Amicrofossil is afossil that is generally between onemicrometre and onemillimetre in size,[2] the visual study of which requires the use of light or electronmicroscopy. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as amacrofossil.
Microfossils are a common feature of thegeological record, from thePrecambrian to theHolocene. They are most common in deposits ofmarine environments, but also occur in brackish water, fresh water and terrestrialsedimentary deposits. While everykingdom oflife is represented in the microfossil record, the most abundant forms areprotist skeletons ormicrobial cysts from theChrysophyta,Pyrrhophyta,Sarcodina,acritarchs andchitinozoans, together withpollen andspores from thevascular plants.
| Part of a series on |
| Paleontology |
|---|
 |
A 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.
Microfossils are found in rocks and sediments as the microscopic remains of what were once life forms such as plants, animals, fungus, protists, bacteria and archaea. Terrestrial microfossils includepollen andspores. Marine microfossils found inmarine sediments are the most common microfossils. Everywhere in the oceans, microscopicprotist organisms multiply prolifically, and many growtiny skeletons which readily fossilise. These includeforaminifera,dinoflagellates andradiolarians.Palaeontologists (geologists who study fossils) are interested in these microfossils because they can use them to determine how environments and climates have changed in the past, and where oil and gas can be found today.[3]
Some microfossils are formed bycolonial organisms such asBryozoa (especially theCheilostomata), which have relatively largecolonies but are classified by fine skeletal details of the small individuals of the colony. As another example, many fossilgenera ofForaminifera, which are protists are known from shells (calledtests) that were as big as coins, such as the genusNummulites.
In 2017, fossilizedmicroorganisms, or microfossils, were discovered inhydrothermal ventprecipitates in theNuvvuagittuq Belt of Quebec, Canada that may be as old as 4.28 billion years old, theoldest record of life on Earth, suggesting "an almost instantaneous emergence of life" (in a geological time-scale), afterocean formation 4.41 billion years ago, and not long after theformation of the Earth 4.54 billion years ago.[4][5][6][7] Nonetheless, life may have started even earlier, at nearly 4.5 billion years ago, as claimed by some researchers.[8][9]
| Part of a series related to |
| Biomineralization |
|---|
 |
Teeth, scales, tusks etc |
Index fossils, also known as guide fossils, indicator fossils or dating fossils, are the fossilized remains or traces of particular plants or animals that are characteristic of a particular span of geologic time or environment, and can be used to identify and date the containing rocks. To be practical, index fossils must have a limited vertical time range, wide geographic distribution, and rapid evolutionary trends. Rock formations separated by great distances but containing the same index fossil species are thereby known to have both formed during the limited time that the species lived.
Index fossils were originally used to define and identify geologic units, then became a basis for defininggeologic periods, and then for faunal stages and zones.
Species ofmicrofossils such asacritarchs,chitinozoans,conodonts,dinoflagellate cysts,ostracods,pollen,spores andforaminiferans are amongst the many species have been identified as index fossils that are widely used inbiostratigraphy. Different fossils work well for sediments of different ages. To work well, the fossils used must be widespread geographically, so that they can be found in many different places. They must also be short lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the poorer the stratigraphic precision, so fossils that evolve rapidly.
Often biostratigraphic correlations are based on afaunal assemblage, rather than an individual species — this allows greater precision as the time span in which all of the species in the assemblage existed together is narrower than the time spans of any of the members. Further, if only one species is present in a sample, it can mean either that (1) the strata were formed in the known fossil range of that organism; or (2) that the fossil range of the organism was incompletely known, and the strata extend the known fossil range. If the fossil is easy to preserve and easy to identify, more precise time estimating of thestratigraphic layers is possible.
Microfossils can be classified by their composition as: (a)siliceous, as indiatoms andradiolaria, (b)calcareous, as incoccoliths andforaminifera, (c)phosphatic, as in the study of somevertebrates, or (d)organic, as in thepollen andspores studied inpalynology. This division focuses on differences in the mineralogical and chemical composition of microfossil remains rather than ontaxonomic orecological distinctions.
