Characteristics of Fossils

Frequency

• 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

• Trace fossils indicate an organism̵7;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.

Sizes

• 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.

Location

• Fossils are found in sedimentary rock beds, but not in igneous or metamorphic rock, as the temperature and pressure destroys the organisms.

Preservation

• 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.