Fossils (from Classical Latin fossilis; literally, "obtained by digging") [1] are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers ( strata) is known as the fossil record.

The study of fossils across geological time, how they were formed, and the evolutionary relationships between taxa ( phylogeny) are some of the most important functions of the science of paleontology. Such a preserved specimen is called a "fossil" if it is older than some minimum age, most often the arbitrary date of 10,000 years. [2] Hence, fossils range in age from the youngest at the start of the Holocene Epoch to the oldest, chemical fossils from the Archaean Eon, up to 3.48  billion years old, [3] [4] [5] or even older, 4.1 billion years old, according to a 2015 study. [6] [7] The observation that certain fossils were associated with certain rock strata led early geologists to recognize a geological timescale in the 19th century. The development of radiometric dating techniques in the early 20th century allowed geologists to determine the numerical or "absolute" age of the various strata and thereby the included fossils.

Like extant organisms, fossils vary in size from microscopic, even single bacterial cells [8] one micrometer in diameter, to gigantic, such as dinosaurs and trees many meters long and weighing many tons. A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates. Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces ( coprolites). These types of fossil are called trace fossils (or ichnofossils), as opposed to body fossils. Finally, past life leaves some markers that cannot be seen but can be detected in the form of biochemical signals; these are known as chemofossils or biosignatures.

Fossilization processes

The process of fossilization varies according to tissue type and external conditions.


Silicified (replaced with silica) fossils from the Road Canyon Formation (Middle Permian of Texas).

Permineralization is a process of fossilization that occurs when an organism is buried. The empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, 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 decay 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.

Casts and molds

External mold of a bivalve from the Logan Formation, Lower Carboniferous, Ohio

In some cases the original remains of the organism completely dissolve or are otherwise destroyed. The remaining organism-shaped hole in the rock is called an external mold. If this hole is later filled with other minerals, it is a cast. An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull.

Authigenic mineralization

This is a special form of cast and mold formation. If the chemistry is right, the organism (or fragment of organism) can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it. If this happens rapidly before significant decay to the organic tissue, very fine three-dimensional morphological detail can be preserved. Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.

Replacement and recrystallization

Recrystallized scleractinian coral (aragonite to calcite) from the Jurassic of southern Israel

Replacement occurs when the shell, bone or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.

Adpression (compression-impression)

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. Often what remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression. Often, however, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil. In many cases, however, compressions and impressions occur together. For instance, when the rock is broken open, the phytoleim will often be attached to one part (compression), whereas the counterpart will just be an impression. For this reason, one term covers the two modes of preservation: adpression. [9]

Soft tissue, cell and molecular preservation

Because of their antiquity, an unexpected exception to the alteration of an organism's tissues by chemical reduction of the complex organic molecules during fossilization has been the discovery of soft tissue in dinosaur fossils, including blood vessels, and the isolation of proteins and evidence for DNA fragments. [10] [11] [12] [13] In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles ( goethite-aFeO(OH)) associated with soft tissues recovered from dinosaur fossils. Based on various experiments that studied the interaction of iron in haemoglobin with blood vessel tissue they proposed that solution hypoxia coupled with iron chelation enhances the stability and preservation of soft tissue and provides the basis for an explanation for the unforeseen preservation of fossil soft tissues. [14] However, a slightly older study based on eight taxa ranging in time from the Devonian to the Jurassic found that reasonably well-preserved fibrils that probably represent collagen were preserved in all these fossils, and that the quality of preservation depended mostly on the arrangement of the collagen fibers, with tight packing favoring good preservation. [15] There seemed to be no correlation between geological age and quality of preservation, within that timeframe.


Carbonaceous films are thin coatings which consist predominantly of the chemical element carbon. The soft tissues of organisms are made largely of organic carbon compounds and during diagenesis under reducing conditions only a thin film of carbon residue is left which forms a silhouette of the original organism.


The star-shaped holes (Catellocaula vallata) in this Upper Ordovician bryozoan represent a soft-bodied organism preserved by bioimmuration in the bryozoan skeleton. [16]

Bioimmuration occurs when a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton. [17] Usually it is a sessile skeletal organism, such as a bryozoan or an oyster, which grows along a substrate, covering other sessile sclerobionts. Sometimes the bioimmured organism is soft-bodied and is then preserved in negative relief as a kind of external mold. There are also cases where an organism settles on top of a living skeletal organism that grows upwards, preserving the settler in its skeleton. Bioimmuration is known in the fossil record from the Ordovician [18] to the Recent. [17]

Other Languages
Afrikaans: Fossiel
العربية: مستحاثة
azərbaycanca: Fosil
تۆرکجه: فوسیل
বাংলা: জীবাশ্ম
Bân-lâm-gú: Hòa-chio̍h
Basa Banyumasan: Fosil
беларуская: Акамянеласці
български: Вкаменелост
bosanski: Fosili
brezhoneg: Karrekaenn
català: Fòssil
čeština: Fosilie
Cymraeg: Ffosil
dansk: Fossil
Deutsch: Fossil
eesti: Kivistis
Ελληνικά: Απολίθωμα
español: Fósil
Esperanto: Fosilio
euskara: Fosil
فارسی: سنگواره
français: Fossile
Frysk: Fossyl
Gaeilge: Iontaise
galego: Fósil
한국어: 화석
हिन्दी: जीवाश्म
hrvatski: Fosil
Ido: Fosilo
Bahasa Indonesia: Fosil
íslenska: Steingervingur
italiano: Fossile
עברית: מאובן
Basa Jawa: Fosil
қазақша: Қазындылар
Kiswahili: Kisukuku
Kreyòl ayisyen: Fosil
Latina: Fossile
latviešu: Fosilijas
lietuvių: Fosilija
Limburgs: Fossiel
magyar: Fosszília
македонски: Фосил
മലയാളം: ജീവാശ്മം
मराठी: जीवाश्म
Bahasa Melayu: Fosil
Mìng-dĕ̤ng-ngṳ̄: Huá-siŏh
Nederlands: Fossiel
नेपाली: जीवावशेष
日本語: 化石
norsk bokmål: Fossil
norsk nynorsk: Fossil
occitan: Fossil
ଓଡ଼ିଆ: ଜୀବାଶ୍ମ
ਪੰਜਾਬੀ: ਪਥਰਾਟ
português: Fóssil
română: Fosilă
Runa Simi: Rumiyasqa
русский: Фоссилии
Scots: Fossil
shqip: Fosilet
සිංහල: පොසිල
Simple English: Fossil
slovenčina: Fosília
slovenščina: Fosil
српски / srpski: Фосил
srpskohrvatski / српскохрватски: Fosil
Basa Sunda: Fosil
suomi: Fossiili
svenska: Fossil
Tagalog: Posil
తెలుగు: శిలాజము
Türkçe: Fosil
اردو: احفور
Tiếng Việt: Hóa thạch
West-Vlams: Fossiel
Winaray: Posil
ייִדיש: פאסיל
粵語: 化石
中文: 化石