Acritarch

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Acritarchs
Temporal range: 1600–0 Ma
Doushantou Embryo Yinetal2007.jpg
A supposed Ediacaran embryo contained within an acritarch from the Doushantuo Formation
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: incertae sedis
(unranked):† Acritarcha
Evitt, 1963
Synonyms
  • Hystrichospheres p.p.
Acritarch from the Weng'an biota c. 570-609 mya Acritarch from the Weng'an biota.jpg
Acritarch from the Weng'an biota
c. 570–609 mya  

Acritarchs are organic microfossils, known from approximately 1800 million years ago to the present.[ inconsistent ] 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.

Contents

Definition

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. [2] [3] [4] 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 small metazoans 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. [5] The nature of the organisms associated with Precambrian acritarchs is generally not well understood, though many are probably related to unicellular marine algae. 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 the classification of acritarchs into form genera is entirely artificial, it is not without merit, as the form taxa show traits similar to those of genuine taxa—for example an 'explosion' in the Cambrian and a mass extinction at the end of the Permian.[ citation needed ]

Classification

The informal group, Acritarcha Evitt 1963, was originally divided into the Subgroups: Acanthomorphitae, Polygonomorphitae, Prismatomorphitae, Oömorphitae, Netromorphitae, Dinetromorphitae, Stephanomorphitae, Pteromorphitae, Herkomorphitae, Platymorphitae, Sphaeromorphitae, and Disphaeromorphitae. [6] [7] [8]

Acritarchs were most likely eukaryotes. 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. [9]

The recent application of atomic 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, [10] [11] [12] [13] [14] has shown that some acritarchs are actually 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. [15] [16]

Occurrence

Acritarchs are found in sedimentary rocks from the present back into the Archean. [17] 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 Paleozoic 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. [18]

Diversity

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 the Paleozoic. 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 era also show signs of anti-predator defenses. [19]

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. [20]

Etymology

Acritarch was coined in 1963 from the Greek ákritos meaning confused (a kritēs, without critic) and archē meaning origin (confer archaic). [21]

Selected genera

There are over 900 described genera; the following is a partial list, from Fossilid.info. [22]

See also

Related Research Articles

<span class="mw-page-title-main">Cambrian</span> First period of the Paleozoic Era

The Cambrian is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma to the beginning of the Ordovician Period 486.85 Ma.

<span class="mw-page-title-main">Ediacaran</span> Third and last period of the Neoproterozoic Era

The Ediacaran is a geological period of the Neoproterozoic Era that spans 96 million years from the end of the Cryogenian Period at 635 Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last period of the Proterozoic Eon as well as the last of the so-called "Precambrian supereon", before the beginning of the subsequent Cambrian Period marks the start of the Phanerozoic Eon, where recognizable fossil evidence of life becomes common.

The PaleozoicEra is the first of three geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic and ends 251.9 Ma at the start of the Mesozoic Era. The Paleozoic is subdivided into six geologic periods, Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. Some geological timescales divide the Paleozoic informally into early and late sub-eras: the Early Paleozoic consisting of the Cambrian, Ordovician and Silurian; the Late Paleozoic consisting of the Devonian, Carboniferous and Permian.

<span class="mw-page-title-main">Proterozoic</span> Geologic eon, 2500–539 million years ago

The Proterozoic is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozoic, and is the most recent part of the Precambrian "supereon".

<span class="mw-page-title-main">Doushantuo Formation</span> Fossil formation in south-central China

The Doushantuo Formation is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China. It is known for the fossil Lagerstätten in Zigui in Hubei, Xiuning in Anhui, and Weng'an in Guizhou, as one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic they have given their name to "Doushantuo type preservation". The formation, whose deposits date back to the Early and Middle Ediacaran, is of particular interest because it covers the poorly understood interval of time between the end of the Cryogenian geological period and the more familiar fauna of the Late Ediacaran Avalon explosion, as well as due to its microfossils' potential utility as biostratigraphical markers. Taken as a whole, the Doushantuo Formation ranges from about 635 Ma at its base to about 551 Ma at its top, with the most fossiliferous layer predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon Peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion at the beginning of the Phanerozoic.

<span class="mw-page-title-main">Micropaleontology</span> Branch of paleontology that studies microfossils

Micropaleontology is the branch of paleontology (palaeontology) that studies microfossils, or fossils that require the use of a microscope to see the organism, its morphology and its characteristic details.

<span class="mw-page-title-main">Chitinozoan</span> Group of marine microfossils

Chitinozoa are a group of flask-shaped, organic walled marine microfossils produced by an as yet unknown organism. Common from the Ordovician to Devonian periods, 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.

<span class="mw-page-title-main">Microfossil</span> Fossil that requires the use of a microscope to see it

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, such as a hand lens, is referred to as a macrofossil.

