A giant ocean predator that terrorised the seas during the time of the dinosaurs may have also hunted in rivers, a tooth fossil discovered in North Dakota suggests.

The extinct lizard-like reptile grew up to 12 metres (40 ft) in length and may have occupied a similar niche to modern-day saltwater crocodiles, say researchers from Uppsala University in Sweden.

Until now, these terrifying giant aquatic reptiles with a bulky skull and powerful jaws were thought to be sea-dwelling predators, exclusively hunting in the oceans.

Reconstruction of Hell Creek Mosasaur (Christopher DiPiazza)

Now, a 66-million-year-old mosasaur tooth fossil has been found in the Hell Creek Formation in North Dakota that bears signs of exposure to river water.

The finding “adds to the growing evidence that mosasaurs, traditionally considered marine reptiles, could inhabit freshwater environments”, scientists say.

Researchers dub the species the “King of the Riverside” in the new study published in the journal BMC Zoology.

In the latest study, scientists found chemical signatures in the fossil tooth suggesting the mosasaur spent some time in freshwater.

Scientists suspect the site where the tooth was found was once likely a river area connected to an ancient sea known as the Western Interior Seaway.

Mosasaur tooth found in Hell Creek (Trissa Shaw)

Analysis of the tooth also suggests it belongs to a member of the mosasaur group Prognathodontini owing to similarities between textured patterns on its surface and on teeth from other members in this group.

Researchers found variants of oxygen and strontium in the fossil tooth, which are elements linked to freshwater environments.

This could be indicative of the mosasaur preying on freshwater animals and being able to live and hunt away from the sea, they say.

The fossil tooth also showed no signs of being transported, meaning the mosasaur may have lived and died in Hell Creek.

(L) Different sections of the mosasaur tooth, and (R) the tooth photographed in situ next to a t. rex tooth (Melanie During)

Until now, no other mosasaur teeth dating to the same period have been found in this region.

Studies of older mosasaur teeth and other animals from the Western Interior Seaway region point to a freshwater habitat rather than a seawater habitat, hinting that salt levels in the region gradually decreased over time.

Scientists suspect mosasaurs in the region may have adapted to a freshwater environment in response to falling salt levels in the Western Interior Seaway, and gradually entered the river channels of Hell Creek.

“This adaptation may indicate that large rivers of the Hell Creek Formation paleoenvironment could support large-bodied taxa, despite it being more likely for younger, smaller individuals to exploit these nearshore to riverine habitats,” they wrote in the study.

“This adaptability may have been a key factor in their ability to thrive in various ecological niches during the Late Cretaceous,” researchers concluded.