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A supposed Ediacaran embryo contained within an acritarch from the Doushantuo formation

Acritarchs are small organic fossils, present from approximately 2,500 million years ago to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.

Contents

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.

Etymology

Acritarch was coined in 1963 from the Greek ákritos meaning confused (a kritēs, without critic) and arch meaning origin (confer archaic). [1]

Occurrence

Acritarchs are found in sedimentary rocks from the present back into the Precambrian. 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 first appear in rocks about 2.5 billion years old,[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]

References

External links

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Simple English

Acritarchs are early microfossils, the remains, probably, of eukaryote cells. It is the resistant single-layered cell walls, or perhaps a cover secreted by cell walls, that we see in early rocks of the Proterozoic era. These structures are organic in their chemical composition, not calcium carbonate. Their precise nature is not known, but they look like the resting stages of modern dinoflagellates (the 'red tide' organisms).

"These microfossils represent the resting stage in the reproductive cycle of eukaryote algae".[1]p258

Acritarchs appear in quantity in organic-rich shales and siltstone strata 1.4 to 1.6 billion years old.[2]p57 They include a wide range of forms, so it is not clear whether or not they are monophyletic. At about 1 billion years ago they started to increase in abundance, diversity, and especially size and number of spines. Their populations crashed during the Cryogenian period 860 million years ago (the Snowball Earth episodes). They proliferated in the Cambrian explosion and reached their highest diversity in the Paleozoic. They survived at least until the Ordovician period.[1]p256

The increased spininess possibly resulted from the need for defence against 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][4]

Recent discovery

A recent discovery has greatly extended our knowledge of acritarchs. Large (50μm)[5] acritarchs have been found in silica-based rocks of Archaean age in South Africa. These rocks date to 3,200 million years ago, which makes them the second oldest life to be found as fossils.[6][7] The earliest are cyanobacteria found in stromatolites.

The significance of this is that acritarchs are eukaryotes. This means that eukaryotes were present at least 1.5 billion years earlier than had been supposed.

References

  1. 1.0 1.1 Levin, Harold. 2006. The Earth through time. Wiley, Hoboken N.J.
  2. Clarkson E.N.K. 1998. Invertbrate palaeontology and evolution. Blackwell, Oxford.
  3. Bengtson S. (2002), "Origins and early evolution of predation", in Kowalewski M. and Kelley P.H. (Free full text), The fossil record of predation. The Paleontological Society Papers 8, The Paleontological Society, pp. 289–317, http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf 
  4. Stanley S.M. 2008. Predation defeats competition on the seafloor. Paleobiology 34: 1–0. doi:10.1666/07026.1.
  5. 50 millionths of a metre
  6. Buick R. 2010. Early life: ancient acritarchs. Nature 463: 885–886. doi:10.1038/463885a. PMID 20164911.
  7. Javaux E. Marshall C.; Bekker A. 2010. Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits. Nature 463: 934–938. doi:10.1038/nature08793. PMID 20139963.

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