Fossil

A fossil is the preserved remains, impression or trace of an organism from a past geological age, commonly held to be at least ten thousand years old, though the oldest known fossils range from about 3.48 to 4.1 billion years old. Body fossils are direct remains of an organism itself, such as bones, shells, exoskeletons or petrified wood, while trace fossils record an organism's activity rather than its body, such as footprints or fossilized droppings known as coprolites. Fossils form through several distinct processes: permineralization, in which mineral rich groundwater fills the empty spaces in buried remains; replacement, in which the original material is gradually swapped for a different mineral while microstructure survives; carbonization, in which organic matter reduces to a thin film of carbon; and silicification and phosphatization, in which silica or calcium phosphate minerals respectively replace the original material.