Afossil (fromClassical Latinfossilis,lit.'obtained by digging')[1] is any preserved remains, impression, or trace of any once-living thing from a pastgeological age. Examples includebones,shells,exoskeletons, stone imprints of animals ormicrobes, objects preserved inamber,hair,petrified wood andDNA remnants. The totality of fossils is known as thefossil record. Though the fossil record is incomplete, numerous studies have demonstrated that there is enough information available to give a good understanding of the pattern of diversification of life on Earth.[2][3][4] In addition, the record can predict and fill gaps such as the discovery ofTiktaalik in the arctic ofCanada.[5]
Paleontology includes the study of fossils: their age, method of formation, andevolutionary significance. Specimens are sometimes considered to be fossils if they are over 10,000 years old.[6][7][8] The oldest fossils are around 3.48 billion years[9][10][11] to 4.1 billion years old.[12][13] The observation in the 19th century that certain fossils were associated with certain rockstrata led to the recognition of ageological timescale and therelative ages of different fossils. The development ofradiometric dating techniques in the early 20th century allowed scientists to quantitatively measure theabsolute ages of rocks and the fossils they host.
Fossils vary in size from one-micrometre (1 μm) bacteria[14] todinosaurs 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 partiallymineralized during life, such as the bones and teeth ofvertebrates, or thechitinous orcalcareous exoskeletons ofinvertebrates. Fossils may also consist of the marks left behind by the organism while it was alive, such asanimal tracks orfeces (coprolites). These types of fossil are calledtrace fossils orichnofossils, as opposed tobody fossils. Some fossils arebiochemical and are calledchemofossils orbiosignatures.
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 ofevolution and continues to be relevant to thehistory of life on Earth.Paleontologists examine the fossil record to understand the process of evolution and the way particularspecies have evolved.
Ancient civilizations
Ceratopsian skulls are common in theDzungarian 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 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 ofPaleolithic stone knives in Europe, with fossilechinoderms set precisely at the hand grip, dating back toHomo heidelbergensis andNeanderthals.[15] 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 godSet was associated with thehippopotamus, therefore fossilized bones of hippo-like species were kept in that deity's temples.[16] Five-rayed fossil sea urchin shells were associated with the deitySopdu, the Morning Star, equivalent ofVenus in Roman mythology.[15]
Fossil shells from thecretaceous era sea urchin,Micraster, were used in medieval times as both shepherd's crowns to protect houses, and as painted fairy loaves by bakers to bring luck to their bread-making.
Fossils appear to have directly contributed to the mythology of many civilizations, including the ancient Greeks. Classical Greek historianHerodotos wrote of an area nearHyperborea wheregryphons protected golden treasure. There was indeed gold miningin that approximate region, where beakedProtoceratops skulls were common as fossils.
A laterGreek 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 ofvaporousexhalations,[17] whichPersian polymathAvicenna modified into the theory ofpetrifyingfluids (succus lapidificatus). Recognition of fossil seashells as originating in the sea was built upon in the 14th century byAlbert of Saxony, and accepted in some form by mostnaturalists by the 16th century.[18]
Roman naturalistPliny the Elder wrote of "tongue stones", which he calledglossopetra. These were fossil shark teeth, thought by some classical cultures to look like the tongues of people or snakes.[19] He also wrote about thehorns of Ammon, which are fossilammonites, whence the group of shelled octopus-cousins ultimately draws its modern name. Pliny also makes one of the earlier known references totoadstones, thought until the 18th century to be a magical cure for poison originating in the heads of toads, but which are fossil teeth fromLepidotes, aCretaceous ray-finned fish.[20]
ThePlains 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 thethunderbird.[21]
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.[22]
In Japan, fossil shark teeth were associated with the mythicaltengu, thought to be the razor-sharp claws of the creature, documented some time after the 8th century AD.[19]
In medieval China, the fossil bones of ancient mammals includingHomo erectus were often mistaken for "dragon bones" and used as medicine andaphrodisiacs. 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 scholarHuang Tingjian of theSong dynasty during the 11th century, who kept a specific seashell fossil with his own poem engraved on it.[23] In hisDream Pool Essays published in 1088, Song dynasty Chinesescholar-officialShen Kuo hypothesized that marine fossils found in ageological stratum of mountains located hundreds of miles from thePacific Ocean was evidence that a prehistoric seashore had once existed there andshifted over centuries of time.[24][25] His observation ofpetrifiedbamboos in the dry northern climate zone of what is nowYan'an,Shaanxi province, China, led him to advance early ideas of gradualclimate change due to bamboo naturally growing in wetter climate areas.[25][26][27]
In medievalChristendom, fossilized sea creatures on mountainsides were seen as proof of the biblical deluge ofNoah's Ark. After observing the existence of seashells in mountains, theancient Greek philosopherXenophanes (c. 570 – 478 BC) speculated that the world was once inundated in a great flood that buried living creatures in drying mud.[28][29]
If what is said concerning the petrifaction of animals and plants is true, the cause of this (phenomenon) is a powerful mineralizing and petrifying virtue which arises in certain stony spots, or emanates suddenly from the earth during earthquake and subsidences, and petrifies whatever comes into contact with it. As a matter of fact, the petrifaction of the bodies of plants and animals is not more extraordinary than the transformation of waters.[30]
From the 13th century to the present day, scholars pointed out that the fossil skulls ofDeinotherium giganteum, found inCrete and Greece, might have been interpreted as being the skulls of theCyclopes ofGreek mythology, and are possibly the origin of that Greek myth.[31][32] Their skulls appear to have a single eye-hole in the front, just like their modernelephant cousins, though in fact it's actually the opening for their trunk.
