fossil

A fossil is the preserved remains or traces of organisms from geological pasts. Fossils are typically found in sedimentary rocks and can include bones, shells, teeth, imprints, footprints, tracks, burrows, or organic residues. They may also consist of preserved seeds, pollen, or organisms trapped in amber, tar, or ice.

Fossils are broadly categorized as body fossils, which preserve parts of the organism such as skeletons or shells, and trace fossils, which preserve evidence of biological activity, such as footprints, burrows, or coprolites. Microfossils, including pollen, foraminifera, and other small remains, are important for detailed dating and environmental reconstructions.

Formation and preservation occur when organisms are buried and protected from decay. Common preservation processes include permineralization (minerals fill the pores), petrifaction (replacement of organic material), replacement and recrystallization, carbonization (thin carbon film), and the formation of molds and casts. Preservation can also occur in amber, ice, or tar, where delicate structures may be retained. Preservation is more likely for organisms with hard parts and in environments that promote rapid burial and low decay.

Dating and interpretation rely on both relative and absolute methods. Relative dating places fossils within a sequence of rock layers, often using index fossils for correlation. Absolute dating uses radiometric techniques (for example, radiocarbon dating for recent materials; other isotopes such as uranium-lead or potassium-argon for older samples). The fossil record is incomplete and biased toward certain environments and organisms, particularly those with hard parts.

Fossils inform studies of evolution, ancient ecosystems, climate history, and biogeography, helping scientists reconstruct past life and its changes over time. Notable examples include diverse body and trace fossils from sites like the Burgess Shale and La Brea Tar Pits.