Because fossils are preserved pieces of the past they can tell us what animals and plants used to live in Utah and what the climate was like in the past. For example, coral, trilobites and fish fossils that hint that there was once a sea or ocean covering the great basin area. Seashells along the mountains tell us that area may have once been the beach of a sea, ocean or large lake. Petrified leaves and wood tell us that there were tropical plants in some areas of Utah. Coal deposits tell us that part of the eastern part of Utah was once swampy.