A 66-million-year-old tooth discovered in North Dakota, USA, suggests that some mosasaurs — extinct lizard-like reptiles that could grow up to 12 metres long — may have hunted in rivers as well as seas. The authors suggest that the findings, which are published in BMC Zoology, may represent the first evidence of a mosasaur hunting freshwater prey in Hell Creek at this time.
Melanie During, Nathan Van Vranken, and colleagues examined the tooth after it was discovered in 2022 in the Hell Creek Formation in North Dakota, USA, in a river-like area formerly connected to an ancient sea known as the Western Interior Seaway. They suggest that 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. Previous research has proposed that prognathodontids had bulky skulls and powerful jaws, and hunted in oceans.
The authors analysed isotopes within the tooth's enamel to infer the conditions in which the mosasaur lived and found oxygen and strontium isotope signatures associated with freshwater environments. They suggest this could have been caused by the mosasaur preying on freshwater animals, indicating it was able to live and hunt away from the sea. The authors also found that the tooth showed no signs of being transported, suggesting it lived and died in Hell Creek. They note that no previously discovered mosasaur teeth that date to the same period have been found in this region. Additional analyses on older mosasaur teeth and other animals from the Western Interior Seaway revealed a concentration of isotopes more consistent with a freshwater habit than a seawater habitat. This indicates that salt levels in the region gradually decreased over time.
The authors propose that members of Prognathodontini may have been opportunistic predators occupying a similar niche to modern saltwater crocodiles (Crocodylus porosus) and that they may have adapted to a freshwater environment in response to falling salt levels in the Western Interior Seaway, gradually entering the river channels of Hell Creek as the seaway receded.