InNorse mythology, echinoderm shells (the round five-part button left over from a sea urchin) were associated with the godThor, not only being incorporated inthunderstones, representations of Thor's hammer and subsequent hammer-shaped crosses as Christianity was adopted, but also kept in houses to garner Thor's protection.[15]
These grew into theshepherd's crowns of English folklore, used for decoration and as good luck charms, placed by the doorway of homes and churches.[33] InSuffolk, a different species was used as a good-luck charm by bakers, who referred to them asfairy loaves, associating them with the similarly shaped loaves of bread they baked.[34][35]
Early modern explanations
Georges Cuvier's 1812 skeletal reconstruction ofAnoplotherium commune based on fossil remains of the extinctartiodactyl fromMontmartre in Paris, France
More scientific views of fossils emerged during theRenaissance.Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life.[36]: 361 For example, Leonardo noticed discrepancies with the biblical flood narrative as an explanation for fossil origins:
If the Deluge had carried the shells for distances of three and four hundred miles from the sea it would have carried them mixed with various other natural objects all heaped up together; but even at such distances from the sea we see the oysters all together and also the shellfish and the cuttlefish and all the other shells which congregate together, found all together dead; and the solitary shells are found apart from one another as we see them every day on the sea-shores.
And we find oysters together in very large families, among which some may be seen with their shells still joined together, indicating that they were left there by the sea and that they were still living when the strait of Gibraltar was cut through. In the mountains of Parma and Piacenza multitudes of shells and corals with holes may be seen still sticking to the rocks....[37]
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.[19]
Robert Hooke (1635–1703) includedmicrographs of fossils in hisMicrographia and was among the first to observe fossilforams. 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.[38]
William Smith (1769–1839), an English canal engineer, observed that rocks of different ages (based on thelaw 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 offaunal 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 calledcatastrophism. Near the end of his 1796 paper on living and fossil elephants he said:
All of these facts, consistent among themselves, and not opposed by any report, seem to me to prove the existence of a world previous to ours, destroyed by some kind of catastrophe.[39]
Interest in fossils, and geology more generally, expanded during the early nineteenth century. In Britain,Mary Anning's discoveries of fossils, including the first completeichthyosaur and a completeplesiosaurus skeleton, sparked both public and scholarly interest.[40]
Linnaeus and Darwin
Earlynaturalists well understood the similarities and differences of living species leadingLinnaeus 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 wroteOn 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 theCambrian 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.phyla) in the oldest known Cambrian fossiliferous strata.[41]
After Darwin
Since Darwin's time, the fossil record has been extended to between 2.3 and 3.5 billion years.[42] Most of these Precambrian fossils are microscopic bacteria ormicrofossils. However, macroscopic fossils are now known from the late Proterozoic. TheEdiacara biota (also called Vendian biota) dating from 575 million years ago collectively constitutes a richly diverse assembly of early multicellulareukaryotes.
The fossil record and faunal succession form the basis of the science ofbiostratigraphy or determining the age of rocks based on embedded fossils. For the first 150 years ofgeology, biostratigraphy and superposition were the only means for determining therelative age of rocks. Thegeologic time scale was developed based on the relative ages of rock strata as determined by the early paleontologists andstratigraphers.
