“King of the Riverside”, a multi-proxy approach offers a new perspective on mosasaurs before their extinction

The discovery of a mosasaurine tooth (NDGS 12217) in the Hell Creek Formation, Morton County, North Dakota, adds to the growing evidence that mosasaurs, traditionally considered marine reptiles, could inhabit freshwater environments. This tooth was found in a context devoid of other marine taxa, suggesting the individual may have ventured into freshwater environments. The morphology of the tooth supports a conservative referral to the tribe Prognathodontini, and stable isotope analyses on carbonate and phosphate fractions of tooth enamel, along with 87Sr/86Sr ratios, indicate a freshwater origin. These findings challenge the notion of mosasaurs as exclusively marine predators, highlighting their potential ecological flexibility during the Late Maastrichtian. This study employs a multi-proxy approach to compare the tooth’s isotopic signatures with other taxa from the same and nearby formations (including teeth of other mosasaurines), providing insights into the environmental transitions of the Western Interior Seaway (WIS) towards the end of the Cretaceous. This comprehensive approach aims to provide deeper insights into the behavior, ecology, and potential migratory patterns of mosasaurs beyond their known marine habitats, enhancing our understanding of their adaptation to rapidly changing environments during the Late Cretaceous.