The Earth as it appeared during the Cretaceous Period (144 to 65 mya). The climate was warm, and dinosaurs like Tyrannosaurus rex and Triceratops ruled Earth. Mammals also lived during the Cretaceous, but they were small. For the first time flowering plants grew. North America was breaking away from the other continents, and Gondwanaland was splitting up as well. The central portion of the United States was covered by a shallow sea that teemed with swimming monsters like Plesiosaurs and Icthyosaurs. Map courtesy of Ronald C. Blakey at NAU.