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A fossil (from
Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication used by humans, including speech (spoken language), gestur ...
: , literally 'obtained by digging') is any preserved remains, impression, or trace of any once-living thing from a past
geological age The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth sciences, Earth scientists to describe t ...
. Examples include
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
s,
shells Shell may refer to: Architecture and design * Shell (structure), a thin structure **Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses **Thin-shell structure, **Oil company Science Biology * Seashell ...

shells
,
exoskeletons An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton ( endoskeleton) of, for example, a hum ...
, stone imprints of animals or
microbes A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms ar ...
, objects preserved in
amber Amber is fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was ...
,
hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Phy ...

hair
,
petrified wood File:PetrifiedWood.jpg, Petrified log at the Petrified Forest National Park Petrified wood (from Ancient Greek meaning 'rock' or 'stone'; literally 'wood turned into stone') is the name given to a special type of fossilized remains of terrestr ...

petrified wood
,
oil An oil is any nonpolar chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can b ...

oil
,
coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ...

coal
, and
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
remnants. The totality of fossils is known as the ''fossil record''.
Paleontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (geology), epoch (roughly 11,700 years before present). It includes th ...
is the study of fossils: their age, method of formation, and
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
ary significance. Specimens are usually considered to be fossils if they are over 10,000 years old. The oldest fossils are around 3.48 billion years old to 4.1 billion years old. Early edition, published online before print. The observation in the 19th century that certain fossils were associated with certain rock
strata (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half of South America. Sharing the bulk of the Southern Cone with Chile to the west, the country is a ...
led to the recognition of a
geological timescale The geologic time scale (GTS) is a system of chronological dating Chronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously establi ...
and the
relative ages Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e. estimated age). In geology, rock Rock most often re ...
of different fossils. The development of
radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "di ...
techniques in the early 20th century allowed scientists to quantitatively measure the
absolute agesAbsolute dating is the process of determining an age on a specified chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language ...
of rocks and the fossils they host. There are many processes that lead to fossilization, including
permineralization Permineralization is a process of fossil A fossil (from Classical Latin: , literally "obtained by digging") is any preserved remains, impression, or trace of any once- living thing from a past geological age. Examples include bones, shell ...

permineralization
, casts and molds, authigenic mineralization, replacement and recrystallization, adpression,
carbonization Carbonization is the conversion of organic matters like plants and dead animal remains into carbon Carbon (from la, carbo "coal") is a chemical element with the Symbol (chemistry), symbol C and atomic number 6. It is nonmetallic and tetraval ...
, and bioimmuration. Fossils vary in size from one-
micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organ ...
(1 µm) bacteria to
dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) a ...

dinosaur
s 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
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
s, or the
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13O5N)n ( ) is a long-chain polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical su ...

chitin
ous or
calcareous ''Calcareous'' is an adjective meaning "mostly or partly composed of calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecula ...
exoskeletons of
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s. Fossils may also consist of the marks left behind by the organism while it was alive, such as
animal tracks __notoc__ 200px, Bird tracks in snow. An animal track is an imprint left behind in soil File:Stagnogley.JPG, Surface-water-Gley soil, gley developed in glacial till, Northern Ireland. Soil is a mixture of organic matter, minerals, gases, li ...
or
feces Feces ( faeces) is the solid or semi-solid remains of food that was not digested in the , and has been broken down by bacteria in the . Feces contains a relatively small amount of products such as bacterially altered , and dead epithelial cel ...

feces
(
coprolite A coprolite (also known as a coprolith) is fossilized A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the ...

coprolite
s). These types of fossil are called
trace fossil A trace fossil, also ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to ...
s or ''ichnofossils'', as opposed to ''body fossils''. Some fossils are
biochemical Biochemistry or biological chemistry, is the study of chemical process In a scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. L ...

biochemical
and are called ''chemofossils'' or
biosignature A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon ...
s.


Fossilization processes

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


Permineralization

Permineralization Permineralization is a process of fossil A fossil (from Classical Latin: , literally "obtained by digging") is any preserved remains, impression, or trace of any once- living thing from a past geological age. Examples include bones, shell ...

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 Groundwater is the water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

groundwater
. Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within the
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
of a
plant cell Plant cells are eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are class ...

plant cell
. Small scale permineralization can produce very detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death, otherwise the remains are destroyed by scavengers or decomposition. 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 Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biolo ...

skin
,
feather Feathers are epidermis (zoology), epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both Bird, avian (bird) and some non-avian (non-bird) and possibly other archosauromorpha, archosauromorphs. They are conside ...

feather
s or even soft tissues. This is a form of
diagenesis upright=1.35, Permineralization in vertebra from ''Valgipes bucklandi'' Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after the ...
.


Casts and molds

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 void is later filled with sediment, the resulting ''cast'' resembles what the organism looked like. An
endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...
, or ''internal mold'', is the result of sediments filling an organism's interior, such as the inside of a
bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representatio ...
or
snail A snail is, in loose terms, a shelled gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (co ...

snail
or the hollow of a
skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North A ...

skull
. Endocasts are sometimes termed , especially when bivalves are preserved this way.


