Most animals that became fossils either lived in water or were washed into a body of water. After an animal died, its soft parts, such as its fur, skin, muscle and organs, decomposed. The hard parts that remained were buried under moist layers of mud or sand, where there was no oxygen and bacteria to cause them to decay. The sediment that covered the bones eventually turned into solid rock. Over the course of millions of years, minerals in the surrounding rock partly or completely replaced the original animal material, forming a fossil. Sometimes, water seeped into the rocks and dissolved the animal remains. When this happened, the outline of the fossil remained intact between the layers of rock, leaving a fossil in the form of a natural mold. Paleontologists, scientists who study dinosaurs, use the fossils to learn about the creatures who roamed Earth millions of years ago.