The Cretaceous period refers to the final period of the Mesozoic Era and is often associated with the extinction of the dinosaurs. Synonyms for the Cretaceous period include the "Cretaceous System," "Upper Cretaceous," and "Late Cretaceous." It was a time when the Earth's climate was warm and there was a high sea level, allowing for marine life to thrive. The Cretaceous period saw the rise of new species of plants and animals, including birds and mammals. The Cretaceous period is also important for the discovery of valuable resources such as oil and gas. Despite being millions of years in the past, the legacy of the Cretaceous period can still be seen in the world around us.