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 :''Siderite is also the name of a type of iron meteorite Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorites that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of ...

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 The Mazon Creek fossil beds are a conservation ' found near Morris, in Grundy County, Illinois. The fossils are preserved in ironstone 300px, Ironstone (sandstone with iron oxides) from the Mississippian age, Mississippian Breathitt Formatio ...
of Illinois, USA, are among the best documented examples of such mineralization.


Replacement and recrystallization

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 Aragonite is a , one of the three most common naturally occurring of , (the other forms being the s and ). It is formed by biological and physical processes, including precipitation from marine and freshwater environments. The of aragonite d ...

aragonite
to
calcite Calcite is a and the most stable of (CaCO3). The , based on , defines value 3 as "calcite". Other polymorphs of calcium carbonate are the minerals and . Aragonite will change to calcite over timescales of days or less at temperatures exceed ...

calcite
.


Adpression (compression-impression)

Compression fossilImage:FossilFernLeavesPennsylvanianOhio.jpg, upright=1.25, Fossil seed fern leaves from the Late Carboniferous of northeastern Ohio. A compression fossil is a fossil preserved in sedimentary rock that has undergone compression (physical), physical co ...
s, 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 upright=1.35, Permineralization in vertebra from ''Valgipes bucklandi'' Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after the ...
. 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''.


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. In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles (
goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" Polymorphism (materials science), polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite ha ...

goethite
-aFeO(OH)) associated with soft tissues recovered from dinosaur fossils. Based on various experiments that studied the interaction of iron in
haemoglobin Hemoglobin or haemoglobin (spelling differences) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein in the red blood ce ...

haemoglobin
with blood vessel tissue they proposed that solution hypoxia coupled with iron
chelation Chelation is a type of bonding of ions An ion () is an atom or molecule with a net electric charge, electrical charge. The charge of an electron is considered negative by convention and this charge is equal and opposite to charge of a proton ...
enhances the stability and preservation of soft tissue and provides the basis for an explanation for the unforeseen preservation of fossil soft tissues. However, a slightly older study based on eight taxa ranging in time from the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the H ...
to the
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
found that reasonably well-preserved fibrils that probably represent
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

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. There seemed to be no correlation between geological age and quality of preservation, within that timeframe.


Carbonization and coalification

Fossils that are carbonized or coalified consist of the organic remains which have been reduced primarily to the chemical element carbon. Carbonized fossils consist of a thin film which forms a silhouette of the original organism, and the original organic remains were typically soft tissues. Coalified fossils consist primarily of coal, and the original organic remains were typically woody in composition. File:Probable leech from the Waukesha Biota.jpg, Carbonized fossil of a possible
leech Leeches are segmented parasitic Parasitism is a Symbiosis, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it so ...

leech
from the Silurian
Waukesha Biota The Waukesha Biota (also known as Waukesha Lagerstätte, Brandon Bridge Lagerstätte, or Brandon Bridge fauna) refers to the biotic assemblage of the Lagerstätten, Konservat-Lagerstätte of Early Silurian (Telychian to Sheinwoodian) age within the ...
of Wisconsin. File:Lycopod axis.jpg, Partially coalified axis (branch) of a
lycopod Lycopodiopsida is a class of herbaceous vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are called clubmosses, firmosses and quillworts. They have dichotomously branching stems bea ...
from the Devonian of
Wisconsin Wisconsin () is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspaper), a daily newspaper ...

Wisconsin
.


Bioimmuration

Bioimmuration occurs when a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton. Usually it is a sessile skeletal organism, such as a bryozoan or an
oyster Oyster is the common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. It is someti ...

oyster
, which grows along a
substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached **Substrate (locomotion), the surface over which an organism loco ...
, 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 The Ordovician ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other eart ...

Ordovician
to the Recent.


Types


Index

Index fossils (also known as guide fossils, indicator fossils or zone fossils) are fossils used to define and identify
geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth sciences, Earth scientists to describe t ...
s (or faunal stages). They work on the premise that, although different
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...

sediment
s may look different depending on the conditions under which they were deposited, they may include the remains of the same
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
of fossil. The shorter the species' time range, the more precisely different sediments can be correlated, and so rapidly evolving species' fossils are particularly valuable. The best index fossils are common, easy to identify at species level and have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is poor.


