Acritarchs are organic

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, ...

s, known from approximately 1800 million years ago to the present. The classification is a catch all term used to refer to any organic microfossils that cannot be assigned to other groups. Their diversity reflects major ecological events such as the appearance of predation and the

Cambrian explosion The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil record. ...

Definition

Acritarchs were originally defined as non-

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...

soluble (i.e. non-

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

, non-

siliceous Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

) organic-walled microfossils consisting of a central cavity, and whose biological affinities cannot be determined with certainty. Most commonly they are composed of thermally altered acid insoluble carbon compounds (

kerogen Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...

). Acritarchs may include the remains of a wide range of quite different kinds of organisms—ranging from the egg cases of small

metazoan Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

s to resting cysts of many kinds of

chlorophyta Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to a ...

(green algae). It is likely that most acritarch species from the

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...

represent various stages of the life cycle of algae that were ancestral to the

dinoflagellates The dinoflagellates ( Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates ...

. The nature of the organisms associated with older acritarchs is generally not well understood, though many are probably related to unicellular marine

alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

e. 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. While the

classification Classification is a process related to categorization, the process in which ideas and objects are recognized, differentiated and understood. Classification is the grouping of related facts into classes. It may also refer to: Business, organizat ...

of acritarchs into

form genera Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of s ...

is entirely artificial, it is not without merit, as the form taxa show traits similar to those of genuine

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

—for example an '

explosion An explosion is a rapid expansion in volume associated with an extreme outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Supersonic explosions created by high explosives are known ...

' in the

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

and a

mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. I ...

at the

end End, END, Ending, or variation, may refer to: End *In mathematics: ** End (category theory) ** End (topology) **End (graph theory) ** End (group theory) (a subcase of the previous) **End (endomorphism) *In sports and games **End (gridiron footbal ...

of the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

Classification

Acritarchs were most likely

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

s. While archaea and bacteria (

prokaryotes A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Con ...

) 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. The recent application of

atomic force microscopy Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the opt ...

confocal microscopy Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser confocal scanning microscopy (LCSM), is an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of using a ...

Raman spectroscopy Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman ...

, and other sophisticated analytic techniques to the study of the ultrastructure, life history, and systematic affinities of mineralized, but originally organic-walled microfossils, has shown that some acritarchs are actually fossilized

microalgae Microalgae or microphytes are microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine systems, living in both the water column and sediment. They are unicellular species which exist indiv ...

. 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. CC-BY icon

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Creative Commons Attribution 4.0 International License

Occurrence

Acritarchs are found in sedimentary rocks from the present back into the

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth during the Arc ...

. They are typically isolated from siliciclastic sedimentary rocks using

hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepres ...

but are occasionally extracted from carbonate-rich rocks. They are excellent candidates for index fossils used for dating rock formations in the

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 The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided i ...

microfossils termed "acritarchs" may actually be prokaryotes. The earliest eukaryotic acritarchs known (as of 2020) are from between 1950 and 2150 million years ago.

Diversity

At about 1 billion years ago the organisms responsible for acritarchs started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines. Their populations crashed during periods of extensive worldwide glaciations that covered the majority of the planet, but they proliferated in the Cambrian explosion and reached their highest diversity in the

. The increased spininess 1000 million years ago possibly resulted from the need for defense against predators, especially predators large enough to swallow them or tear them apart. Other groups of small organisms from the

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is prec ...

era also show signs of anti-predator defenses. Further evidence that acritarchs were subject to herbivory around this time comes from a consideration of taxon longevity. The abundance of planktonic organisms that evolved between 1,700 and 1,400 million years ago was limited by nutrient availability – a situation which limits the origination of new species because the existing organisms are so specialised to their niches, and no other niches are available for occupation. Approximately 1,000 million years ago, species longevity fell sharply, suggesting that predation pressure, probably by protist herbivores, became an important factor. Predation would have kept populations in check, meaning that some nutrients were left unused, and new niches were available for new species to occupy.

Etymology

''Acritarch'' was coined in 1963 from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

''ákritos'' meaning ''confused'' (''a kritēs'', ''without critic'') and ''archē'' meaning ''origin'' (confer archaic).

Genera

This is a list of genera according to Fossilid.info. * '' Acanthodiacrodium'' (Ordovician) * '' Acrosphaeridium'' (Ordovician) * '' Actipillion'' (Ordovician) * '' Akomachra'' (Ordovician) * '' Ammonidium'' (Silurian) * '' Aranidium'' (Cambrian) * '' Arbusculidium'' (Cambrian-Ordovician) * '' Archaeodiscina'' (Cambrian) * '' Arcosphaeridium'' (Ordovician) * '' Aremoricanium'' (Ordovician) * '' Arkonia'' (Ordovician) * '' Asteridium'' (Cambrian) * '' Athabascaella'' (Tremadocian, Early Ordovician) * '' Axisphaeridium'' (Ordovician) * '' Bacisphaeridium'' (Ordovician) * '' Baltisphaeridium'' (Cambrian-Silurian) * '' Buedingiisphaeridium'' (Ordovician) * '' Caldariola'' (Cambrian) * '' Calyxiella'' * '' Celtiberium'' * ''

Cephalonyx ''Cephalonyx'' is a genus of filamentous acritarchs known from the Precambrian and early Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambria ...