Oldhamia is an ichnogenus describing burrows produced by worm-like organisms mining underneath microbial mats. It was common from the Early Cambrian deep-water deposits.

The end-Ediacaran extinction is a mass extinction believed to have occurred near the end of the Ediacaran period, the final period of the Proterozoic eon. Evidence suggesting that such a mass extinction occurred includes a massive reduction in diversity of acritarchs, the sudden disappearance of the Ediacara biota and calcifying organisms, and the time gap before Cambrian organisms "replaced" them. Some lines of evidence suggests that there may have been two distinct pulses of the extinction event, one occurring 550 million years ago and the other 539 million years ago.

The Cambrian explosion is an interval of time beginning approximately 538.8 million years ago in the Cambrian period of the early Paleozoic, when a sudden radiation of complex life occurred and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 to 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.

Large ornamented Ediacaran microfossils are microscopic acritarchs, usually over 100 μm in diameter, which are common in sediments of the Ediacaran period, 635 to 538.8 million years ago. They largely disappear from the Ediacaran period fossil record before 560 million years ago, roughly coeval with the origin of the Ediacara biota. They differ from Palaeozoic microfossils in many respects; they are larger, often have internal contents, have a differently-constructed cell wall, and differ in shape.

<span class="mw-page-title-main">Small carbonaceous fossil</span>

Small carbonaceous fossils (SCFs) are sub-millimetric organic remains of organisms preserved in sedimentary strata.

<span class="mw-page-title-main">Francevillian biota</span> Possible Palaeoproterozoic multicellular fossils from Gabon

The Francevillian biota are a collection of 2.1-billion-year-old Palaeoproterozoic macroscopic structures, controversially suggested to be fossils, known from the Francevillian B Formation in Gabon, a black shale province notable for its lack of any noticeable metamorphism. The structures have been postulated by some authors to be evidence of the earliest form of multicellular life, and of eukaryotes. They were discovered by an international team led by Moroccan-French geologist Abderrazak El Albani, of the University of Poitiers, France. While they have yet to be assigned to a formal taxonomic position, they have been informally and collectively referred to as the "Gabonionta", including by the Natural History Museum Vienna in 2014. The status of the structures as fossils has been questioned, and they remain a subject of debate.

Ceratophyton is a genus of Cambrian acritarch, around 100–200 μm in length, produced by a eukaryotic (metazoan?) organism.

Paleontology or palaeontology is the study of prehistoric life forms on Earth through the examination of plant and animal fossils. This includes the study of body fossils, tracks (ichnites), burrows, cast-off parts, fossilised feces (coprolites), palynomorphs and chemical residues. Because humans have encountered fossils for millennia, paleontology has a long history both before and after becoming formalized as a science. This article records significant discoveries and events related to paleontology that occurred or were published in the year 2016.

<span class="mw-page-title-main">2017 in paleontology</span> Overview of the events of 2017 in paleontology