Trace

Trace fossil A trace fossil, also ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to ...
s consist mainly of tracks and burrows, but also include
coprolite A coprolite (also known as a coprolith) is fossilized A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the ...

coprolite
s (fossil
feces Feces ( faeces) is the solid or semi-solid remains of food that was not digested in the , and has been broken down by bacteria in the . Feces contains a relatively small amount of products such as bacterially altered , and dead epithelial cel ...

feces
) and marks left by feeding. Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily fossilized hard parts, and they reflect animal behaviours. Many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.e.g. Whilst exact assignment of trace fossils to their makers is generally impossible, traces may for example provide the earliest physical evidence of the appearance of moderately complex animals (comparable to
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to man ...

earthworm
s). Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. They were first described by
William Buckland William Buckland Doctor of Divinity, DD, Royal Society, FRS (12 March 1784 – 14 August 1856) was an English theologian who became Dean of Westminster. He was also a Geology, geologist and paleontology, palaeontologist. Buckland wrote the fi ...

William Buckland
in 1829. Prior to this they were known as "fossil fir cones" and "
bezoar A bezoar is a mass found trapped in the gastrointestinal system The gastrointestinal tract, (GI tract, GIT, digestive tract, digestion tract, alimentary canal) is the tract from the mouth to the anus which includes all the organs of the ...

bezoar
stones." They serve a valuable purpose in paleontology because they provide direct evidence of the predation and diet of extinct organisms. Coprolites may range in size from a few millimetres to over 60 centimetres. File:CambrianRusophycus.jpg,
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
trace fossil A trace fossil, also ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to ...
s including '''', made by a
trilobite Trilobites (; meaning "three lobes") are a group of marine s that form the Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the of the ( ...

trilobite
File:Coprolite.jpg, A coprolite of a carnivorous dinosaur found in southwestern
Saskatchewan ("From Many Peoples Strength") , image_map = Saskatchewan in Canada 2.svg , Label_map = yes , coordinates = , official_lang = English language, English , capital = Regina, S ...
File:Climactichnites wilsoni, densely packed.jpg, Densely packed, subaerial or nearshore trackways (''
Climactichnites ''Climactichnites'' is an enigmatic, Cambrian fossil formed on or within sandy tidal flats around . It has been interpreted in many different ways in the past, but is now thought to be a trace fossil '' footprints in a Triassic sandstone '' fro ...
wilsoni'') made by a putative, slug-like
mollusk Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
on a Cambrian tidal flat


Transitional

A ''transitional fossil'' is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation.


Microfossils

Microfossil A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, su ...
is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cutoff point between "micro" and "macro" fossils is 1 mm. Microfossils may either be complete (or near-complete) organisms in themselves (such as the marine plankters
foraminifera Foraminifera (; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
and
coccolithophore A coccolithophore (or coccolithophorid, from the adjective) is a Unicellular organism, unicellular, eukaryotic phytoplankton (alga). They belong either to the kingdom Protista, according to Robert Whittaker (ecologist), Robert Whittaker's Kingdom ...
s) or component parts (such as small teeth or
spores In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...
) of larger animals or plants. Microfossils are of critical importance as a reservoir of
paleoclimate Paleoclimatology (British spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthograp ...
information, and are also commonly used by biostratigraphers to assist in the correlation of rock units.


Resin

Fossil resin (colloquially called
amber Amber is fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was ...

amber
) is a natural
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
found in many types of strata throughout the world, even the
Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Alaska (United States), Canada, Finland, Greenland (Danish Realm, ...

Arctic
. The oldest fossil resin dates to the
Triassic The Triassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...

Triassic
, though most dates to the
Cenozoic The Cenozoic ( ; ) is Earth's current geological era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the ge ...

Cenozoic
. The excretion of the resin by certain plants is thought to be an evolutionary
adaptation In , adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits s to their environment, enhancing their . Secondly, it is a state reached by the population during that process. Thirdly, it is a or adapti ...

adaptation
for protection from insects and to seal wounds. Fossil resin often contains other fossils called inclusions that were captured by the sticky resin. These include bacteria, fungi, other plants, and animals. Animal inclusions are usually small
invertebrates Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebrat ...
, predominantly
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
s such as insects and spiders, and only extremely rarely a
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
such as a small lizard. Preservation of inclusions can be exquisite, including small fragments of
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
.


Derived, or reworked

A ''derived'', ''reworked'' or is a fossil found in rock that accumulated significantly later than when the fossilized animal or plant died. Reworked fossils are created by erosion exhuming (freeing) fossils from the rock formation in which they were originally deposited and their redeposition in a younger sedimentary deposit.


Wood

Fossil wood is wood that is preserved in the fossil record. Wood is usually the part of a plant that is best preserved (and most easily found). Fossil wood may or may not be
petrified In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the process ...

petrified
. The fossil wood may be the only part of the plant that has been preserved: therefore such wood may get a special kind of
botanical name A botanical name is a formal scientific name In taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specif ...
. This will usually include "xylon" and a term indicating its presumed affinity, such as ''Araucarioxylon arizonicum, Araucarioxylon'' (wood of ''Araucaria'' or some related genus), ''Palmoxylon'' (wood of an indeterminate Arecaceae, palm), or ''Castanoxylon'' (wood of an indeterminate Castanopsis, chinkapin).


