Fossils are the preserved remains of plants of animals. For such remains to considered fossils. Scientists have decided they have to be over 10,000 years old. There are two main types of fossils, body fossils and trace fossils. Body fossils are the preserved remains of a plant or animal's body. Trace fossils are the remains of the activity of animal, such as preserved track ways, footprints fossilized egg shells, and nests.