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A microfossil is a fossil that is generally between one micrometre and one millimetre 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, such as a hand lens, is referred to as a macrofossil.
Microfossils are a common feature of the geological record, from the Precambrian to the Holocene. They are most common in deposits of ocean environments, but also occur in brackish water, fresh water and terrestrial sedimentary deposits. While every kingdom of prehistoric life is represented in the microfossil record, the most abundant forms are protist skeletons or from the Chrysophyta, Pyrrhophyta, Sarcodina, and , together with pollen and spores from the .
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 include pollen and . Marine microfossils found in are the most common microfossils. Everywhere in the oceans, microscopic Marine protists multiply prolifically, and many grow Protist shell which readily fossilise. These include foraminifera, and . (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.Campbell, Hamish (12 Jun 2006) "Fossils - Microfossils", Te Ara - the Encyclopedia of New Zealand. Accessed 11 May 2021.
Some microfossils are formed by colonial organisms such as Bryozoa (especially the Cheilostomata), which have relatively large colonies but are classified by fine skeletal details of the small individuals of the colony. As another example, many fossil genus of Foraminifera, which are protists are known from shells (called tests) that were as big as coins, such as the genus Nummulites.
In 2017, fossilized , or microfossils, were discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada that may be as old as 4.28 billion years old, the oldest record of life on Earth, suggesting "an almost instantaneous emergence of life" (in a geological time-scale), after ocean formation 4.41 billion years ago, and not long after the formation of the Earth 4.54 billion years ago. Nonetheless, life may have started even earlier, at nearly 4.5 billion years ago, as claimed by some researchers.
Index fossils were originally used to define and identify geologic units, then became a basis for defining geologic column, and then for faunal stages and zones.
Species of microfossils such as acritarchs, , , dinoflagellate cysts, ostracods, pollen, spores and are amongst the many species have been identified as index fossils that are widely used in biostratigraphy. 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 a faunal 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 the stratigraphic layers is possible.
Common from the Ordovician to Devonian periods (i.e. the mid-Paleozoic), the millimetre-scale organisms are abundant in almost all types of marine sediment across the globe. This wide distribution, and their rapid pace of evolution, makes them valuable biostratigraphic markers.
Their bizarre form has made classification and ecological reconstruction difficult. Since their discovery in 1931, suggestions of protist, plant, and fungus 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 the eggs or juvenile stage of a marine animal. However, recent research has suggested that they represent the test of a group of protists with uncertain affinities.
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.
Acritarch diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion. Precambrian marine diversity was dominated by acritarchs. They underwent a boom around , 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.
Acritarchs may include the remains of a wide range of quite different kinds of organisms—ranging from the egg cases of small to resting cysts of many kinds of chlorophyta (green algae). It is likely that most acritarch species from the Paleozoic represent various stages of the life cycle of algae that were ancestral to the dinoflagellates. The nature of the organisms associated with older acritarchs is generally not well understood, though many are probably related to unicellular marine . 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 likely . 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.
Acritarchs are found in sedimentary rocks from the present back into the Archean. They are typically isolated from siliciclastic sedimentary rocks using hydrofluoric acid but are occasionally extracted from carbonate-rich rocks. They are excellent candidates for index fossils used for dating rock formations in the Palaeozoic 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 earliest Proterozoic microfossils termed "acritarchs" may actually be prokaryotes. The earliest eukaryotic acritarchs known (as of 2020) are from between 1950 and 2150 million years ago.
Recent application of atomic 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, have shown some acritarchs are fossilized microalgae. 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. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
| Silicon oxide silica quartz glass opal chert | diatom | frustule | 0.002 to 0.2 mm (1996). 9780080534411, Academic Press. ISBN 9780080534411 | diatom microfossil from 40 million years ago | ||
| radiolarian | test or shell | 0.1 to 0.2 mm | elaborate silica shell of a radiolarian | |||
Phytoliths (Greek for plant stones) are rigid, microscopic structures made of silica, 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.Piperno, Dolores R. (2006). Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. AltaMira Press .
| CaCO3 calcite aragonite limestone marble chalk | foraminiferan | test or shell | many under 1 mm | Calcified test of a planktic foraminiferan. There are about 10,000 living species of foraminiferans | ||
| coccolithophore | under 0.1 mm | Coccolithophores are the largest global source of biogenic calcium carbonate, and significantly contribute to the global carbon cycle. They are the main constituent of chalk deposits such as the white cliffs of Dover. | ||||
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.
Conodonts are globally widespread in sediments.Their many forms are considered , 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 for hydrocarbon exploration. Study of microfossils maps extreme global warming and environmental change Phys.org, 7 August 2019. Conodont tooth are the earliest vertebrate teeth found in the fossil record, and some conodont teeth are the sharpest that have ever been recorded. Scientists Discover Sharpest Teeth in History Sci-News.com, 20 March 2012.
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 microbial stromatolites, which are limited to shallow water, and it has been suggested that cloudinids lived embedded in the , 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.
The classification of the cloudinids has proved difficult: they were initially regarded as polychaete worms, and then as coral-like on the basis of what look like Budding 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 of type guts, the oldest on record, supporting the interpretation.
Cloudinids are important in the history of animal evolution for two reasons. They are among the earliest and most abundant of the small shelly fossils with mineralized , 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 some Cloudina 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 the Cloudina specimens, and Sinotubulites 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 the evolutionary arms race which this indicates is commonly cited as a cause of the Cambrian explosion of animal Biodiversity and complexity.
Dinocysts are produced by a proportion of as a Dormancy, 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. Dinocysts have a long geological record with geochemical markers suggest a presence that goes back to the Early Cambrian.
Smaller, microscopic spicules can become microfossils, and are referred to as microscleres. Larger spicules visible to the naked eye are called megascleres. Spicule can be calcareous, Biogenic silica, or composed of spongin. They are found in a range of symmetry types.
Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion of rocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.
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