Some 66 million years ago, a city bus-sized terrifying predator prowled a prehistoric river in what is now North Dakota. 

This finding is based on the analysis of a single mosasaur tooth conducted by an international team of researchers from the United States, Sweden, and the Netherlands. 

The tooth came from a prognathodontine mosasaur — a reptile reaching up to 11 meters long. This makes it an apex predator on par with the largest killer whales.

It shows that massive mosasaurs successfully adapted to life in rivers right up until their extinction.

Isotope analysis

Dating from 98 to 66 million years ago, abundant mosasaur fossils have been uncovered in marine deposits across North America, Europe, and Africa.

However, these marine reptile fossils have been rarely found in North Dakota before. 

In this new study, the large mosasaur tooth was unearthed in a fluvial deposit (river sediment) in North Dakota. 

Its neighbors in the dirt were just as compelling: a tooth from a Tyrannosaurus rex and a crocodylian jawbone. Interestingly, all these fossilized remains came from a similar age, around 66 million years old. 

This unusual gathering — sea monster, land dinosaur, and river croc — raised an intriguing question: If the mosasaur was a sea creature, how did its remains end up in an inland river?

The answer lay in the chemistry of the tooth enamel. Using advanced isotope analysis at the Vrije Universiteit in Amsterdam, the team compared the chemical composition of the mosasaur tooth with its neighbors.

The key was the ratio of oxygen isotopes. 

The mosasaur teeth contained a higher proportion of the lighter oxygen isotope than is typical for mosasaurs living in saltwater. This specific isotopic signature, along with the strontium isotope ratio, strongly suggests that the mosasaur lived in a freshwater habitat.

Analysis also revealed that the mosasaur did not dive as deep as many of its marine relatives and may have fed on unusual prey, such as drowned dinosaurs. 

The isotope signatures indicated that this mosasaur had inhabited this freshwater riverine environment. When we looked at two additional mosasaur teeth found nearby, slightly older sites in North Dakota, we saw similar freshwater signatures. These analyses show that mosasaurs lived in riverine environments in the final million years before going extinct,” explained Melanie During, the study author.

Transformation of the Seaway

The adaptation occurred during the final million years of the Cretaceous period.

It is hypothesized that the mosasaurs were adapting to an enormous environmental shift in the Western Interior Seaway, the vast inland sea that once divided North America.

Increased freshwater influx gradually transformed the ancient sea from saltwater to brackish water, and finally to mostly freshwater, similar to the modern Gulf of Bothnia. 

The researchers hypothesize that this change led to the formation of a halocline: a structure where a lighter layer of freshwater rested atop heavier saltwater. The findings of the isotope analyses directly support this theory.

The analyzed mosasaur teeth belong to individuals who successfully adapted to the shifting environments. 

This transition from marine to freshwater habitats (reverse adaptation) is considered less complex than the opposite shift and is not unique among large predators. 

Modern parallels include river dolphins, which evolved from marine ancestors but now thrive in freshwater, and the estuarine crocodile, which moves freely between freshwater rivers and the open sea for hunting.

Findings were published in the journal BMC Zoology on December 11.