Subfossil

The term subfossil can be used to refer to remains, such as bones, nests, or defecations, whose fossilization process is not complete, either because the length of time since the animal involved was living is too short (less than 10,000 years) or because the conditions in which the remains were buried were not optimal for fossilization. Subfossils are often found in caves or other shelters where they can be preserved for thousands of years. The main importance of subfossil vs. fossil remains is that the former contain organic material, which can be used for radiocarbon dating or extraction and DNA sequencing, sequencing of DNA, protein sequencing, protein, or other biomolecules. Additionally, isotope ratios can provide much information about the ecological conditions under which extinct animals lived. Subfossils are useful for studying the evolutionary history of an environment and can be important to studies in paleoclimatology. Subfossils are often found in depositionary environments, such as lake sediments, oceanic sediments, and soils. Once deposited, physical and chemical weathering can alter the state of preservation.


Chemical fossils

Chemical fossils, or chemofossils, are chemicals found in rocks and fossil fuels (petroleum, coal, and natural gas) that provide an organic signature for ancient life. Molecular fossils and isotope ratios represent two types of chemical fossils. The oldest traces of life on Earth are fossils of this type, including carbon isotope anomalies found in zircons that imply the existence of life as early as 4.1 billion years ago.


Dating


Estimating dates

Paleontology seeks to map out how life evolved across geologic time. A substantial hurdle is the difficulty of working out fossil ages. Beds that preserve fossils typically lack the radioactive elements needed for
radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "di ...
. This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better. Although radiometric dating requires careful laboratory work, its basic principle is simple: the rates at which various radioactive elements radioactive decay, decay are known, and so the ratio of the radioactive element to its decay products shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers, which may provide termini for the intervening sediments.


Stratigraphy

Consequently, palaeontologists rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...

sediment
ary record. Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age is claimed to lie between the two known ages. Because rock sequences are not continuous, but may be broken up by Fault (geology), faults or periods of erosion, it is very difficult to match up rock beds that are not directly adjacent. However, fossils of species that survived for a relatively short time can be used to match isolated rocks: this technique is called ''biostratigraphy''. For instance, the conodont ''Eoplacognathus pseudoplanus'' has a short range in the Middle Ordovician period. If rocks of unknown age have traces of ''E. pseudoplanus'', they have a mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and occupy a short time range to be useful. Misleading results are produced if the index fossils are incorrectly dated. Stratigraphy and biostratigraphy can in general provide only relative dating (''A'' was before ''B''), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of the problems involved in matching rocks of the same age across continents. Family-tree relationships also help to narrow down the date when lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved earlier. It is also possible to estimate how long ago two living clades diverged, in other words approximately how long ago their last common ancestor must have lived, by assuming that DNA mutations accumulate at a constant rate. These "molecular clocks", however, are fallible, and provide only approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques may vary by a factor of two.


Limitations

Organisms are only rarely preserved as fossils in the best of circumstances, and only a fraction of such fossils have been discovered. This is illustrated by the fact that the number of species known through the fossil record is less than 5% of the number of known living species, suggesting that the number of species known through fossils must be far less than 1% of all the species that have ever lived. Because of the specialized and rare circumstances required for a biological structure to fossilize, only a small percentage of life-forms can be expected to be represented in discoveries, and each discovery represents only a snapshot of the process of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, which will never demonstrate an exact half-way point. The fossil record is strongly biased toward organisms with hard-parts, leaving most groups of soft-bodied organisms with little to no role. It is replete with the mollusks, the
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
s, the echinoderms, the brachiopods and some groups of
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
s.


Sites


Lagerstätten

Fossil sites with exceptional preservation—sometimes including preserved soft tissues—are known as Lagerstätten—German for "storage places". These formations may have resulted from carcass burial in an hypoxia (environmental), anoxic environment with minimal bacteria, thus slowing decomposition. Lagerstätten span geological time from the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
period to the Holocene, present. Worldwide, some of the best examples of near-perfect fossilization are the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
Maotianshan shales and Burgess Shale, the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the H ...
Hunsrück Slates, the
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
Solnhofen limestone, and the Carboniferous Mazon Creek localities.


Stromatolites

Stromatolites are layered Accretion (geology), accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria. Stromatolites provide some of the most ancient fossil records of life on Earth, dating back more than 3.5 billion years ago. Stromatolites were much more abundant in Precambrian times. While older, Archean fossil remains are presumed to be colony (biology), colonies of cyanobacteria, younger (that is, Proterozoic) fossils may be prehistoric, primordial forms of the Eukaryota, eukaryote Chlorophyta, chlorophytes (that is, green algae). One genus of stromatolite very common in the geologic time scale, geologic record is ''Collenia''. The earliest stromatolite of confirmed microbial origin dates to 2.724 billion years ago. A 2009 discovery provides strong evidence of microbial stromatolites extending as far back as 3.45 billion years ago. Stromatolites are a major constituent of the fossil record for life's first 3.5 billion years, peaking about 1.25 billion years ago. They subsequently declined in abundance and diversity, which by the start of the Cambrian had fallen to 20% of their peak. The most widely supported explanation is that stromatolite builders fell victims to grazing creatures (the Cambrian substrate revolution), implying that sufficiently complex organisms were common over 1 billion years ago. The connection between grazer and stromatolite abundance is well documented in the younger
Ordovician The Ordovician ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other eart ...

