Fossils are the remains of prehistoric life, especially those preserved in rock before the end of the last ice age. A fossil might be the animal or plant itself, or the bones or shells of animals (sometimes altered), or the imprint of a plant or animal (for example the mold or cast of a shell). Fossilization, the fossil-making process, turns this plant or animal remains into rock, sometimes by replacing the material with new, harder minerals, in a process called “mineralization.” Generally, if an animal has hard parts it is more likely it is to become a fossil, so things like insects, worms, and other soft-bodied creatures are rarely fossilized. Over all, only a very tiny percentage of species that ever existed on Earth have been fossilized.
Trace fossils are evidence of life recorded in sedimentary rocks; a record of things that happened when an animal or plant was alive. Some examples of trace fossils include footprints, burrows, and even coprolites (fossilized feces)!
Where an animal or plant lived is one of the biggest factors in whether or not it will become a fossil. Things that die on dry, open plains are often rapidly consumed by scavengers, or decay completely, leaving little behind to become a fossil. The plants and animals that die in or near water can be quickly covered by sediments, increasing their chance of becoming a fossil. This is more common in oceans than swamps, and in swamps than rivers, so the fossil record is an incomplete picture of the history of life on Earth, heavily weighted toward marine life. In fact, most animals and plants decompose before they can be fossilized!
Fossils have been found on every continent of Earth and in every geologic age and are found almost exclusively in sedimentary rocks. Why? Igneous rocks are too hot when they form (plants and animals can't live in lava or magma) and metamorphic rocks undergo heat and pressure which generally destroy all fossil evidence. There is one exception – sometimes ash falls from a volcanic eruption kill and fossilize plants and animals that they cover, but only if they have cooled enough not to incinerate them first! Can you find a fossil near you? Look at sedimentary rocks, if you have some nearby, or in building stones, polished counter tops, or even sidewalks for fossil analogs.
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