🦕 Mosasaurs, the Cretaceous ocean monsters, also lived in rivers

Giant reptiles, masters of the oceans more than 66 million years ago, swimming in rivers? A recent discovery in North America raises this surprising hypothesis and changes our perception of mosasaurs.

In 2022, paleontologists uncovered a mosasaur tooth in North Dakota, in fluvial sediments. Curiously, it lay near a Tyrannosaurus rex tooth and a crocodilian jaw fragment, fossils typical of terrestrial and freshwater environments. The presence of such a marine reptile in such an environment immediately caught the attention of scientists, raising many questions about its lifestyle.


To clarify this, an international team examined the chemical composition of the tooth enamel. Their approach uses isotope analysis, atoms that carry signatures related to the environment and diet. This work was carried out in collaboration with institutions such as Uppsala University and Vrije Universiteit Amsterdam.

The measurements focused on oxygen, strontium, and carbon isotopes. The data indicate high levels of light oxygen, a marker of freshwater, and strontium ratios compatible with a river habitat. Moreover, the carbon signature differs from that of oceanic mosasaurs, showing surface feeding rather than deep feeding.

At that time, the vast inland sea that crossed North America was undergoing a gradual transformation. The increasing influx of freshwater altered its salinity, creating a mixing zone where a layer of freshwater floated above saltier waters. The isotopic results confirm that mosasaurs, breathing with lungs, occupied this upper layer.

This flexibility is not unique in evolutionary history. Modern animals, such as river dolphins or saltwater crocodiles, show comparable colonization of different habitats. For mosasaurs, the transition from saltwater to freshwater appears to have been a fairly straightforward adjustment, facilitated by changes in their environment.


The size of the tooth suggests an animal that could measure up to 36 feet (11 meters) long, similar to a modern bus. A hunter of this size in ancient rivers opens a new window into the final moments of mosasaurs before their extinction.