Ordovician
evolutionary radiation; stromatolite abundance also increased after the Ordovician-Silurian extinction events, end-Ordovician and Permian extinction, end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered. Fluctuations in metazoan population and diversity may not have been the only factor in the reduction in stromatolite abundance. Factors such as the chemistry of the environment may have been responsible for changes. While prokaryote, prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the timeline of evolution, evolutionary development of more complex eukaryote, eukaryotic organisms. Cyanobacteria (as well as extremophile Gammaproteobacteria) are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis. Cyanobacteria use water, carbon dioxide and sunlight to create their food. A layer of mucus often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented by the calcium carbonate to grow thin laminations of limestone. These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis. Layered spherical growth structures termed oncolites are similar to stromatolites and are also known from the fossil record. Thrombolites are poorly laminated or non-laminated clotted structures formed by cyanobacteria common in the fossil record and in modern sediments. The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of southwestern Namibia provides an extremely well exposed example of the thrombolite-stromatolite-metazoan reefs that developed during the Proterozoic period, the stromatolites here being better developed in updip locations under conditions of higher current velocities and greater sediment influx.


Astrobiology

It has been suggested that Biomineralization, biominerals could be important indicators of extraterrestrial life and thus could play an important role in the search for past or present life on the planet Mars. Furthermore, Organic compounds (minerals), organic components (
biosignature A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon ...
s) that are often associated with biominerals are believed to play crucial roles in both pre-biotic and Biotic material, biotic reactions. On 24 January 2014, NASA reported that current studies by the Curiosity (rover), ''Curiosity'' and Opportunity (rover), ''Opportunity'' Mars rover, rovers on Mars will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or Chemolithotrophs, chemolithoautotrophic microorganisms, as well as ancient water, including Lacustrine plain, fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been Planetary habitability, habitable. The search for evidence of Planetary habitability, habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.


Pseudofossils

''Pseudofossils'' are visual patterns in rocks that are produced by geologic processes rather than biologic processes. They can easily be mistaken for real fossils. Some pseudofossils, such as geological Dendrite (crystal), dendrite crystals, are formed by naturally occurring fissures in the rock that get filled up by percolating minerals. Other types of pseudofossils are kidney ore (round shapes in iron ore) and moss agates, which look like moss or plant leaves. Concretions, spherical or ovoid-shaped nodules found in some sedimentary strata, were once thought to be
dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) a ...

dinosaur
eggs, and are often mistaken for fossils as well.


History of the study of fossils

Gathering fossils dates at least to the beginning of recorded history. The fossils themselves are referred to as the fossil record. The fossil record was one of the early sources of data underlying the study of
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
and continues to be relevant to the Timeline of evolution, history of life on Earth. Paleontologists examine the fossil record to understand the process of evolution and the way particular
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
have evolved.


