First condition: rapid burial after death
When an organism dies, if it is left exposed to the air for long, it usually decomposes or is eaten by other animals, so to become a fossil, it must first be buried rapidly in mud or sand. Only an extremely small fraction of organisms that have ever lived on Earth actually become fossils. Immediately after death, the organism must be rapidly buried in an environment low in oxygen and slow in microbial activity β such as a flood, volcanic ash, or riverbed sediment β to avoid decay and decomposition, which then creates the possibility of moving on to the next stage, mineralization.
Permineralization β the best-known method of fossilization
A process in which mineral content dissolved in groundwater seeps into the empty spaces of bone or wood tissue and hardens, preserving the original shape while turning it hard as stone. As mineral-rich groundwater slowly seeps through remains buried underground, silicate or calcium carbonate compounds crystallize, filling the microscopic gaps in the tissue. When this process repeats over an extremely long time, the original organic material is replaced entirely by minerals and disappears, leaving only its shape preserved in stone β petrified wood, where an entire tree turns to stone, is a classic example.
Molds and casts β fossils that preserve only shape
A case in which the organism's body completely decomposes and disappears, leaving only the shape of the empty space it left behind, or the shape of the mineral that later filled that space, preserved as a fossil. When an organism buried in sediment fully dissolves away, it leaves behind a hollow cavity shaped like the original β this is called a mold (impression fossil). When that empty space is later filled and hardened by another mineral, it forms a cast that reproduces the original organism's outer appearance. This process is especially commonly observed in organisms with hard shells, such as clams or trilobites.
Insects preserved in amber
Formed when sticky resin flowing from a tree envelops an insect or other small organism and hardens, this often preserves even fine structures like legs and wings in near-original condition. When a living insect becomes trapped in tree resin and cannot escape before the resin hardens, it can retain its shape almost unchanged over a long period, protected from air and microorganisms. Amber fossils like these are treated as precious material, allowing scientists to observe soft tissue or fine surface structures that are difficult to preserve through other methods.
Fossils preserved whole by freezing or drying
When an organism becomes frozen in permafrost in the polar regions, or when moisture evaporates rapidly in an extremely dry environment, not just the bones but soft tissue like skin and fur can be exceptionally well preserved. Like mammoths found frozen in Siberian permafrost, being trapped in ice halts the activity of decay-causing microorganisms almost entirely, sometimes leaving flesh and fur intact. Conversely, in extremely dry regions like deserts, rapid moisture loss can preserve tissue in a dried, mummy-like state. Both cases reveal far more information than a typical fossil consisting only of bone.
The difference between index fossils and facies fossils
Fossils of organisms that lived widely across a broad area but only for a short period serve as index fossils that reveal the age of a rock layer, while fossils of organisms that lived only in a specific environment serve as facies fossils that reveal the environmental conditions at the time. Index fossils are useful because their wide geographic distribution combined with a short lifespan means that if the same fossil is found in different rock layers, even far apart, those layers can be inferred to have formed around the same time. Facies fossils, on the other hand, are fossils of organisms that could only survive under specific water temperature, depth, or climate conditions, and are used as clues to infer the environment at the time the rock layer formed.
A limitation of the fossil record: preservation bias
Organisms with hard bones or shells are far more likely to become fossils than organisms with only soft bodies, which means the fossil record is an incomplete dataset that shows only part of past ecosystems. Soft-bodied organisms without hard tissue, such as jellyfish or earthworms, are almost never preserved as fossils except under extremely rare conditions. For this reason, scientists always keep in mind that the fossil record does not represent the full diversity of organisms that actually existed in the past, but is instead skewed toward organisms with characteristics favorable to fossilization.
How the age of a fossil is determined
The relative order of rock layers containing a fossil can be used to estimate its relative sequence, while the decay rate of radioactive isotopes in nearby volcanic rock can be used to calculate a more specific absolute age. Since rock layers generally form with older layers at the bottom and newer layers on top, the vertical position of the layer where a fossil is found alone can reveal a rough relative sequence. In addition, using the property that radioactive isotopes in volcanic ash or igneous rock near a fossil-bearing layer decay at a constant rate allows for a far more precise absolute age estimate.