Key finding: Isotope analysis of a mosasaur tooth indicates a bus-sized prognathodontine mosasaur inhabited North Dakota rivers about 66 million years ago. Evidence: Oxygen and strontium isotope ratios point to freshwater or brackish conditions, and two additional teeth show similar signatures. Implication: Mosasaurs may have adapted to a freshening Western Interior Seaway during the final million years of the Cretaceous, a shift comparable to modern river dolphins and estuarine crocodiles.
Bus-Sized Mosasaur Roamed North Dakota Rivers About 66 Million Years Ago, Isotope Study Shows
The mosasaur tooth was found in 2022 in the Bismarck Area, North Dakota. Credit: Melanie During
About 66 million years ago, a predator the size of a city bus prowled a prehistoric river where North Dakota now stands. An international team of researchers used chemical clues preserved in tooth enamel to show that a large mosasaur lived in freshwater river systems in the final million years of the Cretaceous.
The tooth studied belongs to a prognathodontine mosasaur, a group of marine reptiles that could reach up to 11 metres in length and functioned as apex predators comparable to the largest killer whales. The specimen was recovered from a fluvial deposit (river sediment) in North Dakota alongside a Tyrannosaurus rex tooth and a crocodylian jawbone — an unusual association of sea, land and freshwater animals dated to roughly 66 million years ago.
To resolve how a typically marine predator ended up in an inland riverbed, researchers at Vrije Universiteit Amsterdam performed detailed isotope analyses on the enamel. The decisive measurements were ratios of oxygen and strontium isotopes, which record the chemistry of the water an animal lived in.
The mosasaur teeth showed a higher proportion of the lighter oxygen isotope than is typical of saltwater-dwelling mosasaurs; combined with strontium isotope ratios, this strongly indicates a freshwater or brackish riverine habitat rather than open ocean. Additional teeth from slightly older North Dakota sites produced similar freshwater signatures, implying this behavior was not a one-off.
Further isotopic evidence suggests this individual did not dive as deeply as many marine relatives and may have hunted unusual prey for a mosasaur — possibly including drowned dinosaurs washed into rivers.
"The isotope signatures point to a riverine habitat for this mosasaur," said Melanie During, a study author. "These analyses show that mosasaurs lived in river environments in the final million years before going extinct."
The authors propose that the shift toward freshwater habitation was driven by large-scale environmental change in the Western Interior Seaway. Increased freshwater influx over time would have transformed the seaway from salty to brackish and eventually to predominantly freshwater — a process similar to conditions in today’s Gulf of Bothnia. The formation of a halocline (a freshwater layer above denser saltwater) is consistent with the isotope data.
This transition — a reversal from marine to freshwater life — is considered less complex than evolving from freshwater into the sea and has modern parallels, including river dolphins (descended from marine ancestors) and estuarine crocodiles (which move between rivers and the open sea).
The study was published in BMC Zoology on December 11.
