Fossil

A fossil is the preserved remains or traces of an organism that lived in the past, kept in a geological setting such as rock or sediment. Body fossils preserve part of the organism itself, such as bones, teeth, shells, wood or leaves, or an impression or mold of them. Trace fossils record activity instead: footprints, burrows, trails, nests and fossilized dung, called coprolites.

Fossilization is rare, because most dead organisms are eaten or decay. The best chance comes when remains are buried quickly in sand, mud or volcanic ash, which slows decay by keeping oxygen out. Over long periods, groundwater can deposit minerals in the pores of bone or wood, a process called permineralization, or replace the original material entirely. Sometimes the buried remains dissolve and leave a hollow mold, which later fills to form a cast. Hard parts survive far more often than soft tissue, so the fossil record favors animals with shells and bones, especially those that lived in water.

Fossils are found mostly in sedimentary rocks. They show how life has changed over time, provide evidence for evolution and extinction, and help geologists date and match rock layers.