Oxygen isotopes from the teeth of Cretaceous marine lizards reveal their migration and consumption of freshwater in the Western Interior Seaway, North America

Mosasaurs were widespread predators of the vast Cretaceous seas, and it is debated whether these extinct marine reptiles migrated to coastal environments to feed or reproduce. Here we investigate the potential for migration of mosasaurs through novel high-resolution sclerochronology that samples incremental growth lines in fossil mosasaur teeth and extracts oxygen isotopes from pristine enamel. Oxygen isotope trends of consecutive teeth are spliced to reconstruct one to seven month-long life histories of Platecarpus tympaniticus and Clidastes propython mosasaurs, respectively, collected from time-equivalent chalk deposits of the Western Interior Seaway and Mississippi Embayment of North America. The records of all individuals—two adults and one juvenile—are characterized by semi-regular depletions in oxygen isotope values, indicating travel from marine to freshwater coastal environments. Weekly to bi-weekly consumption of freshwater by two genera at diverse life stages implies that mosasaur osmoregulatory function was similar to their living sea snake relatives that drink freshwater periodically. •We reconstruct palaeoecology of mosasaurs from coeval Kansas and Alabama chalk.•Oxygen isotopes in pristine fossil tooth enamel record changing aquatic habitats.•Two adults and one juvenile show a pattern of bi-weekly migration to freshwater.•Freshwater consumption suggests osmoregulation similar to living sea snakes.