A fossil represents the remains of ancient life. It must also be at least 11,000 years old. The one exception is the remains of any extinct species regardless of age.
Most fossils are hard parts such as shells, bone and teeth.
These are by far the most commonly preserved organisms. The group includes brachiopods, trilobites, corals, gastropods, etc. These include dinosaurs, reptiles, sharks, humans and other mammals. They are more rare as fossils than are invertebrates. The most common plant fossils are leaves, seedpods and woody parts.