Ancient civilizations

Fossils have been visible and common throughout most of natural history, and so documented human interaction with them goes back as far as recorded history, or earlier. There are many examples of paleolithic stone knives in Europe, with fossil echinoderms set precisely at the hand grip, going all the way back to ''Homo heidelbergensis'' and Neanderthals. These ancient peoples also drilled holes through the center of those round fossil shells, apparently using them as beads for necklaces. The ancient Egyptians gathered fossils of species that resembled the bones of modern species they worshipped. The god Set (deity), Set was associated with the hippopotamus, therefore fossilized bones of hippo-like species were kept in that deity's temples. Five-rayed fossil sea urchin shells were associated with the deity Sopdu, the Morning Star, equivalent of Venus (mythology), Venus in Roman mythology. file:hyperborean-gryphon-persepolis-protoceratops-psittacosaurus-skeletons.jpg, Ceratopsian skulls are common in the Dzungarian Gate mountain pass in Asia, an area once famous for gold mines, as well as its endlessly cold winds. This has been attributed to legends of both gryphons and the land of Hyperborea Fossils appear to have directly contributed to the mythology of many civilizations, including the ancient Greeks. Classical Greek historian Herodotos wrote of an area near Hyperborea where gryphons protected golden treasure. There was indeed gold mining Dzungarian Gate#Hyperborean connection, in that approximate region, where beaked ''Protoceratops'' skulls were common as fossils. A later Ancient Greece, Greek scholar, Aristotle, eventually realized that fossil seashells from rocks were similar to those found on the beach, indicating the fossils were once living animals. He had previously explained them in terms of vaporous exhalations, which Persian polymath Avicenna modified into the theory of Petrifaction, petrifying fluids (). Recognition of fossil seashells as originating in the sea was built upon in the 14th century by Albert of Saxony (philosopher), Albert of Saxony, and accepted in some form by most naturalists by the 16th century. Roman naturalist Pliny the Elder wrote of "tongue stones", which he called glossopetra. These were fossil shark teeth, thought by some classical cultures to look like the tongues of people or snakes. He also wrote about the horns of Ammon, which are fossil ammonites, from whence the group of shelled octopus-cousins ultimately draws its modern name. Pliny also makes one of the earlier known references to toadstones, thought until the 18th century to be a magical cure for poison originating in the heads of toads, but which are fossil teeth from ''Lepidotes'', a Cretaceous ray-finned fish. The Plains tribes of North America are thought to have similarly associated fossils, such as the many intact pterosaur fossils naturally exposed in the region, with their own mythology of the thunderbird (mythology), thunderbird. There is no such direct mythological connection known from prehistoric Africa, but there is considerable evidence of tribes there excavating and moving fossils to ceremonial sites, apparently treating them with some reverence. In Japan, fossil shark teeth were associated with the mythical tengu, thought to be the razor-sharp claws of the creature, documented some time after the 8th century AD. In medieval China, the fossil bones of ancient mammals including ''Homo erectus'' were often mistaken for "dragon bones" and used as medicine and aphrodisiacs. In addition, some of these fossil bones are collected as "art" by scholars, who left scripts on various artifacts, indicating the time they were added to a collection. One good example is the famous scholar Huang Tingjian of the Song Dynasty during the 11th century, who kept a specific seashell fossil with his own poem engraved on it. In his ''Dream Pool Essays'' published in 1088, Song dynasty Chinese scholar-official Shen Kuo hypothesized that marine fossils found in a stratum, geological stratum of mountains located hundreds of miles from the Pacific Ocean was evidence that a prehistoric seashore had once existed there and Geomorphology, shifted over centuries of time.Needham, Joseph. (1959). ''Science and Civilization in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth''. Cambridge University Press. pp. 603–618. His observation of petrified bamboos in the dry northern climate zone of what is now Yan'an, Shaanxi province, China, led him to advance early ideas of gradual climate change (general concept), climate change due to bamboo naturally growing in wetter climate areas.Rafferty, John P. (2012). ''Geological Sciences; Geology: Landforms, Minerals, and Rocks''. New York: Britannica Educational Publishing, p. 6. In medieval Christendom, fossilized sea creatures on mountainsides were seen as proof of the biblical deluge of Noah's Ark. After observing the existence of seashells in mountains, the List of ancient Greek philosophers, ancient Greek philosopher Xenophanes (c. 570 – 478 BC) speculated that the world was once inundated in a great flood that buried living creatures in drying mud.Rafferty, John P. (2012). ''Geological Sciences; Geology: Landforms, Minerals, and Rocks''. New York: Britannica Educational Publishing, pp 5–6. In 1027, the Persian people, Persian Avicenna explained fossils' stoniness in ''The Book of Healing'': From the 13th century to the present day, scholars pointed out that the fossil skulls of Deinotherium giganteum, found in Crete and Greece, might have been interpreted as being the skulls of the Cyclopes of Greek mythology, and are possibly the origin of that Greek myth. Their skulls appear to have a single eye-hole in the front, just like their modern elephant cousins, though in fact it's actually the opening for their trunk. In Norse mythology, echinoderm shells (the round five-part button left over from a sea urchin) were associated with the god Thor, not only being incorporated in Thunderstone (folklore)#Fossils as thunderstones, thunderstones, representations of Thor's hammer and subsequent hammer-shaped crosses as Christianity was adopted, but also kept in houses to garner Thor's protection. These grew into the shepherd's crowns of English folklore, used for decoration and as good luck charms, placed by the doorway of homes and churches. In Suffolk, a different species was used as a good-luck charm by bakers, who referred to them as fairy loaf, fairy loaves, associating them with the similarly shaped loaves of bread they baked.


Early modern explanations

More scientific views of fossils emerged during the Renaissance. Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life. For example, da Vinci noticed discrepancies with the biblical flood narrative as an explanation for fossil origins: In 1666, Nicholas Steno examined a shark, and made the association of its teeth with the "tongue stones" of ancient Greco-Roman mythology, concluding that those were not in fact the tongues of venomous snakes, but the teeth of some long-extinct species of shark. Robert Hooke (1635-1703) included micrographs of fossils in his ''Micrographia'' and was among the first to observe fossil forams. His observations on fossils, which he stated to be the petrified remains of creatures some of which no longer existed, were published posthumously in 1705. William Smith (geologist), William Smith (1769–1839), an English canal engineer, observed that rocks of different ages (based on the law of superposition) preserved different assemblages of fossils, and that these assemblages succeeded one another in a regular and determinable order. He observed that rocks from distant locations could be correlated based on the fossils they contained. He termed this the principle of ''faunal succession''. This principle became one of Darwin's chief pieces of evidence that biological evolution was real. Georges Cuvier came to believe that most if not all the animal fossils he examined were remains of extinct species. This led Cuvier to become an active proponent of the geological school of thought called catastrophism. Near the end of his 1796 paper on living and fossil elephants he said: Interest in fossils, and geology more generally, expanded during the early nineteenth century. In Britain, Mary Anning's discoveries of fossils, including the first complete ichthyosaur and a complete plesiosaurus skeleton, sparked both public and scholarly interest.


