwater signatures rewrite mosasaur habitats

To understand how a marine reptile ended up in a river deposit, the team compared the mosasaur tooth with nearby fossils of the same age. At the Vrije Universiteit Amsterdam, they analyzed isotopes of oxygen, strontium and carbon preserved in enamel, a method that can reveal both habitat and feeding behavior.

The results clearly pointed far offshore. The tooth contained high levels of the lighter isotope of oxygen ¹⁶O, and its strontium ratios matched freshwater conditions rather than those of the sea. Together, they suggest an animal living primarily in rivers.

“The carbon isotopes in the teeth generally reflect what the animal ate. Many mosasaurs have low ¹³C values ​​because they dive deep. The mosasaur tooth found with the T. rex The tooth, on the other hand, has a higher ¹³C value than all known mosasaurs, dinosaurs and crocodiles, suggesting that it did not dive deeply and may have sometimes fed on drowned dinosaurs,” Durant said.

There was no evidence that the tooth had been carried away by distant marine sediments, implying that the animal’s life – and death – played out in the river itself. Similar freshwater signatures in two slightly older teeth in the region indicate a recurring presence rather than a single wandering one, suggesting that some mosasaurs used these rivers in the latter part of the Cretaceous.

Learn more: The end of the dinosaurs: what was the mass extinction at the end of the Cretaceous?

Why changing waters have opened new habitats for giant mosasaurs

The freshwater signal in the tooth aligns with larger environmental changes that reshaped North America during the Late Cretaceous. As the Western Interior Seaway retreated, rivers discharged increasing amounts of fresh water into its basin, creating a surface layer that gradually lost salinity. Comparisons with nearby fossils show that gill breathers are tied to brackish water, while lung breathers – including mosasaurs – lack these signatures, indicating a reliance on the surface freshwater layer.

In this changing environment, it would have been difficult to overlook a predator the size of the Hell Creek mosasaur. The tooth indicates an animal about 11 meters long – about the size of a city bus – and probably part of the animal. Prognathodontinia group with large heads, sturdy jaws and a reputation for opportunistic hunting.

“Its size means the animal would rival larger killer whales, making it an extraordinary predator to encounter in riverine environments not previously associated with such giant marine reptiles,” said Per Ahlberg, co-author of the study.

Together, the chemical evidence and environmental context suggest that as the seaway receded, some mosasaurs adapted to it, following newly formed rivers and exploiting habitats long thought to be beyond their reach.

Learn more: A Real Dragon: How a Newly Classified Mosasaur Got Its Legendary Name

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