Information about plants and animals that lived long ago can be found in rock layers. They contain the remains of organisms at least 10,000 years old called fossils. These are usually found in sedimentary rocks. Scientists even discovered that rocks with dinosaur remains are older than those with saber-toothed tiger remains. Rocks and fossils also give us clues about Earth’s past. Regions where limestone occurs were most likely once under water. Places where fossil corals are found were once tropical areas covered by shallow sea. Fossil ferns in rocks found in polar regions tell us these areas were warmer than they are today.