Acritarch ecology

Poor understanding of taxonomy, biological affinity and rarity in modern environments hinder palaeoecological interpretations. Acritarchs have mostly been found in marine strata, especially in shales and mudstones, but also occur in sandstones and limestones. Non-marine examples are first reported from Recent strata.

Lagoonal facies are characterized by low diversity and monospecific assemblages of sphaeromorph and netromorph acritarchs and prasinophytes. The boundary between nutrient-rich coastal waters and nutrient-poor oceanic water is reflected in inshore-offshore trends in plankton communities, abundance and diversity. Dorning (1981, 1997) documented acritarch distribution across the Ludlow (Silurian) shelf in Wales and the Welsh Borderland. Based on the relative abundances of 17 genera he concluded that acritarch assemblages in nearshore and deep offshore environments had low diversity, dominated by sphaeromorphs. He found a much higher diversity in mid-shelf environments. Inshore facies contained abundant Micrhystridium, whereas quieter offshore facies are reflected in assemblages with longer, more delicate and elaborate processes and crests. A similar pattern was described for Middle Ordovician acritarchs (Wright & Meyers 1991). This simple palaeoecological model has been widely accepted (e.g. Hill & Molyneux 1988; Wicander et al. 1999) though probably belies much more complex physico-chemical and relative sea-level changes (e.g. Jacobsen 1979; Colbath 1990). Vecoli (2000) reported some Early Ordovician, high latitude acritarchs, including Acanthodiachrodium, may have been facies controlled implying a benthonic mode of life. A number of other acritarchs are known to be facies controlled. Neoveryhachium (Fig. 9.1u) occurs in turbid environments whereas Pulvinosphaeridium (Fig. 9.1q) and Estiastra (Fig. 9.1r) are most common in warm-water carbonate facies. In the Late Devonian reefs of western Canada sphaeromorphs predominate in near-reef facies further away from the reef thin-spined acantho-morphs and finally thick-spined acanthomorphs and polygonomorphs came to dominate assemblages (Staplin 1961). Salinity control on acritarchs has not yet been widely demonstrated, though Servais et al. (1996) did suggest that process length may vary with salinity, a feature found in some dinoflagellate cysts. Acritarchs along with dinoflagellates track transgress-ive and regressive sequences in the Jurassic of Britain and France. In the Mesozoic, acanthomorphs appear to have favoured inshore environments whilst poly-gonomorphs and some netromorphs favoured open marine environments.

Though temperature range had a primary control on acritarch distribution, evidence for provinciality is patchy in the Palaeozoic. It appears that acritarchs did not show marked provinciality until the Cambrian-Ordovician boundary. By the Tremadoc-Arenig two well-developed provinces were established, in warm tropical and cool temperate-boreal latitudes. Ordovic-ian acritarchs appear following a pattern similar to that of modern dinoflagellate cysts, primarily controlled by latitude but also following continental margins and modified by surface ocean currents (Li & Servais 2002; Servais et al. 2003). Later in the Palaeozoic, geographically restricted assemblages have been reported from the Silurian (Le Hérissé & Gourvennec 1995). Cramer & Diez (1974) suggested Silurian acritarchs were provincial assemblages paralleling palaeolatitude. The Deunffia-Domasia Assemblage is thought to be characteristic of low latitudes during the mid-Silurian and has been shown to be associated with outer-shelf environments independent of temperature control. However, it is now known the group had a wide overall tolerance, being found from periglacial to tropical palaeoenvironments. Late in the Devonian provinciality broke down with the appearance of many cosmopolitan forms (Le Hérissé et al. 1997).

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  • Melissa Trevisani
    Where is acritarch found in the sea?
    8 years ago

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