Mosasaurs were apex predators of the Late Cretaceous seas closely related to modern lizards, like the Komodo dragon. During the Late Cretaceous, much of North Dakota was covered by a warm, shallow sea. The Pierre Shale formation, deposited in that sea, preserves some of the last mosasaurs that lived before their extinction, but North Dakota’s record is unstudied. This project will clarify what mosasaur species lived in North Dakota and what these fossils tell us about this ancient ecosystem.