Since the early years of the twentieth century,absolute dating methods, such asradiometric dating (includingpotassium/argon,argon/argon,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
The fossil record is life's evolutionary epic that unfolded over four billion years as environmental conditions and genetic potential interacted in accordance with natural selection.
Paleontology has joined withevolutionary 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."[44] Molecular biologists, usingphylogenetics, can compare proteinamino acid ornucleotide 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.[45]
Niles Eldredge's study of thePhacopstrilobite 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 theDevonian.[46] Eldredge's interpretation of thePhacops fossil record was that the aftermaths of the lens changes, but not the rapidly occurring evolutionary process, were fossilized. This and other data ledStephen Jay Gould and Niles Eldredge to publish their seminal paper onpunctuated equilibrium in 1971.
SynchrotronX-raytomographic analysis of early Cambrian bilaterianembryonic microfossils yielded new insights ofmetazoan 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-likeMarkuelia and a putative, primitiveprotostome,Pseudooides, provide a peek atgerm layer embryonic development. These 543-million-year-old embryos support the emergence of some aspects ofarthropod development earlier than previously thought in the late Proterozoic. The preserved embryos fromChina andSiberia underwent rapiddiagenetic phosphatization resulting in exquisite preservation, including cell structures.[jargon] 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 suggestsMarkuelia has closest affinity to priapulid worms, and is adjacent to the evolutionary branching ofPriapulida,Nematoda andArthropoda.[47][jargon]
Despite significant advances in uncovering and identifying paleontological specimens, it is generally accepted that the fossil record is vastly incomplete.[48][49] Approaches for measuring the completeness of the fossil record have been developed for numerous subsets of species, including those grouped taxonomically,[50][51] temporally,[52] environmentally/geographically,[53] or in sum.[54][55] This encompasses the subfield oftaphonomy and the study of biases in the paleontological record.[56][57][58]
Stratigraphy of the Montañita-Olón locality of theDos Bocas Formation. Stratigraphy is a useful branch when it comes to the understanding of the successive layers of rock and their fossiliferous content, giving insight into the relative age of fossils
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 forradiometric dating. 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.[59] Although radiometric dating requires careful laboratory work, its basic principle is simple: the rates at which various radioactive elementsdecay 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.[59]
Consequently, palaeontologists rely onstratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is thesedimentary record.[60] 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.[61] Because rock sequences are not continuous, but may be broken up byfaults or periods oferosion, 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 calledbiostratigraphy. For instance, the conodontEoplacognathus pseudoplanus has a short range in the Middle Ordovician period.[62] If rocks of unknown age have traces ofE. pseudoplanus, they have a mid-Ordovician age. Suchindex 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.[63] Stratigraphy and biostratigraphy can in general provide only relative dating (A was beforeB), 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 acrosscontinents.[63] 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 DNAmutations 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 theCambrian explosion first evolved,[64] and estimates produced by different techniques may vary by a factor of two.[65]
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.[66] 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.[67]
A fossil is said to berecrystallized when the original skeletal compounds are still present but in a different crystal form, such as fromaragonite tocalcite.[69]
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. Scientists can use such fossils when researching the anatomical structure of ancient species.[70] Several species of saurids have been identified from mineralized dinosaur fossils.[71][72]
Permineralization
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-richgroundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within thecell wall of aplant cell. Small scale permineralization can produce very detailed fossils.[73] For permineralization to occur, the organism must become covered by sediment soon after death, otherwise the remains are destroyed by scavengers or decomposition.[74] 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 ofskin,feathers or even soft tissues.[75] This is a form ofdiagenesis.
Phosphatization
It involves the process of fossilization where organic matter is replaced by abundantcalcium-phosphateminerals. The produced fossils tend to be particularly dense and have a dark coloration that ranges from dark orange to black.[76]
Pyritization
This fossil preservation involves the elementssulfur andiron. Organisms may become pyritized when they are in marine sediments saturated with iron sulfides. As organic matter decays it releases sulfide which reacts with dissolved iron in the surrounding waters, formingpyrite. Pyrite replaces carbonate shell material due to an undersaturation of carbonate in the surrounding waters. Some plants become pyritized when they are in a clay terrain, but to a lesser extent than in a marine environment. Some pyritized fossils includePrecambrian microfossils, marinearthropods and plants.[77][78]
Insilicification, the precipitation ofsilica from saturated water bodies is crucial for the fossil preservation. The mineral-laden water permeates the pores and cells of some dead organism, where it becomes agel. Over time, the gel will dehydrate, forming asilica-rich crystal structure, which can be expressed in the form ofquartz,chalcedony,agate,opal, among others, with the shape of the original remain.[79][80]
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 anexternal mold. If this void is later filled with sediment, the resultingcast resembles what the organism looked like. Anendocast, orinternal mold, is the result of sediments filling an organism's interior, such as the inside of abivalve orsnail or the hollow of askull.[81] Endocasts are sometimes termedSteinkerns, especially when bivalves are preserved this way.[82]
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 assiderite, 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 CarboniferousMazon Creek fossil beds of Illinois, US, are among the best documented examples of such mineralization.[83]
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 ofdiagenesis. Often what remains is acarbonaceous 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.[84]
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.
