Acritarchsa Review

Abstract

Summary 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 ...

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