![\"An](\"https:\/\/cdn.sci.news\/images\/enlarge13\/image_14423e-Hell-Creek-Mosasaur.jpg\")
<\/a><\/p>\nAn artist\u2019s reconstruction of the Hell Creek mosasaur. Image credit: Christopher DiPiazza.<\/p>\n
\u201cMosasaurs were apex marine predators that diversified during the Late Cretaceous epoch, dominating the seas and occupying a variety of marine niches,\u201d said Dr. Melanie During, a paleontologist at Uppsala University and Vrije Universiteit Amsterdam, and her colleagues.<\/p>\n
\u201cWithin Mosasauridae, three subfamilies are recognized: Mosasaurinae, Plioplatecarpinae, and Tylosaurinae, each characterized by distinct morphological adaptations that allowed them to exploit different ecological opportunities.\u201d<\/p>\n
\u201cThough mainly associated with shallow seas, mosasaur remains in estuarine and freshwater environments challenge their traditional strictly marine classification.\u201d<\/p>\n
In 2022, the researchers discovered a large tooth from a mosasaur at a multi-species fossil locality in the Hell Creek Formation.<\/p>\n
Formerly connected to an ancient sea known as the Western Interior Seaway, this locality is notable for its lack of marine species, with fossils dominated by terrestrial and freshwater species.<\/p>\n
The fossil was found in a fluvial deposit, together with a tooth from a Tyrannosaurus rex, and a crocodylian jawbone in an area known for remains of the duck-billed dinosaur Edmontosaurus.<\/p>\n
The specimen belonged to a member of the mosasaur group Prognathodontini<\/a> owing to similarities between textured patterns on its surface and on teeth from other members in this group.<\/p>\n The authors also analyzed isotopes within the tooth\u2019s enamel to infer the conditions in which the mosasaur lived and found oxygen and strontium isotope signatures associated with freshwater environments.<\/p>\n 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.<\/p>\n \u201cCarbon isotopes in teeth generally reflect what the animal ate,\u201d Dr. During said.<\/p>\n \u201cMany mosasaurs have low 13C values because they dive deep.\u201d<\/p>\n \u201cThe mosasaur tooth found with the Tyrannosaurus rex tooth, on the other hand, has a higher 13C value than all known mosasaurs, dinosaurs and crocodiles, suggesting that it did not dive deep and may sometimes have fed on drowned dinosaurs.\u201d<\/p>\n \u201cThe isotope signatures indicated that this mosasaur had inhabited this freshwater riverine environment.\u201d<\/p>\n \u201cWhen we looked at two additional mosasaur teeth found at nearby, slightly older, sites in North Dakota, we saw similar freshwater signatures.\u201d<\/p>\n \u201cThese analyses shows that mosasaurs lived in riverine environments in the final million years before going extinct.\u201d<\/p>\n 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.<\/p>\n This indicates that salt levels in the region gradually decreased over time.<\/p>\n The authors propose that members of Prognathodontini may have been opportunistic predators occupying a similar niche to modern saltwater crocodiles (Crocodylus porosus)<\/a> 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.<\/p>\n \u201cFor comparison with the mosasaur teeth, we also measured fossils from other marine animals and found a clear difference,\u201d said Dr. Per Ahlberg, a paleontologist at Uppsala University.<\/p>\n \u201cAll gill-breathing animals had isotope signatures linking them to brackish or salty water, while all lung-breathing animals lacked such signatures.\u201d<\/p>\n \u201cThis shows that mosasaurs, which needed to come to the surface to breathe, inhabited the upper freshwater layer and not the lower layer where the water was more saline.\u201d<\/p>\n