Partially coalified axis (branch) of alycopod from the Devonian ofWisconsin.
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.[86][87][88][89] 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 inhaemoglobin with blood vessel tissue they proposed that solution hypoxia coupled with ironchelation enhances the stability and preservation of soft tissue and provides the basis for an explanation for the unforeseen preservation of fossil soft tissues.[90] However, a slightly older study based on eighttaxa ranging in time from theDevonian to theJurassic found that reasonably well-preserved fibrils that probably representcollagen 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.[91] There seemed to be no correlation between geological age and quality of preservation, within that timeframe.
Bioimmuration
The star-shaped holes (Catellocaula vallata) in this Upper Ordovician bryozoan represent a soft-bodied organism preserved by bioimmuration in the bryozoan skeleton.[92]
Bioimmuration occurs when a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton.[93] Usually it is asessile skeletal organism, such as abryozoan or anoyster, which grows along asubstrate, covering other sessilesclerobionts. 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 theOrdovician[94] to the Recent.[93]
Index fossils (also known as guide fossils, indicator fossils or zone fossils) are fossils used to define and identifygeologic periods (or faunal stages). They work on the premise that, although differentsediments may look different depending on the conditions under which they were deposited, they may include the remains of the samespecies 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 fossils consist mainly of tracks and burrows, but also includecoprolites (fossilfeces) and marks left by feeding.[95][96] 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.[97] 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 toearthworms).[96]
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 byWilliam Buckland in 1829. Prior to this they were known as "fossilfir cones" and "bezoar stones." They serve a valuable purpose in paleontology because they provide direct evidence of the predation and diet of extinct organisms.[98] Coprolites may range in size from a few millimetres to over 60 centimetres.
Atransitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.[99] 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.
Microfossil 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 planktersforaminifera andcoccolithophores) or component parts (such as small teeth orspores) of larger animals or plants. Microfossils are of critical importance as a reservoir ofpaleoclimate information, and are also commonly used bybiostratigraphers to assist in the correlation of rock units.
Fossil resin (colloquially calledamber) is a naturalpolymer found in many types of strata throughout the world, even theArctic. The oldest fossil resin dates to theTriassic, though most dates to theCenozoic. The excretion of the resin by certain plants is thought to be an evolutionaryadaptation 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 smallinvertebrates, predominantlyarthropods such as insects and spiders, and only extremely rarely avertebrate such as a small lizard. Preservation of inclusions can be exquisite, including small fragments ofDNA.
ErodedJurassicplesiosaur vertebral centrum found in the LowerCretaceous Faringdon Sponge Gravels in Faringdon, England. An example of aremanié fossil.
Aderived,reworked orremanié fossil is a fossil found in rock that accumulated significantly later than when the fossilized animal or plant died.[100] 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.
Polished section of petrified wood showing annual rings
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 bepetrified. The fossil wood may be the only part of the plant that has been preserved;[101] therefore such wood may get a special kind ofbotanical name. This will usually include "xylon" and a term indicating its presumed affinity, such asAraucarioxylon (wood ofAraucaria or some related genus),Palmoxylon (wood of an indeterminatepalm), orCastanoxylon (wood of an indeterminatechinkapin).[102]
The term subfossil can be used to refer to remains, such as bones, nests, orfecal deposits, whose fossilization process is not complete, either because the length of time since the animal involved was living is too short or because the conditions in which the remains were buried were not optimal for fossilization.[103] Subfossils are often found in caves or other shelters where they can be preserved for thousands of years.[104] The main importance of subfossil vs. fossil remains is that the former contain organic material, which can be used forradiocarbon dating or extraction andsequencing of DNA,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 inpaleoclimatology.
