Acritarcha

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The acritarcha (Greek ἄκριτος, unsafe '; αρχη, origin'), also acritars ( singular : the acritarch ) are a group of microfossils whose systematic classification is largely unknown. The name was introduced by William R. Evitt in 1963. The planktonic organisms are counted among the acritars , whose resistant shells, for example based on sporopollenin or similar highly polymeric organic compounds , can be detected as fossils in sediments . The diverse shapes and the ability to be preserved in otherwise fossil-free rocks make them valuable tools in paleontology . In the past, the acritars, together with some forms that are now dinoflagellates , were classified as the so-called hystrichosphaeroids.

shape

Morphologically, the acritarcha show a wide range of variation, ranging from simple spheroidal shapes to those with complex sculptured and appendage shells. The cell size is typically in the range from 10 to 50  µm . This diameter, which is significantly larger than that of bacteria, and the chemical structure of the envelopes indicate that the acritarcha are eukaryotes . The individual representatives of the very heterogeneous group of acritars seem to be cysts of algae (including the dinoflagellates, which are important today), spores of mosses ( Bryophyta ) and ferns ( Pteridophyta ) and, less frequently, permanent forms of zooplankton .

Temporal spread

The oldest finds of acritars date back to the Paleoproterozoic, around two billion years old . During the general mass extinction in the Marino Ice Age 635 million years ago there was a considerable decrease in the number of species, which reduced the biodiversity of the Acritarcha by around 70%. In the following warm period , these species losses were more than compensated for by extensive new emergence, so that the Acritarcha showed their greatest diversity from the Cambrian to the Devonian . In this period they represent the dominant group of fossil phytoplankton . Towards the end of the Upper Devonian there was a second major extinction event, from which the acritarcha did not recover. Acritarcha occur sporadically up to the Tertiary , but only very rarely and with a small number of species. Freshwater acritarcha are known from the Quaternary .

meaning

With their great abundance in connection with the time-varying composition of the species, the acritars are important key fossils in the biostratigraphy of Proterozoic and Paleozoic sediments. This is especially true for the Proterozoic period , since other fossils that can be used for dating are rarely found in this period . In addition, the stable shell prevented the fossils from being destroyed even in rocks that were deposited in an environment that was detrimental to the preservation of fossils, or that were subject to metamorphosis after their deposition .

The resilience of the casings allows easy extraction by dissolving the rock with hydrofluoric acid . The insoluble residues remain, among them the acritars, which can then be sorted out and determined under the microscope.

literature

  • Ulrich Lehmann: Paleontological Dictionary . 4th edition. Ferdinand Enke Verlag, Stuttgart 1996, ISBN 3-432-83573-6 , pp. 3 .
  • Michael Montenari, Ursula Leppig: The acritarcha: its classification, morphology, ultrastructure and paleoecological / paleogeographical distribution. In: Paleontological Journal. Volume 77, No. 1, 2003, pp. 173-193 ( doi: 10.1007 / BF03004567 ).

Web links

Individual evidence

  1. William R. Evitt (1963): A discussion and proposals Concerning fossil dinoflagellates, hystrichospheres, and acritarchs. Proceedings of the National Academy of Sciences USA 49: 298-302 ( doi: 10.1073 / pnas.49.3.298 ).
  2. Ulrich Lehmann and Gero Hillmer: Invertebrates of prehistoric times: Guide to the systematic paleontology of invertebrates. 2nd edition, Stuttgart: Ferdinand Enke-Verlag Stuttgart, 1988; ISBN 3-432-90652-8
  3. ^ Ulrich Lehmann: Paleontological Dictionary . 4th edition. Ferdinand Enke Verlag, Stuttgart 1996, ISBN 3-432-83573-6 , pp. 3 .
  4. Michael Montenari, Thomas Servais and Florentin Paris: Palynological dating (acritarchs and chitinozoans) of Lower Paleozoic phyllites from the Black Forest / southwestern Germany . In: Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science . tape 330 , no. 7 , 2000, pp. 493-499 , doi : 10.1016 / S1251-8050 (00) 00182-8 .