Fossils are preserved remains, impressions, traces, or other evidence of ancient organisms that become incorporated into geological materials and survive long enough to be recognised in the rock record. Fossilisation is uncommon because most organisms are destroyed by scavenging, decomposition, physical erosion, chemical dissolution, or biological activity before burial. The most important general requirement for preservation is relatively rapid burial, because sediment covering an organism can isolate it from oxygen, scavengers, currents, and many decomposers.
Burial commonly occurs in environments such as river floodplains, lakes, deltas, beaches, shallow seas, volcanic ash deposits, and caves. Sediment accumulates around the remains and eventually becomes compacted and cemented into sedimentary rock. Fossils can also occur in some volcanic or metamorphic settings, although intense heat, pressure, deformation, and chemical alteration commonly destroy original biological structures. The fossil record therefore represents a strongly biased sample of past life, favouring organisms and environments in which preservation was physically and chemically possible.
The most familiar form of fossil preservation involves hard biological structures such as bones, teeth, shells, coral skeletons, wood, and some plant tissues. Soft tissues are usually rapidly decomposed, but under exceptional conditions they can be preserved when decay is slowed dramatically. Low oxygen environments, very fine sediment, rapid burial, freezing, drying, resin entombment, and chemical environments that inhibit microbial activity can all contribute to exceptional preservation.
An organism may therefore be preserved as an essentially unchanged body, although such cases are rare. More commonly, the original biological material is altered or replaced by minerals. During burial, groundwater can move through sediment and carry dissolved ions into pores and cavities within remains. Minerals may precipitate inside these spaces, producing a three dimensional mineral replica. This process can preserve detailed anatomical features even when much of the original organic material has disappeared.
Permineralisation is one of the major processes responsible for fossil preservation. In permineralisation, mineral-rich water enters the microscopic pores and cavities of buried bones, wood, shells, or other structures, and minerals precipitate within those spaces. The original framework may remain while additional mineral material strengthens and stabilises it. Petrification is a related term often used for extensive mineral replacement or mineralisation, especially in fossil wood.
Replacement occurs when the original material is progressively dissolved and its space is occupied by newly precipitated minerals such as silica, calcite, or pyrite. Recrystallisation can alter the internal structure of a shell or skeleton without necessarily replacing it completely. These processes can preserve shape, microscopic textures, and sometimes cellular details. The chemical composition of groundwater, temperature, pressure, acidity, and availability of dissolved ions strongly influence which minerals participate in fossilisation.
Moulds and casts preserve organisms even when the original body material disappears. If a shell or other hard structure becomes buried and later dissolves, it can leave a cavity in the surrounding sediment. This cavity is called an external mould when it records the outside shape and surface features of the organism. If sediment or minerals subsequently fill the cavity, the resulting object is a cast.
Internal moulds can form when sediment fills the inside of a shell and later remains after the shell dissolves, preserving the internal geometry of the organism. These processes are especially important for marine invertebrates with shells, because shells commonly become buried in sediment and can leave detailed impressions. Trace fossils are preserved differently because they record biological activity rather than the body itself. Footprints, burrows, feeding marks, nests, trails, borings, coprolites, and root traces can become buried and lithified, providing evidence of behaviour, movement, feeding, and ecological interactions.
Fossils are most commonly preserved in sedimentary rocks because sedimentary environments provide the burial conditions required for long-term preservation. Mudstone and shale can preserve delicate impressions because fine particles settle around organisms and can reproduce small surface details. Sandstone can preserve tracks, burrows, wood, and other robust remains when burial and cementation occur rapidly enough. Limestone is particularly important for marine fossils because it can form from accumulated shells, skeletal fragments, carbonate mud, and chemically precipitated calcium carbonate.
Fossils can also occur in conglomerates, although physical abrasion during transport can damage or fragment remains. Igneous rocks generally contain very few fossils because molten magma and lava reach temperatures capable of destroying biological material. Metamorphic rocks may preserve distorted or recrystallised fossils when temperatures and pressures are not sufficiently extreme to erase all biological structures, but increasing metamorphic grade generally reduces fossil preservation.
Fossil preservation continues after burial through the processes that transform loose sediment into rock and alter the buried remains. Compaction reduces pore space as layers of sediment become compressed by overlying material. Cementation occurs when minerals precipitate from groundwater and bind sediment grains together. These processes constitute lithification and can convert fossil-bearing sediment into solid rock. With deeper burial, increasing temperature and pressure can cause additional chemical and structural changes.
Diagenesis can modify both the surrounding sediment and the fossil, sometimes improving preservation by mineralising the remains and sometimes destroying microscopic or chemical information. Fossils can later be exposed at the surface when tectonic uplift, erosion, faulting, or changes in sea level bring fossil-bearing strata within reach of weathering. Paleontologists therefore often find fossils in naturally eroded cliffs, river cuts, quarries, deserts, and coastal exposures.
Fossilisation can preserve much more than the visible shape of an organism. Chemical residues, microscopic structures, isotopic compositions, growth rings, pollen, spores, and molecular fragments can sometimes survive under favourable conditions. Such evidence can reveal diet, climate, age, growth rate, migration, environmental chemistry, and relationships among ancient organisms. Fossil footprints can provide information about locomotion and body size, while coprolites can reveal aspects of diet and digestive ecology.
Some exceptionally preserved deposits contain organisms with soft tissues, feathers, skin impressions, or gut contents, allowing scientists to reconstruct anatomy and behaviour with unusual precision. Amber can preserve insects and other small organisms in three dimensional form because resin can entomb them rapidly and isolate them from many decompositional processes. Frozen remains in permanently cold environments can preserve tissues for very long periods, although this type of preservation is distinct from conventional fossilisation in sedimentary rock.
Fossils are therefore geological records produced by a combination of biological, environmental, chemical, and geological processes rather than simply dead organisms that have turned into stones. Preservation requires a favourable sequence of events, commonly beginning with death, followed by burial, reduced destruction, chemical alteration or mineralisation, lithification, geological stability, and eventual exposure. The likelihood of preservation varies greatly among organisms, with hard parts generally having much higher preservation potential than soft tissues.
Marine organisms are often well represented because large areas of the seafloor receive continuous sediment deposition, while terrestrial organisms are frequently underrepresented because exposed environments promote decay and erosion. Fossilisation can also be incomplete, producing fragmented, compressed, distorted, mineralised, moulded, or chemically altered remains. Understanding these preservation biases is essential when interpreting biodiversity through geological time, because absence from the fossil record does not necessarily mean that an organism never existed.
