Platecarpus sp. cf. P. somenensis Class: Reptilia Subclass: Diapsida Infraclass: Lepidosauromorpha Order: Squamata Suborder: Sauria Family: Mosasauridae Age: Late Cretaceous Period (approx. 80 mya) Locality: South Dakota, USA Formation: Pierre Formation Mosasaurs were a group of lizards that evolved to live in the ocean. Their legs and feet became completely modified into flippers, which it is believed they used to steer as they swam in a snake-like fashion. Mosasaurs lived near the shores of a shallow sea, where they used their powerful teeth to prey on fish. Once a tyrant of the ocean, Mosasaurs were driven to extinction at the end of the Cretaceous Period around the same time dinosaurs disappeared from the land.