| Acritarchs | |
|---|---|
 | |
| A supposedEdiacaranembryo contained within an acritarch from theDoushantuo Formation | |
Scientific classification | |
| Domain: | Eukaryota |
| (unranked): | †Acritarcha Evitt, 1963 |
| Synonyms | |
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Acritarchs (from Greek achritos 'uncertain' and arché 'origin')[2] are organicmicrofossils, known from theArchean eon of thePrecambrian to the present.[3] 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 theCambrian explosion. Many acritarchs likely representresting cysts of single-celled marinephytoplanktonicalgae, similar to those produced by livingdinoflagellates.[4]
Acritarchs were originally defined as non-acid soluble (i.e. non-carbonate, non-siliceous) organic-walled microfossils consisting of a central cavity, and whose biological affinities cannot be determined with certainty.[5][6][7] Most commonly they are composed of thermally altered acid-insoluble carbon compounds (kerogen).
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.[8] The nature of the organisms associated withPrecambrian 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.[citation needed]
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 an 'explosion' in theCambrian and amass extinction at theend of thePermian.[citation needed]
The informal group Acritarcha Evitt 1963 was originally divided into these Subgroups: Acanthomorphitae, Polygonomorphitae, Prismatomorphitae, Oömorphitae, Netromorphitae, Dinetromorphitae, Stephanomorphitae, Pteromorphitae, Herkomorphitae, Platymorphitae, Sphaeromorphitae, and Disphaeromorphitae.[9][10][11]
Acritarchs were most likelyeukaryotes. While archaea and bacteria (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.[12]
The recent application ofatomic force microscopy,confocal microscopy,Raman spectroscopy, and other sophisticated analytic techniques to the study of the ultrastructure, life history, and systematic affinities of mineralized, but originally organic-walled microfossils,[13][14][15][16][17] has shown that some acritarchs are actually 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.[18][19]
Acritarchs are found in sedimentary rocks from the present back into theArchean.[3] 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[update]) are from between 1950 and 2150 million years ago.[20]
At about 1 billion years ago the organisms responsible for acritarchs started to increase in abundance, diversity, size, complexity of shape, and especially, the 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 thePaleozoic. 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 theNeoproterozoic era also show signs of anti-predator defenses.[21]
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.[22]
Acritarch was coined in 1963 from theGreekákritos meaningconfused (a kritēs,without critic) andarchē meaningorigin (conferarchaic).[23]
There are over 900 described genera; the following is a partial list, from Fossilid.info.[24]
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