'' * '' Ceratophyton'' (Cambrian) * ''

Cheleutochroa ''Cheleutochroa'' is an extinct genus of acritarchs from the Ordovician. ''C. elegans'' was recovered from Rapla Rapla (German: ''Rappel'') is a town in central Estonia, the capital of Rapla County and the centre of Rapla Parish. The old ...

'' (Ordovician) * '' Chlamydosphaeridia'' (Ordovician) * '' Comasphaeridium'' (Ordovician) * '' Coronitesta'' (Ordovician) * '' Coryphidium'' (Ordovician) * '' Costatilobus'' (Ordovician) * '' Cristallinium'' (Cambrian) * '' Cycloposphaeridium'' (Ordovician) * '' Cymatiogalea'' (Cambrian) * '' Cymatiosphaera'' (Cambrian-Ordovician) * '' Dactylofusa'' (Ordovician) * '' Dasydiacrodium'' (Cambrian) * '' Dicommopalla'' (Ordovician) * '' Dictyosphaera'' (Paleozoic) * '' Dictyosphaeridium'' (Ordovician) * '' Diexallophasis'' (Silurian) * '' Dilatisphaera'' (Ordovician) * '' Domasia'' (Ordovician-Silurian) * '' Dongyesphaera'' (Paleozoic) * '' Elektroriskos'' (Cambrian) * '' Elenia'' * '' Eliasum'' * '' Estiastra'' (Ordovician) * '' Excultibrachium'' (Ordovician) * '' Fimbriaglomerella'' * '' Florisphaeridium'' (Ordovician) * '' Globosphaeridium'' (Cambrian) * '' Goniosphaeridium'' (Cambrian-Ordovician) * '' Gorgonisphaeridium'' (Ordovician) * '' Granomarginata'' (Cambrian) * '' Gyalorhethium'' (Ordovician) * '' Hapsidopalla'' (Ordovician) * '' Helosphaeridium'' (Ordovician) * '' Impluviculus'' (Cambrian) * '' Introvertocystis'' (Late Cretaceous) * '' Izhoria'' * '' Joehvisphaera'' (Ordovician) * '' Korilophyton'' (Cambrian) * '' Kundasphaera'' (Ordovician) * '' Labyrinthosphaeridium'' (Ordovician) * '' Lacunosphaeridium'' (Ordovician) * '' Ladogella'' * '' Leiofusa'' (Cambrian-Silurian) * '' Leiosphaeridia'' (Cambrian-Silurian) * '' Leiovalia'' (Ordovician) * '' Liepaina'' (Cambrian) * '' Liliosphaeridium'' (Ordovician) * '' Lophosphaeridium'' (Ordovician-Silurian) * '' Lunulidia'' * '' Micrhystridium'' (Ordovician-Silurian) * '' Multiplicisphaeridium'' (Cambiran-Silurian) * '' Nanocyclopia'' (Ordovician) * '' Nodosus'' * '' Oppilatala'' (Silurian) * '' Ordovicidium'' (Ordovician) * '' Orthosphaeridium'' (Ordovician) * '' Ovulum'' (Cambrian) * '' Palaeocladophora'' (Cambrian) * '' Palaeomonostroma'' (Cambrian) * '' Peteinosphaeridium'' (Ordovician) * '' Pheoclosterium'' (Ordovician) * '' Pirea'' (Cambrian) * '' Poikilofusa'' * '' Polyancistrodorus'' (Ordovician) * '' Polyedryxium'' (Ordovician) * '' Polygonium'' (Cambrian-Ordovician) * '' Portalites'' (Permian) * '' Priscogaleata'' (Ordovician) * '' Priscotheca'' * '' Protosphaeridium'' (Silurian) * '' Pterospermella'' * '' Pterospermopsis'' (Ordovician) * '' Pulvinosphaeridium'' (Ordovician) * '' Quadrisporites'' (Permian) * '' Raplasphaera'' (Ordovician) * '' Revinotesta'' (Ordovician) * '' Rhopaliophora'' (Ordovician) * '' Saharidia'' (Ordovician) * '' Salopidium'' (Silurian) * ''

Satka Satka (russian: Са́тка) is a town and the administrative center of Satkinsky District in Chelyabinsk Oblast, Russia, located on the western slope of the Southern Ural Mountains on the bank of the Satka River, from Chelyabinsk, the administra ...

'' (Paleozoic) * '' Skiagia'' (Cambrian) * '' Solisphaeridium'' (Ordovician) * '' Stellechinatum'' (Ordovician) * '' Stelliferidium'' (Cambrian) * '' Taeniosphaeridium'' (Ordovician) * '' Tasmanites'' (Ordovician-Silurian) * '' Tetraporina'' (Permian) * '' Timofeevia'' (Cambrian-Ordovician) * '' Tranvikium'' (Ordovician) * '' Trichosphaeridium'' * '' Tunisphaeridium'' (Ordovician-Silurian) * '' Tylotopalla'' (Ordovician) * '' Vernanimalcula'' (Pre-Cambrian) * '' Veryhachium'' (Cambrian-Silurian) * '' Villosacapsula'' (Ordovician) * '' Visbysphaera'' (Silurian) * '' Volkovia'' * '' Vulcanisphaera'' (Cambrian-Ordovician) * '' Winwaloeusia'' (Ordovician)

References

External links

CIMP Subcommission on Acritarchs
Commission Internationale de Microflore du Paléozoique (CIMP), international commission for Palaeozoic palynology. * * The American Association of Stratigraphic Palynologists (AASP) {{Taxonbar, from=Q342747 Acritarchs Incertae sedis