Pollen has an outer sheath, called asporopollenin, which affords it some resistance to the rigours of the fossilisation process that destroy weaker objects. It is produced in huge quantities. There is an extensive fossil record of pollen grains, often disassociated from their parent plant. The discipline ofpalynology is devoted to the study of pollen, which can be used both for biostratigraphy and to gain information about the abundance and variety of plants alive — which can itself yield important information about paleoclimates. Also, pollen analysis has been widely used for reconstructing past changes in vegetation and their associated drivers.[11]Pollen is first found in thefossil record in the lateDevonian period,[12][13] but at that time it is indistinguishable from spores.[12] It increases in abundance until the present day.
Aspore is a unit ofsexual orasexual reproduction that may be adapted fordispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of thelife cycles of manyplants,algae,fungi andprotozoa.[14]Bacterial spores are not part of a sexual cycle but are resistant structures used for survival under unfavourable conditions.
Chitinozoa are ataxon offlask-shaped,organic walledmarine microfossils produced by an as-yet-unknown organism.[15]
Common from theOrdovician toDevonian periods (i.e. the mid-Paleozoic), the millimetre-scale organisms are abundant in almost all types ofmarine sediment across the globe.[16] This wide distribution, and their rapid pace of evolution, makes them valuablebiostratigraphic markers.
Their bizarre form has madeclassification and ecological reconstruction difficult. Since their discovery in 1931, suggestions ofprotist,plant, andfungal affinities have all been entertained. The organisms have been better understood as improvements in microscopy facilitated the study of their fine structure, and it has been suggested that they represent either theeggs or juvenile stage of a marine animal.[17] However, recent research has suggested that they represent thetest of a group of protists with uncertain affinities.[18]
The ecology of chitinozoa is also open to speculation; some may have floated in the water column, where others may have attached themselves to other organisms. Most species were particular about their living conditions, and tend to be most common in specific paleoenvironments. Their abundance also varied with the seasons.
Acritarchs, Greek forconfused origins,[20] are organic-walled microfossils, known from about2,000 million years ago to the present. Acritarchs are not a specific biological taxon, but rather a group with uncertain or unknown affinities.[21][22][23] Most commonly they are composed of thermally altered acid insoluble carbon compounds (kerogen). While theclassification of acritarchs intoform genera is entirely artificial, it is not without merit, as the form taxa show traits similar to those of genuinetaxa — for example the 'explosion' in theCambrian and themass extinction at theend of thePermian.
Acritarch diversity reflects major ecological events such as the appearance of predation and theCambrian explosion. Precambrian marine diversity was dominated by acritarchs. They underwent a boom around1,000 million years ago, increasing in abundance, diversity, size, complexity of shape, and especially size and number of spines. Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation.[24]
Acritarchs may include the remains of a wide range of quite different kinds of organisms—ranging from the egg cases of smallmetazoans to resting cysts of many kinds ofchlorophyta (green algae). It is likely that most acritarch species from thePaleozoic represent various stages of the life cycle of algae that were ancestral to thedinoflagellates.[25] The nature of the organisms associated with older acritarchs is generally not well understood, though many are probably related to unicellular marinealgae. In theory, when the biological source (taxon) of an acritarch does become known, that particular microfossil is removed from the acritarchs and classified with its proper group.
