Fossils: How They Form and Why They Matter

In short: Fossils are preserved remains or traces of living things from past geological ages, such as petrified bones, shells or footprints. They are an important archive of the history of life on Earth.

In more detail: For a fossil to form, an organism usually has to be covered quickly by sediment before it decays completely. Over long periods, minerals can then replace or take the shape of the original structures.

In detail

• Rare strokes of luck: The vast majority of living things decay without a trace; what survives is mainly hard parts such as bones, teeth, shells and armour.
• Petrification: If remains are quickly buried under mud, sand or volcanic ash, minerals from groundwater can seep into cavities or gradually replace the material.
• Impressions and casts: If an embedded organism dissolves, a cavity may remain as an impression, which later fills with rock (gesteine-und-gesteinskreislauf).
• Trace fossils: Tracks, burrows, feeding marks or fossilised droppings also show how animals moved and fed.
• Exceptional preservation: Amber sometimes encloses insects with fine details intact, and mammoths have been found in permafrost with skin and hair.
• Dating: Deeper rock layers are generally older than the ones above them; together with radiometric measurements, for example of volcanic layers, the age of fossils can be narrowed down well.
• Index fossils: Species that lived for only a short time but were widespread help to match rock layers in different places to the same period.
• Transitional forms: Fossils such as the early bird Archaeopteryx show features of two groups and document how living things have changed over time (evolution-grundlagen, phylogenetik-und-stammbaeume).
• An incomplete archive: Because fossils form only under special conditions, the record is incomplete, and every new find makes the picture more accurate (dinosaurier-und-massenaussterben).