What Is a Fossil? Body Fossils, Traces and Preservation

A fossil is physical evidence of past life preserved in a geological context. It may be part of an organism, an impression left by it, or a trace of something it did while alive. Fossils are therefore not limited to dinosaur skeletons. Shells, leaves, pollen, footprints, burrows, eggshell, tooth marks and fossilised dung can all preserve different parts of ancient life.

Body fossils preserve organisms

Body fossils are the preserved remains of plants, animals and other organisms. Bones, teeth, shells and woody tissues are common because they contain durable structures. They can be altered after burial: minerals may fill pores, original material may be replaced, or the fossil may retain a complex mixture of original and new substances. A fossil is not always a body that has simply “turned to stone”.

Some preservation pathways retain more detail. Fine-grained sediment can capture a leaf impression or a skin surface. Resin may preserve small organisms as amber. In frozen or unusually oxygen-poor settings, soft tissues can survive longer than they usually would. These are exceptional windows, not the typical fate of a dead animal.

Trace fossils record activity

Trace fossils, also called ichnofossils, record an organism’s activity rather than its body. Footprints and trackways show where an animal touched soft ground; burrows record excavation; borings preserve animals that drilled into shells or rock. Coprolites are fossilised faeces and can preserve fragments of food, although identifying the producer may be difficult. A trace may belong to an animal whose bones have never been found at that site.

Tracks are especially useful because they form at the place an animal moved across a surface. They can record steps, stride length and turns. A trackmaker’s identity is usually less certain than the existence of the track itself. Similar footprints can be made by different animals, and soft mud can distort a print before it is buried. See how palaeontologists read dinosaur trackways and compare those traces with body fossils.

Impressions, moulds and casts

A fossil can preserve shape without preserving the original body. If a shell is buried and later dissolves, the surrounding sediment may retain a hollow mould. If that hollow fills with minerals or sediment, it can form a cast. Compression fossils preserve flattened outlines, sometimes with organic films. The same organism can therefore leave a record of its shape even when its original tissues have disappeared.

Some fossils are microscopic. Pollen grains, spores, tiny shells and mineralised microbial structures can reveal organisms or environments that a field observer would overlook. Chemical fossils and isotope ratios can also preserve evidence associated with ancient biological processes, but they require careful geological and contamination checks before they are attributed to life.

Why most life leaves no fossil

Fossilisation is rare. Scavengers, microbes, weather, currents and chemistry destroy or disperse most remains. Burial can protect a body, but the sediment must later survive erosion and alteration, and someone must eventually find it. Hard parts are more likely to persist than soft tissues, and water-rich settings with rapid burial are often better for preservation than exposed ground.

That bias shapes the fossil record. It contains far fewer organisms than once lived, and some habitats, body types and time intervals are better represented than others. Fossils reveal genuine events in life’s history, but gaps are not evidence that nothing lived there. The complementary article on how fossils form follows the chemical and geological pathways from burial to preservation, while our guide to fossil preparation and study explains how specimens are documented after discovery.