Acritarchs: a review
In: Biological Reviews of the Cambridge Philosophical Society. Cambridge University Press: London. ISSN 0006-3231; e-ISSN 1469-185X, meer
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Acritarcha [WoRMS]
Marien/Kust |
| Abstract |
1. Acritarchs are a polyphyletic group of unicellular organisms, essentially marine and fossil, with a very resistant organic membrane; the majority probably represent the cysts of microscopic, extinct eukaryote algae. This review gives a general account for the non-specialist of their characteristics and affinities, but focuses, using selected examples, on their role as biostratigraphic tools for the specialist.2. Invisible to the naked eye, up to several tens of thousands of acritarchs per gram of rock may be extracted and concentrated from a wide variety of sediments, especially argillaceous or even calcareous, but preferably fine-grained, unweathered and only slightly recrystallized or metamorphosed.3. Always hollow and without unequivocal intracellular structures, acritarchs are extremely variable inoverall size, from a few to several hundred µm, with numerous divergent morphological modifications from a basic spherical form; the type and development of ornamentation; the number of cellular walls; and the method of opening, attributed to excystment. Acritarchs are classified according to criteria that are relatively simple compared with the modern demands of phycologists. For convenience they are treated under the International Code of Botanical Nomenclature, recognizing the existence of form genera of uncertain position. The lack of a comprehensive taxonomic framework is not surprising, given the number and variety of unclassifiable microorganisms resistant to HF that may be included in the acritarchs.4. The sporopollenin-like wall of acritarchs, like the sporopollenin of modern plants, is chemically very inert except to oxidation, carbonization and bacterial or fungal activity. Of poorly-known composition but very probably including highly polymerized polyterpenes, it may form an abundant component of Palaeozoic kerogen, a potential source of hydrocarbons. The codification of colour changes and preservation in selected acritarchs may enable the optical evaluation of palaeotemperatures lower than about 120–150 °C and of the degree of maturity of possible oils.5. The first known acritarchs sensu stricto, although discovered in 1862, were designated as such in 1963, after having been given a variety of names reflecting mainly assessments of their biological affinities. In spite of some attempts to abandon it, the name acritarch is still the most correct as it is the least ambiguous for designating the great majority of examples.6. The reclassifying of acritarchs among microorganisms of known systematic position remains speculative or tentative. It is possible that many acritarchs represent cysts of extinct dinoflagellates, without archaeopyle or indication of a stable tabulation. Laboratory culture of Pterosperma has shown that Cymatiosphaera and Pterospermella have to be considered not as acritarchs but as phycoma of prasinophytes. The ultrastructure of the wall in Tasmanites is similar to that of Pachysphaera, another recent prasinophyte. Comparisons with euglenoids or spore-like bodies of the first terrestrial plants are indirect and that with eggs of recent crustaceans remains fortuitous.7. The composition of live acritarch assemblages is most often heavily biased in taphocoenosis. In fact, because of their very small size and low density, these microfossils are frequently found reworked in strata younger than those in which they were originally deposited. If their distributions are sufficiently documented, they can be useful as provenance indicators in palaeogeographic reconstructions.8. Acritarchs' mode of life is thought to be best compared with that of planktonic photosynthetic algae. General schemes seeking to explain variations in their abundance and distribution in deposits formed during the distant geological past are based especially on extrapolations from complex combinations of factors that govern the distribution of modern marine phytoplankton.9. With a worldwide geographic distribution and a record only partly influenced by facies control, the acritarchs exhibit, geologically speaking, an extraordinarily long life span, from the Mesoproterozoic to the present day. In spite of the examples of reworking, rarely objectively verifiable, and the still relatively small number of detailed data with reliable independent age control, it is known that acritarchs, among a great number of ubiquitous forms, include time index taxa whose levels of appearance permit the calibration of very remote geological time and the establishment of regional or global correlations. These biostratigraphic indices, certainly present in the Neoproterozoic but still little known, are best demonstrated from around the beginning of the Cambrian to slightly before the end of the Upper Devonian, a time of maximum abundance and diversity for the group.At the beginning of the Early Cambrian in the East European Platform, and probably slightly above the international systemic boundary, drawn at the appearance of the ichnofossil Phycodes pedum in eastern Newfoundland, the acritarchs display a radiation of original diversity which occurs at three levels and contrasts with the worldwide impoverished sphaeromorph assemblages of the latest subjacent Neoproterozoic. The first level is marked especially by the appearance of Annulum squamaceum, the second by the diversification of Comasphaeridium, and the third, which is the clearest and most geographically widespread, by the appearance of Skiagia orbicularis, S. ornata and S. scottica, which coincides approximately with that of the trilobites.The Cambrian-Ordovician boundary is not yet agreed internationally but should be near the appearance of the Cordylodus lindstromi conodont Biozone, slightly below the first occurrence of nematophorous planktonic graptolites. Corollasphaeridium wilcoxianum is the index acritarch whose appearance is closest to, and slightly below, this boundary, in the upper part of the Cordylodus proavus Biozone. The species enters at this level in the north Sino-Korean Platform (Jilin province) and northern Laurentia (Alberta). It has not been recorded in Baltica, Avalonia and Gondwana, where the acritarch assemblages are better documented, more varied and different on the whole from those of northeastern China and western Canada. With reservations, it may be that in marine deposits associated with these three palaeocontinents, the lower limit of the range of Acanthodiacrodium angustum is located within the Cordylodus proavus Biozone.In the Late Devonian, the Frasnian—Famennian boundary is fixed internationally by means of conodonts, at the base of the Early Palmatolepis triangularis Biozone, which succeeds the Palmatolepis linguiformis Biozone. Regionally, in the Dinant Basin, Belgium, no index acritarch is known to appear at the base of the Lower Famennian. On the other hand, at Senzeilles the appearances of Visbysphaera?occulta and of Ephelopalla media occur successively at the end of the Frasnian in deposits undated by means of conodonts but attributable to the end of the late Palmatolepis zhenana Biozone and to the P. linguiformis Biozone.In the course of the upper Famennian, and from the end of the Late Devonian onwards, known assemblages are essentially sporadic, unvaried and of reduced or local stratigraphic value. The last species that is autochthonous, morphologically unmistakable and of worldwide distribution appears in the middle Neogene (Ypresian). |
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