Linnaeus and Darwin

Early Natural science, naturalists well understood the similarities and differences of living species leading Carl Linnaeus, Linnaeus to develop a hierarchical classification system still in use today. Darwin and his contemporaries first linked the hierarchical structure of the tree of life with the then very sparse fossil record. Darwin eloquently described a process of descent with modification, or evolution, whereby organisms either adapt to natural and changing environmental pressures, or they perish. When Darwin wrote ''On the Origin of Species, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life'', the oldest animal fossils were those from the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
Period, now known to be about 540 million years old. He worried about the absence of older fossils because of the implications on the validity of his theories, but he expressed hope that such fossils would be found, noting that: "only a small portion of the world is known with accuracy." Darwin also pondered the sudden appearance of many groups (i.e. phylum, phyla) in the oldest known Cambrian fossiliferous strata.


After Darwin

Since Darwin's time, the fossil record has been extended to between 2.3 and 3.5 billion years. Most of these Precambrian fossils are microscopic bacteria or Micropaleontology, microfossils. However, macroscopic fossils are now known from the late Proterozoic. The Ediacara biota (also called Vendian biota) dating from 575 million years ago collectively constitutes a richly diverse assembly of early multicellular eukaryotes. The fossil record and faunal succession form the basis of the science of biostratigraphy or determining the age of rocks based on embedded fossils. For the first 150 years of geology, biostratigraphy and superposition were the only means for determining the Relative dating, relative age of rocks. The geologic time scale was developed based on the relative ages of rock strata as determined by the early paleontologists and Stratigraphy, stratigraphers. Since the early years of the twentieth century, absolute dating methods, such as
radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "di ...
(including K–Ar dating, potassium/argon, Argon–argon dating, argon/argon, uranium–lead dating, uranium series, and, for very recent fossils, radiocarbon dating) have been used to verify the relative ages obtained by fossils and to provide absolute ages for many fossils. Radiometric dating has shown that the earliest known stromatolites are over 3.4 billion years old.


Modern era

Paleontology has joined with evolutionary biology to share the interdisciplinary task of outlining the tree of life, which inevitably leads backwards in time to Precambrian microscopic life when cell structure and functions evolved. Earth's deep time in the Proterozoic and deeper still in the Archean is only "recounted by microscopic fossils and subtle chemical signals." Molecular biologists, using phylogenetics, can compare protein amino acid or nucleotide sequence homology (i.e., similarity) to evaluate taxonomy and evolutionary distances among organisms, with limited statistical confidence. The study of fossils, on the other hand, can more specifically pinpoint when and in what organism a mutation first appeared. Phylogenetics and paleontology work together in the clarification of science's still dim view of the appearance of life and its evolution. Niles Eldredge's study of the ''Phacops''
trilobite Trilobites (; meaning "three lobes") are a group of marine s that form the Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the of the ( ...

trilobite
genus supported the hypothesis that modifications to the arrangement of the trilobite's eye lenses proceeded by fits and starts over millions of years during the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the H ...
. Eldredge's interpretation of the ''Phacops'' fossil record was that the aftermaths of the lens changes, but not the rapidly occurring evolutionary process, were fossilized. This and other data led Stephen Jay Gould and Niles Eldredge to publish their seminal paper on punctuated equilibrium in 1971. Synchrotron X-ray Tomography, tomographic analysis of early Cambrian bilaterian embryonic microfossils yielded new insights of Animal, metazoan evolution at its earliest stages. The tomography technique provides previously unattainable three-dimensional resolution at the limits of fossilization. Fossils of two enigmatic bilaterians, the worm-like ''Markuelia'' and a putative, primitive protostome, ''Pseudooides'', provide a peek at germ layer embryonic development. These 543-million-year-old embryos support the emergence of some aspects of
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
development earlier than previously thought in the late Proterozoic. The preserved embryos from China and Siberia underwent rapid Diagenesis, diagenetic phosphatization resulting in exquisite preservation, including cell structures. This research is a notable example of how knowledge encoded by the fossil record continues to contribute otherwise unattainable information on the emergence and development of life on Earth. For example, the research suggests ''Markuelia'' has closest affinity to priapulid worms, and is adjacent to the evolutionary branching of Priapulida, Nematode, Nematoda and Arthropoda. Despite significant advances in uncovering and identifying paleontological specimens, it is generally accepted that the fossil record is vastly incomplete. Approaches for measuring the completeness of the fossil record have been developed for numerous subsets of species, including those grouped taxonomically, temporally, environmentally/geographically, or in sum. This encompasses the subfield of taphonomy and the study of biases in the paleontological record.


