Fossilization is actually a rare occurrence because most components of formerly-living things tend to decompose relatively quickly following death. In order for an organism to be fossilized, the remains normally need to be covered by sediment as soon as possible. However there are exceptions to this, such as if an organism becomes frozen, desiccated, or comes to rest in an anoxic or oxygen-free environment such as at the bottom of a lake. There are several different types of fossils and fossilization processes. Due to the combined effect of taphonomic processes and simple mathematical chance, fossilization tends to favor organisms with hard body parts, those that were widespread, and those that lived for a long time. On the other hand, it is very unusual to find fossils of small, soft bodied, geographically restricted and geologically ephemeral organisms, because of their relative rarity and low likelihood of preservation. Larger specimens or macrofossils are more often observed, dug up and displayed, although microscopic remains also known as the microfossils are actually far more common in the fossil record. Some casual observers have been perplexed by the rarity of transitional species within the fossil record. The conventional explanation for this rarity was given by Darwin, who stated that “the extreme imperfection of the geological record,” combined with the short duration and narrow geographical range of transitional species, made it unlikely that many such fossils would be found. Simply put, the conditions under which fossilization takes place are quite rare; and it is highly unlikely that any given organism will leave behind a fossil. Eldredge and Gould developed their theory of punctuated equilibrium in part to explain the pattern of stasis and sudden appearance in the fossil record.
Permineralization. Permineralization occurs after burial, as the empty spaces within an organism or spaces filled with liquid or gas during life become filled with mineral-rich groundwater and the minerals precipitate from the groundwater, thus occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce very detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decaying process. The degree to which the remains are decayed when covered determines the later details of the fossil. Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. This is a form of diagenesis.
Replacement and compression fossils. In some cases the original remains of the organism have been completely dissolved or otherwise destroyed. When all that is left is an organism-shaped hole in the rock, it is called a mould fossil or typolite. If this hole is later filled with other minerals, it is called a cast fossil and is considered a replacement fossil since the original materials have been completely replaced by new, unrelated ones. In some cases replacement occurs so gradually and at such fine scales that no hole in the rock can ever be discerned and microstructural features are preserved despite the total loss of original material. Compression fossils such as those of fossil ferns are the result of chemical reduction of the complex organic molecules composing the organism’s tissues. In this case the fossil consists of original material, albeit in a geochemically altered state. This chemical change is an expression of diagenesis.
To sum up, fossilization processes proceed differently for different kinds of tissues and under different kinds of conditions.