Plant and animal remains that have been preserved in or imprinted onto rock are called fossils; they can be a few thousand to hundreds of millions of years old.
Fossils are extremely rare, since most organisms decompose before they can become fossilized; the organisms that do survive fossilization usually have hard parts that are resistant to weathering and protected from being crushed.
Trace fossils indicate an organism’s activity at the time when it was alive, providing evidence of movement, like burrows or footprints: a mammoth footprint, for example, is a trace fossil.
Practically every size fossil has been found, from full skeletons of dinosaurs to single-cell organism fossils called foraminifera that are useful in petroleum exploration.
Fossils are found in sedimentary rock beds, but not in igneous or metamorphic rock, as the temperature and pressure destroys the organisms.
There are five ways an organism is fossilized: petrification, when the original tissue is replaced with minerals; carbonization, when all tissues are removed but the carbon remains; mold and cast formation, when rock fills or surrounds an organism causing it to decompose, thereby leaving the rock; recrystallization, which occurs when the organism is replaced by a crystallized copy of itself; and finally, unaltered preservation, which occurs when an organism is preserved in its original state, such as insects trapped in amber.
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