Acritarchs were most likelyeukaryotes. While archaea, bacteria and cyanobacteria (prokaryotes) usually produce simple fossils of a very small size, eukaryotic unicellular fossils are usually larger and more complex, with external morphological projections and ornamentation such as spines and hairs that only eukaryotes can produce; as most acritarchs have external projections (e.g., hair, spines, thick cell membranes, etc.), they are predominantly eukaryotes, although simple eukaryote acritarchs also exist.[26]
Acritarchs are found in sedimentary rocks from the present back into theArchean.[27] They are typically isolated from siliciclastic sedimentary rocks usinghydrofluoric acid but are occasionally extracted from carbonate-rich rocks. They are excellent candidates for index fossils used for dating rock formations in thePaleozoic Era and when other fossils are not available. Because most acritarchs are thought to be marine (pre-Triassic), they are also useful for palaeoenvironmental interpretation. The Archean and earliestProterozoic microfossils termed "acritarchs" may actually be prokaryotes. The earliest eukaryotic acritarchs known (as of 2020) are from between 1950 and 2150 million years ago.[28]
Recent application ofatomic force microscopy,confocal microscopy,Raman spectroscopy, and other analytic techniques to the study of the ultrastructure, life history, and systematic affinities of mineralized, but originally organic-walled microfossils,[29][30][31][32][33] have shown some acritarchs are fossilizedmicroalgae. In the end, it may well be, as Moczydłowska et al. suggested in 2011, that many acritarchs will, in fact, turn out to be algae.[34][35]
Cells can be preserved in therock record because their cell walls are made of proteins which convert to the organic materialkerogen as the cell breaks down after death. Kerogen isinsoluble in mineralacids,bases, andorganic solvents.[36] Over time, it is mineralised intographite or graphite-likecarbon, or degrades into oil and gas hydrocarbons.[37] There are three main types of cell morphologies. Though there is no established range of sizes for each type, spheroid microfossils can be as small as about 8 micrometres, filamentous microfossils have diameters typically less than 5 micrometres and have a length that can range from tens of micrometres to 100 micrometres, and spindle-like microfossils can be as long as 50 micrometres.[38][39]
Siliceous ooze is a type of biogenicpelagic sediment located on thedeepocean floor. Siliceous oozes are the least common of the deep sea sediments, and make up approximately 15% of the ocean floor.[40] Oozes are defined as sediments which contain at least 30% skeletal remains of pelagic microorganisms.[41] Siliceous oozes are largely composed of the silica based skeletons of microscopic marine organisms such asdiatoms andradiolarians. Other components of siliceous oozes near continental margins may include terrestrially derived silica particles and sponge spicules. Siliceous oozes are composed of skeletons made from opal silicaSi(O2), as opposed tocalcareous oozes, which are made from skeletons of calcium carbonate organisms (i.e.coccolithophores). Silica (Si) is a bioessential element and is efficiently recycled in the marine environment through thesilica cycle.[42] Distance from land masses, water depth and ocean fertility are all factors that affect the opal silica content in seawater and the presence of siliceous oozes.
| mineral forms | protist involved | name of skeleton | typical size | ||||
|---|---|---|---|---|---|---|---|
| SiO2 silica quartz glass opal chert | diatom |  | frustule | 0.002 to 0.2 mm [43] |  | diatom microfossil from 40 million years ago | |
| radiolarian | .jpg) | test or shell | 0.1 to 0.2 mm | %2c_Haeckel_(28187768550).jpg) | elaborate silica shell of a radiolarian | ||
.jpg)
.jpg)
Phytoliths (Greek forplant stones) are rigid, microscopic structures made ofsilica, found in some plant tissues and persisting after the decay of the plant. These plants take up silica from the soil, whereupon it is deposited within different intracellular and extracellular structures of the plant. Phytoliths come in varying shapes and sizes. The term "phytolith" is sometimes used to refer to all mineral secretions by plants, but more commonly refers to siliceous plant remains.[45]
The termcalcareous can be applied to a fossil, sediment, or sedimentary rock which is formed from, or contains a high proportion of,calcium carbonate in the form ofcalcite oraragonite. Calcareous sediments (limestone) are usually deposited in shallow water near land, since the carbonate is precipitated by marine organisms that need land-derived nutrients. Generally speaking, the farther from land sediments fall, the less calcareous they are. Some areas can have interbedded calcareous sediments due to storms, or changes in ocean currents.Calcareous ooze is a form of calcium carbonate derived from planktonic organisms that accumulates on thesea floor. This can only occur if the ocean is shallower than thecarbonate compensation depth. Below this depth, calcium carbonate begins to dissolve in the ocean, and only non-calcareous sediments are stable, such assiliceous ooze orpelagic red clay.
