Acritarch: Difference between revisions
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[[Image:Doushantou_Embryo_Yinetal2007.jpg|thumb|A supposed [[Ediacaran]] [[embryo]] contained within an acritarch from the [[Doushantuo formation]]]] |
[[Image:Doushantou_Embryo_Yinetal2007.jpg|thumb|A supposed [[Ediacaran]] [[embryo]] contained within an acritarch from the [[Doushantuo formation]]]] |
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'''Acritarchs''' are small organic [[fossil]]s, present from approximately {{Ma| |
'''Acritarchs''' are small organic [[fossil]]s, present from approximately {{Ma|3200}} to the present. Their diversity reflects major ecological events such as the appearance of predation and the [[Cambrian explosion]]. |
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==Definition== |
==Definition== |
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While the [[biological classification|classification]] of acritarchs into [[form taxon|form genera]] is entirely artificial, it is not without merit, as the form taxa show similar traits to genuine [[taxon|taxa]] - for example an '[[cambrian explosion|explosion]]' in the [[Cambrian]] and a [[mass extinction]] at the [[Permian-Triassic_extinction_event|end]] of the [[Permian]]. |
While the [[biological classification|classification]] of acritarchs into [[form taxon|form genera]] is entirely artificial, it is not without merit, as the form taxa show similar traits to genuine [[taxon|taxa]] - for example an '[[cambrian explosion|explosion]]' in the [[Cambrian]] and a [[mass extinction]] at the [[Permian-Triassic_extinction_event|end]] of the [[Permian]]. |
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==Affinity== |
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| ⚫ | |||
Acritarchs may represent the remains of any of the three domains of life, the archaea, the bacteria, and the eukaryotes. Archaea and bacteria usually produce fossils of a very small size, although the sheaths of some bacteria can reach the millimetre scale. Eukaryotes can sometimes be identified by complex traits such as ornamentation or projections, although simple eukaryote acritarchs also exist.<ref>{{cite doi|10.1038/463885a}}</ref> |
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==Occurrence== |
==Occurrence== |
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Acritarchs are found in sedimentary rocks from the present back into the [[ |
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|Paleozoic]] Era and when other fossils are not available. Because most acritarchs are thought to be marine, they are also useful for palaeoenvironmental interpretation. |
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==Diversity== |
==Diversity== |
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Acritarchs |
Acritarchs have been recovered from sediments deposited as long as 3.2 billion years ago,<ref>{{cite doi|10.1038/nature08793}}</ref> but at about 1 billion years ago they started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines. Their populations crashed during the [[Snowball Earth]] episodes, when all or very nearly all of the Earth's surface was covered by ice or snow, but they proliferated in the Cambrian explosion and reached their highest diversity in the [[Paleozoic]]. The increased spininess 1 billion years ago possibly resulted from the need for defence 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 defences.<ref name="Bengtson2002OriginsOfPredation">{{ Citation |
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| author=Bengtson, S. |
| author=Bengtson, S. |
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| date=2002 |
| date=2002 |
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}}</ref> |
}}</ref> |
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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.<ref name=Stanley2008>{{Cite |
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.<ref name=Stanley2008>{{Cite doi | 10.1666/07026.1}}</ref> |
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| title = Predation defeats competition on the seafloor |
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==Etymology== |
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| author = Stanley |
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| year = 2008 |
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| journal = Paleobiology |
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| volume = 34 |
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| pages = 1 |
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| url = http://paleobiol.geoscienceworld.org/cgi/content/extract/34/1/1 |
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| format = extract |
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| doi = 10.1666/07026.1 |
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}}</ref> |
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==References== |
==References== |
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Revision as of 19:05, 17 February 2010
Acritarchs are small organic fossils, present from approximately 3,200 million years ago to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.
Definition
In general, any small, non-acid soluble (i.e. non-carbonate, non-siliceous) organic structure that can not otherwise be accounted for is classified as an acritarch.
Acritarchs 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 different kinds of chlorophyta (green algae). It is likely that some acritarch species represent the resting stages (cysts) of algae that were ancestral to the dinoflagellates. The nature of the organisms associated with older acritarchs is generally not clear, 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.
While the classification of acritarchs into form genera is entirely artificial, it is not without merit, as the form taxa show similar traits to genuine taxa - for example an 'explosion' in the Cambrian and a mass extinction at the end of the Permian.
Affinity
Acritarchs may represent the remains of any of the three domains of life, the archaea, the bacteria, and the eukaryotes. Archaea and bacteria usually produce fossils of a very small size, although the sheaths of some bacteria can reach the millimetre scale. Eukaryotes can sometimes be identified by complex traits such as ornamentation or projections, although simple eukaryote acritarchs also exist.[1]
Occurrence
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 Paleozoic Era and when other fossils are not available. Because most acritarchs are thought to be marine, they are also useful for palaeoenvironmental interpretation.
Diversity
Acritarchs have been recovered from sediments deposited as long as 3.2 billion years ago,[2] but at about 1 billion years ago they started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines. Their populations crashed during the Snowball Earth episodes, when all or very nearly all of the Earth's surface was covered by ice or snow, but they proliferated in the Cambrian explosion and reached their highest diversity in the Paleozoic. The increased spininess 1 billion years ago possibly resulted from the need for defence 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 defences.[3]
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.[4]
Etymology
Acritarch was coined in 1963 from the Greek ákritos meaning confused (a kritēs, without critic) and arch meaning origin (confer archaic). [5]
References
- ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1038/463885a, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with
|doi=10.1038/463885ainstead. - ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1038/nature08793, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with
|doi=10.1038/nature08793instead. - ^ Bengtson, S. (2002), "Origins and early evolution of predation", The fossil record of predation. The Paleontological Society Papers 8 (Free full text), The Paleontological Society, pp. 289–317
{{citation}}: Unknown parameter|editors=ignored (|editor=suggested) (help) - ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi: 10.1666/07026.1, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with
|doi= 10.1666/07026.1instead. - ^ definition of acritarch at dictionary.com
External links
- CIMP Subcommission on Acritarchs
- Commission Internationale de Microflore du Paléozoique (CIMP), international commission for Palaeozoic palynology.
- The Micropalaeontological Society
- The American Association of Stratigraphic Palynologists (AASP)