Fossils are the remains of a once living organism.Fossils are mostly from extinct animals that once lived on earth. Fossils are classified into different categories: mold and cast, true form fossils, trace fossils. Earth's history is split into three eras: Cenozoic, Mesozoic and Mesozoic Era. Scientists use pieces of evidence left from organisms to get a clue of what their ecosystems were like. During the Paleozoic Era was the Cambrian Explosion, also during this period of time there was a huge increase in the diversity of marine animals. The Mesozoic Era is considered the "Age of Reptiles." This era also ended with an extinction but scientists don't know why. Also during the Mesozoic Era, the continents were breaking apart from a supercontinent to their current locations. The Cenozoic era is known as the "Age of Mammals."