The Cretaceous is a geological period of the Mesozoic Era, spanning 79 million years — from 145 million years ago to 66 million years ago. This period of time had a warm climate and was host to dinosaurs, mammals, birds, flowering plants, and inland seas. The Pierre formation was deposited during the Cretaceous in a shallow marine environment in eastern Colorado. The end of the Cretaceous saw a major extinction event which lead to the extinction of one-third of the plant and animal species on Earth.

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