Subfossils are often found in depositionary environments, such as lake sediments, oceanic sediments, and soils. Once deposited, physical and chemicalweathering can alter the state of preservation, and small subfossils can also be ingested by livingorganisms. Subfossil remains that date from theMesozoic are exceptionally rare, are usually in an advanced state of decay, and are consequently much disputed.[105] The vast bulk of subfossil material comes fromQuaternary sediments, including many subfossilizedchironomid head capsules,ostracodcarapaces,diatoms, andforaminifera.
For remains such as molluscanseashells, which frequently do not change their chemical composition over geological time, and may occasionally even retain such features as the original color markings for millions of years, the label 'subfossil' is applied to shells that are understood to be thousands of years old, but are ofHolocene age, and therefore are not old enough to be from thePleistocene epoch.[106]
Chemical fossils, or chemofossils, are chemicals found in rocks andfossil 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.[107] The oldest traces of life on Earth are fossils of this type, including carbon isotope anomalies found inzircons that imply the existence of life as early as 4.1 billion years ago.[12][13]
Stromatolites are layeredaccretionarystructures formed in shallow water by the trapping, binding and cementation of sedimentary grains bybiofilms ofmicroorganisms, especiallycyanobacteria.[108] Stromatolites provide some of the most ancient fossil records of life on Earth, dating back more than 3.5 billion years ago.[109]
A 2009 discovery provides strong evidence of microbial stromatolites extending as far back as 3.45 billion years ago.[111][112]
Stromatolites are a major constituent of the fossil record for life's first 3.5 billion years, peaking about 1.25 billion years ago.[111] They subsequently declined in abundance and diversity,[113] 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 (theCambrian substrate revolution), implying that sufficiently complex organisms were common over 1 billion years ago.[114][115][116]
The connection between grazer and stromatolite abundance is well documented in the youngerOrdovicianevolutionary radiation; stromatolite abundance also increased after theend-Ordovician andend-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered.[117] Fluctuations inmetazoan 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.[118]
Whileprokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for theevolutionary development of more complexeukaryotic organisms. Cyanobacteria (as well asextremophileGammaproteobacteria) are thought to be largely responsible for increasing the amount ofoxygen in the primeval Earth'satmosphere through their continuingphotosynthesis. Cyanobacteria usewater,carbon dioxide andsunlight to create their food. A layer ofmucus 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 oflimestone. 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 termedoncolites are similar to stromatolites and are also known from thefossil record.Thrombolites are poorly laminated or non-laminated clotted structures formed by cyanobacteria common in the fossil record and in modern sediments.[110]
The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of southwesternNamibia 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.[119]
Pseudofossils
An example of a pseudofossil: Manganese dendrites on a limestone bedding plane fromSolnhofen, Germany; scale in mm
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 geologicaldendrite 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) andmoss agates, which look like moss or plant leaves.Concretions, spherical or ovoid-shaped nodules found in some sedimentary strata, were once thought to bedinosaur eggs, and are often mistaken for fossils as well.
Astrobiology
It has been suggested thatbiominerals could be important indicators ofextraterrestrial life and thus could play an important role in the search for past or present life on the planetMars. Furthermore,organic components (biosignatures) that are often associated with biominerals are believed to play crucial roles in both pre-biotic andbiotic reactions.[120]
According to one hypothesis, a Corinthian vase from the 6th century BCE is the oldest artistic record of a vertebrate fossil, perhaps a Miocene giraffe combined with elements from other species.[125] However, a subsequent study using artificial intelligence and expert evaluations reject this idea, because mammals do not have the eye bones shown in the painted monster. Morphologically, the vase painting correspond to a carnivorous reptile of the Varanidae family that still lives in regions occupied by the ancient Greek.[126]
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.[127] The problem is quite pronounced in China, where many specimens have been stolen.[128]
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 intraditional medicine and include the use of fossils astalismans. 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 aplacebo effect, though fossil material might conceivably have someantacid activity or supply someessential minerals.[129] The use of dinosaur bones as "dragon bones" has persisted inTraditional Chinese medicine into modern times, with mid-Cretaceous dinosaur bones being used for the purpose inRuyang County during the early 21st century.[130]
Gallery
Marine fossils found high in the Himalayas. Collection of the Abbot ofDhankar Gompa, HP, India
Three smallammonite fossils, each approximately 1.5 cm across
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![Carbonized fossil of a cycloneuralian worm that was once misidentified as leech[85] from the Silurian Waukesha Biota of Wisconsin.](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aProbable_leech_from_the_Waukesha_Biota.jpg)
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