There are a number of different ways fossils are formed. Sometimes the actual remains of plants and animals are preserved. For instance, the frozen remains of a baby Woolly Mammoth were found at Cripple Creek, Alaska. In Florida, shells are often found when digging canals near the coast. Appearing as they do at the beach, these buried shells are actually fossils from a time when the sea covered the area where they were found.
Petrified wood represents another kind of fossil. In the formation of petrified wood, dissolved mineral matter fills in the pores of the wood. Over many years, the wood is replaced, leaving only the mineral matter behind. A similar process occurs in the case of bones and teeth. With bones and teeth, mineral matter fills in the pore spaces, but the bone matter or dental enamel does not fully dissolve. Instead, the mineral matter makes the bone or tooth more durable, and it is thus able to survive for millions of years. In rare cases, whole skeletons are preserved in this way, but usually the bones are found disarticulated, meaning separated from one another.
Another kind of fossil is called a mold. Molds are formed when plants or animals are encased in mud or some other sediment. The sediment hardens and eventually becomes rock. Meanwhile, the organism decays away, leaving behind a mold of itself inside the rock. If some substance fills this mold and hardens inside, a cast can be created. Casts look like the original organism externally, but lack any internal detail.
Similar in appearance to the inside of a mold are imprints and impressions. Imprints are made when an animal or plant is pressed into a clay or mud layer. Often this is how tracks of animals are preserved. Paleontologists can then tell how much an animal weighed by the track depth and how the animal moved by reconstructing its steps.
A number of conditions are necessary for fossils to form. Since scavengers, the elements, bacteria, and fungi quickly destroy or decay the remains of living things; the decay process must be slowed for fossils to form. Hard parts like bones, teeth, and shells decay more slowly than wood, flesh, or other soft tissue so most fossils come from organisms that had hard parts. They are most commonly found in layers of sedimentary rock formed from mud and silt at the bottom of lakes and shallow seas because plants and animals that are buried quickly decay more slowly.
Source: Florida Industrial and Phosphate Research Institute