Calcareous ooze | |||||||
|---|---|---|---|---|---|---|---|
| mineral forms | protist involved | name of skeleton | typical size | ||||
| CaCO3 calcite aragonite limestone marble chalk | foraminiferan |  | test or shell | many under 1 mm |  | Calcifiedtest of a planktic foraminiferan. There are about 10,000 living species of foraminiferans[46] | |
| coccolithophore |  | coccoliths | under 0.1 mm [47] |  | Coccolithophores are the largest global source of biogenic calcium carbonate, and significantly contribute to the globalcarbon cycle.[48] They are the main constituent of chalk deposits such as thewhite cliffs of Dover. | ||
Ostracods are widespread crustaceans, generally small, sometimes known asseed shrimps. They are flattened from side to side and protected with a calcareous or chitinousbivalve-like shell. There are about 70,000 known species, 13,000 of which areextant.[51] Ostracods are typically about 1 mm (0.039 in) in size, though they can range from 0.2 to 30 mm (0.008 to 1.181 in), with some species such asGigantocypris being too large to be regarded as microfossils.
_of_the_Illinois_basin_(1958)_(20654535006).jpg)
Conodonts (cone tooth in Greek) are tiny, extinct jawless fish that resemble eels. For many years, they were known only from tooth-like microfossils found in isolation and now called conodont elements. The evolution ofmineralized tissues has been a puzzle for more than a century. It has been hypothesized that the first mechanism of chordate tissue mineralization began either in the oral skeleton of conodont or the dermal skeleton of earlyagnathans.[52] Conodont elements are made of a phosphatic mineral,hydroxylapatite.[53]
The element array constituted a feeding apparatus that is radically different from the jaws of modern animals. They are now termed "conodont elements" to avoid confusion. The three forms of teeth (i.e., coniform cones, ramiform bars, and pectiniform platforms) probably performed different functions. For many years, conodonts were known only from enigmatic tooth-like microfossils (200 micrometres to 5 millimetres in length) which occur commonly, but not always in isolation, and were not associated with any other fossil.[54]
Conodonts are globally widespread in sediments.Their many forms are consideredindex fossils, fossils used to define and identify geological periods and date strata. Conodonts elements can be used to estimate the temperatures rocks have been exposed to, which allows the thermal maturation levels of sedimentary rocks to be determined, which is important forhydrocarbon exploration.[55][56] Conodontteeth are the earliest vertebrate teeth found in the fossil record,[52] and some conodont teeth are the sharpest that have ever been recorded.[57][58]
Scolecodonts (worm jaws in Latin) are tiny jaws ofpolychaeteannelids of the orderEunicida - a diverse and abundant group of worms which has been inhabiting different marine environments in the past 500 million years. Composed of highly resistant organic substance, the scolecodonts are frequently found as fossils from the rocks as old as the lateCambrian. Since the worms themselves were soft-bodied and hence extremely rarely preserved in the fossil record, their jaws constitute the main evidence of polychaetes in the geological past, and the only way to restore the evolution of this important group of animals. Small size of scolecodonts, usually less than 1 mm, puts them into a microfossil category. They are common by-product of conodont,chitinozoan andacritarch samples, but sometimes they occur in the sediments where other fossils are very rare or absent.[59]
Thecloudinids were an earlymetazoanfamily that lived in the lateEdiacaranperiod about 550 million years ago,[60][61] and became extinct at the base of theCambrian.[62] They formed small millimetre size conical fossils consisting ofcalcareous cones nested within one another; the appearance of the organism itself remains unknown. The nameCloudina honorsPreston Cloud.[63] Fossils consist of a series of stacked vase-likecalcite tubes, whose original mineral composition is unknown,[64] Cloudinids comprise two genera:Cloudina itself is mineralized, whereasConotubus is at best weakly mineralized, whilst sharing the same "funnel-in-funnel" construction.[65]
Cloudinids had a wide geographic range, reflected in the present distribution of localities in which their fossils are found, and are an abundant component of some deposits.Cloudina is usually found in association with microbialstromatolites, which are limited to shallow water, and it has been suggested that cloudinids lived embedded in themicrobial mats, growing new cones to avoid being buried by silt. However no specimens have been found embedded in mats, and their mode of life is still an unresolved question.
