“Mosasaurs were peak marine predators that diversified throughout the Late Cretaceous date, controling the seas and inhabiting a range of marine specific niches,” stated Dr. Melanie During, a paleontologist at Uppsala University and Vrije Universiteit Amsterdam, and her coworkers.

“Within Mosasauridae, 3 subfamilies are acknowledged: Mosasaurinae, Plioplatecarpinae, and Tylosaurinae, each defined by unique morphological adjustments that enabled them to make use of various eco-friendly chances.”

“Though generally connected with shallow seas, mosasaur stays in estuarine and freshwater environments challenge their conventional strictly marine category.”

In 2022, the scientists found a big tooth from a mosasaur at a multi-species fossil area in the Hell Creek Formation.

Previously linked to an ancient sea called the Western Interior Seaway, this region is significant for its absence of marine types, with fossils controlled by terrestrial and freshwater types.

The fossil was discovered in a fluvial deposit, together with a tooth from a Tyrannosaurus rexand a crocodylian jawbone in a location understood for remains of the duck-billed dinosaur Edmontosaurus

The specimen came from a member of the mosasaur group Prognathodontini owing to resemblances in between textured patterns on its surface area and on teeth from other members in this group.

The authors likewise examined isotopes within the tooth’s enamel to presume the conditions in which the mosasaur lived and discovered oxygen and strontium isotope signatures connected with freshwater environments.

They recommend this might have been triggered by the mosasaur taking advantage of freshwater animals, showing it had the ability to live and hunt far from the sea.

“Carbon isotopes in teeth usually show what the animal consumed,” Dr. During stated.

“Many mosasaurs have low ¹³C worths due to the fact that they dive deep.”

“The mosasaur tooth discovered with the Tyrannosaurus rex tooth, on the other hand, has a greater ¹³C worth than all understood mosasaurs, dinosaurs and crocodiles, recommending that it did not dive deep and might in some cases have actually fed upon drowned dinosaurs.”

“The isotope signatures showed that this mosasaur had actually populated this freshwater riverine environment.”

“When we took a look at 2 extra mosasaur teeth discovered at close by, somewhat older, websites in North Dakota, we saw comparable freshwater signatures.”

“These analyses reveals that mosasaurs resided in riverine environments in the last million years before going extinct.”

Extra analyses on older mosasaur teeth and other animals from the Western Interior Seaway exposed a concentration of isotopes more constant with a freshwater routine than a seawater environment.

This shows that salt levels in the area slowly reduced in time.

The authors propose that members of Prognathodontini might have been opportunistic predators inhabiting a comparable specific niche to modern-day saltwater crocodiles (Crocodylus porosuswhich they might have adjusted to a freshwater environment in action to falling salt levels in the Western Interior Seaway, slowly going into the river channels of Hell Creek as the seaway declined.

“For contrast with the mosasaur teeth, we likewise determined fossils from other marine animals and discovered a clear distinction,” stated Dr. Per Ahlberg, a paleontologist at Uppsala University.

“All gill-breathing animals had isotope signatures connecting them to brackish or salted water, while all lung-breathing animals did not have such signatures.”

“This reveals that mosasaurs, which required to come to the surface area to breathe, lived in the upper freshwater layer and not the lower layer where the water was more saline.”

The group’s paper was released December 12, 2025 in the journal BMC Zoology

_____

M.A.D. During et al2025. ‘King of the Riverside,’ a multi-proxy method uses a brand-new viewpoint on mosasaurs before their termination. BMC Zool 10, 25; doi: 10.1186/ s40850-025-00246-y