Acritarch

The Acritarcha ( gr ἄχριτος uncertain; αρχη origin), also acritarchs (sing.: that Acritarch ' ) represent an artificially defined group of microfossils whose precise systematic classification is largely unknown. The acritarchs the planktonic organisms are counted, which enter in the form of their resistant cases, for instance based on sporopollenin or similar high- polymeric organic compounds in the fossil record. The various forms and the preservation ability in otherwise empty fossil rocks make them valuable tools of paleontology. Previously, the acritarchs were asked along with some today dinoflagellates attributed to the so-called forms Hystrichosphaeroideen.

Shape

Morphologically show the Acritarcha a high variation, ranging from simple spheroidal shapes to those with complex sculptured and fitted with extensions shells. The cell size is typically in the range of 10-50 microns. This point compared to the bacteria significantly larger in diameter and the chemical structure of the shells indicate that it is in eukaryotes Acritarcha. In the individual representatives of the very heterogeneous group of acritarchs appears to be cysts of algae (including today's important dinoflagellates ), spores of mosses ( Bryophyta ) and ferns ( Pteridophyta ) and less frequently to act permanent forms of zooplankton.

Temporal distribution

The oldest finds of acritarchs rich with an age of about two billion years back to the Paleoproterozoic. During the general mass extinction in the Varanger Ice Age million years ago, 600 there was a significant decrease in the number of species, which reduced the biodiversity of Acritarcha by about 70 %. In the following interglacial period this Artverluste were more than offset by extensive Neuentstehungen, so the Acritarcha starting from the Cambrian to the Devonian had their greatest diversity. They provide in this period, the dominant group of fossil phytoplankton dar. the end of the Late Devonian it came to a second major extinction event, of which the Acritarcha not recovered. Although come to the Tertiary into isolated Acritarcha before, but only very rarely and with little style number. From the Quaternary freshwater Acritarcha are known.

Importance

With its high incidence in conjunction with the time-varying assortment of styles make the acritarchs important index fossils in biostratigraphy of Proterozoic and Paleozoic sediments Represents the is especially true for the period of the Proterozoic, as other usable for dating fossils are rarely available in this time range. In addition, the stable shell prevented the destruction of fossils in rocks that were deposited in one of the preservation of fossils adverse environment, or subject after deposition of a metamorphosis.

The resistance of the shells allows for easy production, by the stone is dissolved by hydrofluoric acid. The insoluble residues remain, among them the acritarchs, which are then sorted under a microscope and can be determined.