How do Fossils Form?
Most organisms die and decay to leave no remains at all, but on very rare occasions a dead organism can become fossilized. The most important step to become a fossil is to get buried quickly after death. Once buried, water trickling through the ground penetrates the shell or bone, and the minerals in that ground water can begin replacing the original shell or bone. After a very long time, the shell or bone is turned to stone, making a fossil. Of course, some more recent fossils—such as the frozen mammoths found in Siberia—are not altered but contain the original bone and tissue.
They form when plant or animal remains are covered by sediment, which then turns into stone. The best-known fossils are bones, which absorb minerals from the sediment and turn into rock that differs from the surrounding matrix. Sometimes a dead animal is buried between layers of rock, which form an impression or cast that can be split apart along the layer containing the bones, producing a part and counterpart. In a few cases, the casts include impressions of feathers or skin that decayed long ago. Footprints can also become fossilised if the mud where they form is buried and turns to rock. Fossilisation requires burial by accumulating sediment, so it is much more likely in the sea, a stream, or a lake than on dry land.
