How do fossil layers help scientists read Earth’s history?

Sedimentary layers are deposited over time, so lower layers are usually older than those above them. Fossils within the layers can show when organisms lived and what conditions existed, such as shallow seas, river plains, or deserts. The principle of superposition helps geologists build a relative timeline even before absolute dates are known. Famous fossils like trilobites, ammonites, and dinosaur bones have helped shape the geologic time scale.