Trading and collecting

Fossil trading is the practice of buying and selling fossils. This is many times done illegally with artifacts stolen from research sites, costing many important scientific specimens each year. The problem is quite pronounced in China, where many specimens have been stolen. Fossil collecting (sometimes, in a non-scientific sense, fossil hunting) is the collection of fossils for scientific study, hobby, or profit. Fossil collecting, as practiced by amateurs, is the predecessor of modern paleontology and many still collect fossils and study fossils as amateurs. Professionals and amateurs alike collect fossils for their scientific value.


As medicine

The use of fossils to address health issues is rooted in traditional medicine and include the use of fossils as talismans. The specific fossil to use to alleviate or cure an illness is often based on its resemblance to the symptoms or affected organ. The usefulness of fossils as medicine is almost entirely a placebo effect, though fossil material might conceivably have some antacid activity or supply some essential minerals. The use of dinosaur bones as "dragon bones" has persisted in Traditional Chinese medicine into modern times, with mid-Cretaceous dinosaur bones being used for the purpose in Ruyang County during the early 21st century.


Gallery

File:Marine fossils found high in the Himalayas. Collection of the Abbot of Dhankar Gompa, HP, India.jpg, Marine fossils found high in the Himalayas. Collection of the Abbot of Dhankar Gompa, HP, India Amonite Cropped.jpg, Three small ammonite fossils, each approximately 1.5 cm across Priscacara liops Green River Formation.jpg, Eocene fossil fish ''Priscacara liops'' from the Green River Formation of Wyoming Trilobite2.jpg, A permineralized
trilobite Trilobites (; meaning "three lobes") are a group of marine s that form the Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the of the ( ...

trilobite
, ''Asaphus kowalewskii'' Carcharodontosaurus and Megalodon teeth.jpg, Megalodon and ''Carcharodontosaurus'' teeth. The latter was found in the Sahara Desert. The fossils from Cretaceous age found in Lebanon.jpg, Fossil shrimp (Cretaceous) PetrifiedWood.jpg, Petrified wood in Petrified Forest National Park, Arizona Petrified Araucaria cone from patagonia-Edit3.jpg, Petrified Conifer cone, cone of ''Araucaria mirabilis'' from Patagonia, Argentina dating from the Jurassic, Jurassic Period (approx. 210 Ma (unit), Ma) CyprusPlioceneGastropod.JPG, A fossil gastropod from the Pliocene of Cyprus. A Serpulidae, serpulid worm is attached. OrhtocerasNautiloid092313.jpg, Silurian Orthoceras fossil Eocene fossil flower, Clare Family Florissant Fossil Quarry, Florissant, Colorado, USA - 20100807.jpg, Eocene fossil flower from Florissant, Colorado File:RoyLindmanMicraster.JPG, ''Micraster'' echinoid fossil from Englandf File:Productid Permian Texas.JPG, Productid brachiopod ventral valve; Roadian, Guadalupian (Middle Permian); Glass Mountains, Texas. File:Fossil agatized coral Florida.JPG, Agate, Agatized coral from the Hawthorn Group (Oligocene–Miocene), Florida. An example of preservation by replacement. File:Fossils_from_Gotland_beaches.jpg, Fossils from beaches of the Baltic Sea island of Gotland, placed on paper with 7 mm (0.28 inch) squares File:Dinosaur footprints in ToroToro Bolivia.jpg, Dinosaur footprints from Torotoro National Park in Bolivia.


See also

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References


Further reading


"Grand Canyon cliff collapse reveals 313 million-year-old fossil footprints"
21 Aug 2020, ''CNN''
"Hints of fossil DNA discovered in dinosaur skull"
by Michael Greshko, 3 Mar 2020, ''National Geographic''
"Fossils for Kids , Learn all about how fossils are formed, the types of fossils and more!"
Video (2:23), 27 Jan 2020, ''Clarendon Learning''
"Fossil & their formation"
Video (9:55), 15 Nov 2019, ''Khan Academy''
"How are dinosaur fossils formed?
by Lisa Hendry, ''Natural History Museum, London''
"Fossils 101"
Video (4:27), 22 Aug 2019, ''National Geographic''
"How to Spot the Fossils Hiding in Plain Sight"
by Jessica Leigh Hester, 23 Feb 2018, ''Atlas Obscura''
"It's extremely hard to become a fossil"
by Olivia Judson, 30 Dec 2008, ''The New York Times''
"Bones Are Not the Only Fossils"
by Olivia Judson, 4 Mar 2008, ''The New York Times''


External links

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The Virtual Fossil Museum throughout Time and Evolution

Paleoportal, geology and fossils of the United States

The Fossil Record, a complete listing of the families, orders, class and phyla found in the fossil record
* * * {{Authority control Fossils,