The term fossil, which comes from the Latin fossil (that which is excavated), refers to remains of organisms that lived long ago and are preserved petrified in sedimentary rocks of the earth’s crust. The set of all the fossils on the planet, including the undiscovered ones, is known as the fossil record and is studied by paleontology.
Fossils can be divided into two large groups:
- corporeal fossils: formed by the body of the organism, by fragments or by molds that the body has left in the sedimentary rock.
- ichnofossils: formed by signals of the activity of the organism. For example footprints, nests, feces, etc.
The process of fossil formation, or fossilization, is studied by the branch of Paleontology called taphonomy. It is a rare phenomenon in nature, it is normal for the remains of living beings to disappear completely after their death. However, if the right conditions are met, they can be preserved for millions of years and reach our days, becoming an incredible source of information to learn about the history of life.
In very general terms, fossilization consists of a series of physical, chemical and biological transformations that occur in the dead organism, its remains or signs in the environment, until it is petrified and integrated into the sedimentary rock of the earth’s crust.
The entire fossilization process can be divided into two large taphonomic phasesthe biostratinomic phasewhich encompasses everything that occurs from the moment a trace that could become a fossil is generated until he is buriedand the fossil diagenetic phasewhich covers the processes from which the rest is buried until it is found (not just until it becomes a fossil).
Biostratinomy is the branch of taphonomy specialized in the study of the processes that occur from the time any remains that could become a fossil are generated until their taphonomic accumulation, that is, until they become part of the lithosphere by burial. This phase may be absent in organisms that live buried during some part of their life cycle.
The rest likely to become a fossil can be a dead organism or some organic remains of it, for example faeces, as well as any mark in the external environment that it has left, for example a footprint.
In the case of dead organisms, some authors incorporate the biostratinomic phase into the necrobiosiswhich are the processes of agony that produce the death of the organism and that would be prior to the generation of the rest pro-fossil in itself.
The most notable changes during the biostratinomic phase are:
- Physical changes: for example fragmentation, disarticulation, necrokinesis (post-mortem transport by natural means such as wind or water currents). Physical changes are also considered freezing, encapsulation in resins, etc.
- chemical changes: includes early changes at the chemical level. Oxidation and dissolution are the most prominent.
- biological changes: includes changes in the rest itself, such as putrefaction and decomposition, but also changes in the external environment such as bioerosion or bioturbation. The autolithificationa process of mineralization and sealing by microbial activity, seems to have been very important in some very well preserved fossils and is probably more common in the biostratinomic phase, although it could also occur in the fossil diagenetic phase.
Most soft organic remains are completely destroyed during the biostratinomic phase and only hard and mineralized parts remain, such as skeletons and shells, which are finally buried and integrated into the substrate, which is known as taphonomic accumulation.
It is important to clarify that taphonomic accumulation does not imply piling up or adding matter, but rather refers specifically to the information transfer to the lithosphere. One of the clearest examples of taphonomic accumulation without the accumulation of matter would be in the formation of fossil footprints.
Once taphonomic accumulation has occurred, the fossil diagenetic phase is entered. The first processes that occur in this phase are shared with the biostratinomic phase, for example decomposition or dissolution. But other new processes begin and, given the right conditions, can cause the rest susceptible to becoming fossil eventually becomes one and is petrified and embedded in the lithosphere.
These changes occur together with diagenesis, which the process of sedimentary rock formation, hence this phase is known as fossildiagenesis. The changes, alterations and fossil diagenetic processes are very varied, from compaction and mineralization to deformations due to tectonic action. Among the most common are the following:
Permineralization, or petrification, is one of the most common methods of fossil formation. It occurs when spaces previously filled with liquid or gas are flooded by mineral-rich groundwater. Subsequently, the minerals precipitate and occupy these spaces.
For permineralization to occur, the organism must be buried quickly after death or in the early stages of decomposition. The degree of decomposition determines the degree of subsequent permineralization
Permineralization can occur in very small spaces, for example in plant cell walls. This small scale allows highly detailed plant fossils to be produced. Some fossils preserve only hard parts such as teeth and bones, while others may retain skin, feathers, and even soft tissues.
The high pressures and temperatures to which a buried organic remains can be subjected can cause it to release hydrogen and oxygen from the tissue leaving a carbon residue. Charring can leave very detailed impressions of the organism in the sedimentary rock that forms later.
In some cases, the remains of the organism are completely destroyed, usually by dissolution, but their form remains in the rock forming what is known as external mold. If the hole left by the organism is filled by other minerals, a countermold is formed, known as cast. If the minerals fill a cavity of the organism, for example a skull, an internal mold is formed, better known as endocast.
Sometimes, the organism itself or its remains act as a nucleus for the precipitation of minerals. Around the body a thin mineral layer is formed and when the body disappears an external mold remains that can preserve small morphological details if it occurs shortly after death. This phenomenon is known as authigenic mineralization or cementation.
Replacement and recrystallization
Replacement occurs when molecules in shells, bones, or other mineral tissues are replaced by a different mineral. The replacement can be so gradual and slow that microstructural features are preserved even though the original composition has been lost.
For its part, recrystallization takes place when the composition of the tissues remains constant but the minerals crystallize in a different way than they do in the original structure; for example, calcium carbonate crystallizes as aragonite in the shells of mollusks and in the skeleton of corals and can recrystallize as calcite, more common in sedimentary rocks.
Compression fossils occur when the fossil is deposited on soft sediments, for example sand, and is then covered by another layer of sediment and a rock is formed by compression. Plant compression fossils are much more common than animal compression fossils and generally appear deformed by compression.
Impression fossils are similar to compression fossils but are produced when the organism deposits or leaves a mark on mud or mud that will later form a rock.
Both types are common in places where there is water and fine sediment deposition, such as river and lake deltas.
Inclusion fossils are those preserved included within some substance. The best known example is fossil preservation in amber and those preserved in ice (cryofossilization).
Bioimaging is a type of fossilization in which an organism, usually soft, is covered by another hard organism. In the fossil of the hard organism a kind of external molds left by the soft organisms are preserved. Most commonly, this type of fossilization occurs in sclerobionts or encrusting organisms.
Conservation of organic material
In addition to minerals, There are fossils that contain organic material.. For example, fossils of millions of years old dinosaurs have been found with preserved soft tissue, including blood vessels.
Exactly how organic matter can be preserved for so long in a petrified fossil is unknown. One of the possible mechanisms is iron chelation under reducing conditions (of little oxygen), which would help the formation of cross-links between proteins or other macromolecules such as DNA. These cross-links would make the bacteria that feed on these substances unable to recognize them.
Another possible mechanism is the bacterial autolithification. In this case, the very bacteria that initially feed on the dead organism secrete minerals as a waste product, and these minerals hermetically seal the rest of the organism.
In any case, the exceptional conservation of organic remains in fossils is a source of incalculable value to learn about the history of life.