Theclassification of the cloudinids has proved difficult: they were initially regarded aspolychaete worms, and then as coral-likecnidarians on the basis of what look likebuds on some specimens. Current scientific opinion is divided between classifying them as polychaetes and regarding it as unsafe to classify them as members of any broader grouping. In 2020, a new study showed the presence ofNephrozoan typeguts, the oldest on record, supporting thebilaterian interpretation.[61]
Cloudinids are important in the history of animal evolution for two reasons. They are among the earliest and most abundant of thesmall shelly fossils withmineralizedskeletons, and therefore feature in the debate about why such skeletons first appeared in the Late Ediacaran. The most widely supported answer is that their shells are a defense against predators, as someCloudina specimens from China bear the marks of multiple attacks, which suggests they survived at least a few of them. The holes made by predators are approximately proportional to the size of theCloudina specimens, andSinotubulites fossils, which are often found in the same beds, have so far shown no such holes. These two points suggest that predators attacked in a selective manner, and theevolutionary arms race which this indicates is commonly cited as a cause of theCambrian explosion of animaldiversity and complexity.
Some dinoflagellates produceresting stages, called dinoflagellate cysts ordinocysts, as part of their lifecycles. Dinoflagellates are mainly represented in the fossil record by these dinocysts, typically 15 to 100 micrometres in diameter, which accumulate in sediments as microfossils. Organic-walled dinocysts have resistant cell walls made out ofdinosporin. There are alsocalcareous dinoflagellate cysts andsiliceous dinoflagellate cysts.
Dinocysts are produced by a proportion ofdinoflagellates as adormant,zygotic stage of their lifecycle. These dinocyst stages are known to occur in 84 of the 350 described freshwater dinoflagellate species, and in about 10% of the known marine species.[66][67] Dinocysts have a long geological record with geochemical markers suggest a presence that goes back to theEarly Cambrian.[68]
Spicules are structural elements found in mostsponges. They provide structural support and deterpredators.[69] The meshing of many spicules serves as the sponge'sskeleton, providing structural support and defense against predators.
Smaller,microscopic spicules can become microfossils, and are referred to asmicroscleres. Larger spicules visible to the naked eye are calledmegascleres. Spicule can becalcareous,siliceous, or composed ofspongin. They are found in a range of symmetry types.
.jpg)
Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous.
Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion ofrocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.
Biogenous sediments account for the other 55% of the total sediment, and originate in the skeletal remains ofmarine protists (single-celled plankton and benthos microorganisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present.Ooze, in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used byJohn Murray, the "father of modern oceanography", who proposed the termradiolarian ooze for the silica deposits of radiolarian shells brought to the surface during theChallenger expedition.[70] Abiogenic ooze is apelagic sediment containing at least 30 per cent from the skeletal remains of marine organisms.
![Opal can include microfossil diatoms, radiolarians, silicoflagellates and ebridians[71]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCoober_Pedy_Opal_Doublet.jpg)
.jpg)
The study of microfossils is calledmicropaleontology. In micropaleontology, what would otherwise be distinct categories are grouped together based solely on their size, including microscopic organisms and minute parts of larger organisms. Numerous sediments have microfossils, which serve as significantbiostratigraphic,paleoenvironmental, and paleoceanographic markers.[73] Their widespread presence around the world and physical toughness makes microfossils important for biostratigraphy, while the manner in which they have reacted to environmental changes makes them helpful when reconstructing past environments.[74]
Material was copied from this source, which is available under aCreative Commons Attribution 4.0 International License.
.jpg)
{{citation}}: CS1 maint: work parameter with ISBN (link) Material was copied from this source, which is available under aCreative Commons Attribution 4.0 International License.{{cite book}}: CS1 maint: location missing publisher (link){{cite book}}: CS1 maint: location missing publisher (link){{cite book}}: CS1 maint: location missing publisher (link)