The Cretaceous is a geological period of the Mesozoic Era, spanning 79 million years – from 145 million to 66 million years ago. This period had a warm climate and the region was host to dinosaurs, mammals, flowering plants, and <a href=””>inland seas</a>. The Pierre formation was deposited during the Cretaceous in a shallow marine environment in eastern Colorado. Fossilized marine dinosaurs such as the mosasaur and plesiosaur have been found in Cretaceous rocks in the Western Slopes and eastern foothills. The end of the Cretaceous period saw a major extinction event which lead to the extinction of one-third of the plant and animal species on Earth. The period ended with the Cretaceous-Paleogene (K-T) extinction event in which a massive asteroid was believed to collide with the earth, killing 75 percent of all species. The K-T boundary can be seen in Trinidad State Park in the Raton Basin of Colorado.