Acritarchs are small organic structures found as fossils. In general, any small, non-acid soluble (i.e. non carbonate, non-silicate) organic structure that can not otherwise be accounted for is an acritarch. Most acritarchs are surely remains of single celled lifeforms. They are found in sedimentary rocks from the present back into the Precambrian. They are easily isolated from limestones with hydrochloric acid, and can also be isolated from silica rich rocks using hydrofluoric acid. They are excellent candidates for index fossils to be used for formation dating in the Palaeozoic and when other fossils are not available. They are also useful for palaeoenvironmental interpretation. Acritarchs include the remains of several quite different kinds of organisms including bacteria and dinoflagellates. The nature of the creatures associated with older acritarchs is generally not clear, though many are probably related to unicellular marine algae.
Acritarchs are known from 1400Ma and had achieved considerable diversity by 1300Ma. Diversity crashed during the Sturtian-Varanger glacial event around 800Ma. Diversity increased again during the Ediacaran period. Diversity declined suddenly at the end of the Precambrian. The acritarchs show their greatest diversity during the Cambrian, Ordovician, Silurian and Devonian. The nature of some Acritarchs can be identified by their structure. A few can be tentatively identified by the presence of specific chemicals associated with the fossils.
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CIMP Subcommission on Acritarchs (http://www.shef.ac.uk/~cidmdp/cimpsuba.html)
Commission Internationale de Microflore du Paléozoique (CIMP) (http://www.shef.ac.uk/~cidmdp/), international commission for Palaeozoic palynology.
Centre for Palynology, University of Sheffield, UK (http://www.shef.ac.uk/uni/academic/N-Q/palysc/index.html)
In this new scenario, diversified acritarch assemblages from the Cordillera Oriental correspond to the platform facies of the foreland basin.
Identification of the Ordovicianâ€“Silurian boundary on the basis of acritarchs is complicated because of the extinction caused by the Hirnantian glaciation.
Acritarchs from both formations are middle Arenig in age and independently dated by conodonts from the upper part of the Oepikodus evae Zone (Rubinstein and Astini, 2000; Rubinstein 2001a, 2001b and unpublished data).
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