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References

  1. ↑ Cunningham, John A.; Vargas, Kelly; Yin, Zongjun; Bengtson, Stefan; Donoghue, Philip C. J. (2017). "The Weng'an Biota (Doushantuo Formation): An Ediacaran window on soft-bodied and multicellular microorganisms". Journal of the Geological Society. 174 (5): 793–802. Bibcode:2017JGSoc.174..793C. doi: 10.1144/jgs2016-142 . hdl: 1983/d874148a-f20e-498a-97d2-379b3feaa18a .
  2. ↑ Evitt, William R. (1963). "A discussion and proposals concerning fossil dinoflagellates, hystrichospheres, and acritarchs, II" (PDF). Proceedings of the National Academy of Sciences. 49 (3): 298–302. Bibcode:1963PNAS...49..298E. doi: 10.1073/pnas.49.3.298 . PMC   299818 . PMID   16591055 – via PNAS.
  3. ↑ Martin, Francine (1993). "Acritarchsa Review". Biological Reviews. 68 (4): 475–537. doi:10.1111/j.1469-185X.1993.tb01241.x. S2CID   221527533.
  4. ↑ Colbath, G.Kent; Grenfell, Hugh R. (1995). "Review of biological affinities of Paleozoic acid-resistant, organic-walled eukaryotic algal microfossils (Including "acritarchs")". Review of Palaeobotany and Palynology. 86 (3–4): 287–314. Bibcode:1995RPaPa..86..287C. doi:10.1016/0034-6667(94)00148-D.
  5. ↑ Colbath, G.Kent; Grenfell, Hugh R. (1995). "Review of biological affinities of Paleozoic acid-resistant, organic-walled eukaryotic algal microfossils (including "acritarchs")". Review of Palaeobotany and Palynology. 86 (3–4): 287–314. Bibcode:1995RPaPa..86..287C. doi:10.1016/0034-6667(94)00148-d. ISSN   0034-6667.
  6. ↑ Strother, Paul. "Palynomorph focus: Acritarchs". AASP – The Palynological Society Newsletter. 56 (3): 29-33.
  7. ↑ Strother, Paul. "Acritarchs". The Palynological Society. AASP-The Palynological Society. Retrieved 12 July 2024.
  8. ↑ Evitt, William R. (1963). "A discussion and proposals concerning fossil dinoflagellates, hystrichospheres, and acritarchs, II" (PDF). Proceedings of the National Academy of Sciences. 49 (3): 298–302. Bibcode:1963PNAS...49..298E. doi: 10.1073/pnas.49.3.298 . PMC   299818 . PMID   16591055 – via PNAS.
  9. ↑ Buick, R. . (2010). "Early life: Ancient acritarchs". Nature. 463 (7283): 885–886. Bibcode:2010Natur.463..885B. doi: 10.1038/463885a . PMID   20164911.
  10. ↑ Kempe, A.; Schopf, J. W.; Altermann, W.; Kudryavtsev, A. B.; Heckl, W. M. (2002). "Atomic force microscopy of Precambrian microscopic fossils". Proceedings of the National Academy of Sciences. 99 (14): 9117–9120. Bibcode:2002PNAS...99.9117K. doi: 10.1073/pnas.142310299 . PMC   123103 . PMID   12089337.
  11. ↑ Kempe, A.; Wirth, R.; Altermann, W.; Stark, R.; Schopf, J.; Heckl, W. (2005). "Focussed ion beam preparation and in situ nanoscopic study of Precambrian acritarchs". Precambrian Research. 140 (1–2): 36–54. Bibcode:2005PreR..140...36K. doi:10.1016/j.precamres.2005.07.002.
  12. ↑ Marshall, C.; Javaux, E.; Knoll, A.; Walter, M. (2005). "Combined micro-Fourier transform infrared (FTIR) spectroscopy and micro-Raman spectroscopy of Proterozoic acritarchs: A new approach to Palaeobiology". Precambrian Research. 138 (3–4): 208–224. Bibcode:2005PreR..138..208M. doi:10.1016/j.precamres.2005.05.006.
  13. ↑ Kaźmierczak, Józef; Kremer, Barbara (2009). "Spore-Like Bodies in Some Early Paleozoic Acritarchs: Clues to Chlorococcalean Affinities". Acta Palaeontologica Polonica. 54 (3): 541–551. doi: 10.4202/app.2008.0060 .
  14. ↑ Schopf, J. William; Kudryavtsev, Anatoliy B.; Sergeev, Vladimir N. (2010). "Confocal laser scanning microscopy and Raman imagery of the late Neoproterozoic Chichkan microbiota of South Kazakhstan". Journal of Paleontology. 84 (3): 402–416. Bibcode:2010JPal...84..402S. doi:10.1666/09-134.1. S2CID   130041483.
  15. ↑ Moczydłowska, Małgorzata; Landing, ED; Zang, Wenlong; Palacios, Teodoro (2011). "Proterozoic phytoplankton and timing of Chlorophyte algae origins". Palaeontology. 54 (4): 721–733. Bibcode:2011Palgy..54..721M. doi: 10.1111/j.1475-4983.2011.01054.x .
  16. ↑ Chamberlain, John; Chamberlain, Rebecca; Brown, James (2016). "A Mineralized Alga and Acritarch Dominated Microbiota from the Tully Formation (Givetian) of Pennsylvania, USA". Geosciences. 6 (4): 57. Bibcode:2016Geosc...6...57C. doi: 10.3390/geosciences6040057 . CC-BY icon.svg Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
  17. ↑ "Montenari, M. & Leppig, U. (2003): The Acritarcha: their classification morphology, ultrastructure and palaeoecological/palaeogeographical distribution". Paläontologische Zeitschrift. 77: 173–194. 2003. doi:10.1007/bf03004567. S2CID   127238427.
  18. ↑ Yin, Leiming (February 2020). "Microfossils from the Paleoproterozoic Hutuo Group, Shanxi, North China: Early evidence for eukaryotic metabolism". Precambrian Research. 342: 105650. Bibcode:2020PreR..34205650Y. doi: 10.1016/j.precamres.2020.105650 .
  19. ↑ Bengtson, S. (2002). "Origins and early evolution of predation" (PDF). In Kowalewski, M.; Kelley, P.H. (eds.). The fossil record of predation. The Paleontological Society Papers. Vol. 8. The Paleontological Society. pp. 289–317. Retrieved 29 December 2014.
  20. ↑ Stanley, S. M. (2008). "Predation defeats competition on the seafloor". Paleobiology. 34 (1): 1–21. Bibcode:2008Pbio...34....1S. doi:10.1666/07026.1. S2CID   83713101.
  21. ↑ definition of acritarch at dictionary.com
  22. ↑ "Actipillion".
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