426 MICROFOSSILS/Acritarchs acritarchs of small size, and they are notable in the Late Cretaceous pelagic sediments, including the Chalk Acritarchs and dinoflagellate cysts were first studied in detail within flints in the Chalk, and continue to be of value in the sourcing of archaeological flint and chert Assemblages in the Cenozoic are also dominated by dinoflagellate cysts, although the marine phytoplankton record also includes the calcareous nanoplankton (Haptophyta, Coccosphaerales) and diatoms (Bacillariophyta) Acritarchs are recorded together with dinoflagellate cysts in Holocene marine sediments, although some may include copepod eggs Palaeoenvironmental Distribution In common with present-day marine phytoplankton, the acritarchs show distributions related to water masses, so that different nearshore, offshore shelf, and basinal assemblages can be recognized (Figure 7) Forms that are interpreted as benthonic cysts are most common in shelf areas, whereas forms with planktonic cysts, including photosynthetic phycoma, are widespread and common in deep-water oceanic sediments On a global basis, some forms are apparently restricted to broad latitudinal belts, comparable to modern phytoplankton The palaeoenvironmental interpretation of Late Precambrian acritarchs is uncertain, although it is probable that different sizes of sphaeromorph acritarch were influenced by nutrient availability In the Cambrian, many acritarchs are widespread, although Micrhystridium favoured inshore areas, whereas more complex forms, including Skiagia, are found in offshore areas During the Ordovician and Silurian, clear palaeoenvironmental distribution patterns become established, with distinct assemblages associated with nearshore shelf, offshore shelf, and deep-water areas In the Late Ordovician, nearshore areas contain many Leiosphaeridia, with Peteinosphaeridium and Dicommopalla characteristic of shoal areas, and diverse acritarchs with Baltisphaeridium, Veryhachium, and Polygonium in open sea shelf areas In the Silurian, nearshore assemblages are of low diversity and low to moderate abundance, with many small, thin-walled Leiosphaeridia, together with Veryhachium with three short processes, Micrhystridium, and Diexallophasis, together with land plant spores The offshore shelf assemblage is of high diversity and moderate abundance, with no one taxa dominating the assemblage Species of Veryhachium often have three or four medium to long processes This pattern of increasing length of process in an offshore direction is also documented from the Early Jurassic The assemblages associated with deep water are of low diversity and low to moderate abundance, with Leiosphaeridia dominant, particularly thick-walled forms Other genera present in low numbers include Cymatiosphaera and Pterospermella that have planktonic phycoma, together with occasional Diexallophasis, Micrhystridium, and Diexallophasis, which are the most common acritarchs on the shelf There is some evidence from the Silurian to suggest that acritarch assemblages may be used to predict oceanic phytoplankton productivity and palaeoclimate, as certain forms are associated with argillaceous sediments formed during cool, wet, primo episodes, with others associated with more calcareous sediments deposited during drier, warmer, secundo episodes During the Silurian and Devonian, distribution patterns are also recognized within carbonate environments, with distinct assemblages associated with patch reef, inter-reef and deeper non-reef areas Figure Diagrammatic section from a shallow to deep shelf tropical sea during the mid Silurian, illustrating the distribution of acritarchs Data in part from Dorning KJ (1981) Silurian acritarch distribution in the Ludlovian shelf sea of South Wales and the Welsh Borderland In: Neale JW and Brasier MD (eds.) Microfossils from Recent and Fossil Shelf Seas, pp 31 36 Chichester: Ellis Horwood Ltd