» SILURIAN FOSSILS in STRATIGRAPHY 20. Acritarchs b y FRANCINE MARTIN T he purpose of this chapter is to give readers with no Llandovery, from the end of the Rhuddanian onwards, specialised knowledge of acritarchs an overview of Silu­and the Wenlock, particularly in southern Ontario and rian palynofloras, to list references leading to addi­ western New York State. The oldest samples from the tional information, and to gauge the possibilities for lowest Llandovery at Anticosti Island, Quebec, Canada, worldwide correlation based on this group. are only partly published (Ref. 1). Palynological data Acritarchs are often common and highly varied in for the Ludlow and Pridoli Series in North America marine Silurian deposits. Tappan (1980) and Downie refer to boreholes in Florida and lack adequate chrono- (1984) give general comments on these organic-walled stratigraphical control (Ref. 4, p. 122). microfossils of disputed origin, some of which are Data from South America (Argentina, Bolivia, considered as cysted stages of phytoplankton and all ofBrazil; Refs. 9 to 14) and from Asia (Siberia and Tuva; which are treated taxonomically under the International Refs. 65, 66) are very dispersed and usually have little Code of Botanical Nomenclature. Systematic study ofindependent age control. The most abundant data are acritarchs began during the 1930s. Following the from Europe, particularly the western part, where pioneer studies of Eisenack, from 1931 onwards and numerous localities, the age control of which is covering, inter alia, the Silurian of the Baltic region, it variable, give a glimpse of the whole Silurian System. In was only towards the end of the 1960s and particularly particular, partially published palynological study of following the example of Loeblich (1970) that the indis­the British stratotypes (Refs. 20 to 25) has permitted the pensable detailed descriptions were provided, supported establishment of a regional scale of four assemblage by observations using the scanning electron microscope. biozones in the Llandovery, from the end of the Rhudd­ The present state of taxonomy is extremely chaotic, theanian onwards, and of seven range zones in the com­ result on the one hand of the accumulation of too brief bined Wenlock and Ludlow (Ref. 23). In addition seven diagnoses, leading to the production of often long rang­ biozones were recognised in the Llandovery Series of ing ‘balloon’ taxa, and on the other of a lack ofthe type Llandovery area (Ref. 22). The basal Rhuddan­ unanimity in hierarchial choice of morphological ian contains indeterminable acritarchs (personal criteria (see Loeblich 1970, pp. 708-710; Cramer and observation) at the type locality of Dob’s Linn, in Diez 1972b, pp. 139-146: Tappan 1980, pp. 205-206). southern Scotland, and long ranging forms in the Llan­ Moreover, relatively few detailed monographs have dovery area of Wales (Refs. 21, 22). The base of the been published on type sections and/or stratigraphically Downton Group, at the type locality in the Welsh continuous deposits dated by means of macrofossils. Borderland, is very poor in acritarchs and no new These general comments apply particularly to the appearance of any taxon has been formally established Silurian System. (Ref. 24). In the Baltic region (Refs. 28 to 32), as in Fig. 148 shows principal Silurian acritarch localities, Belgium (Refs. 38 to 40), where acritarchs trom the excluding regions for which suitable data are lacking. acuminatus Biozone are partially known (Ref. 39), The numbers (see list in explanation of Fig. 148) indicate analyses refer especially to the Llandovery and Wenlock bibliographic references corresponding generally to the Series. In Norway, Austria, Romania, and Sardinia most recent publications that clearly include previously (Refs. 26, 36, 37, 53) the data are very preliminary and published information. Localities that have furnished often based on isolated samples. In Spain (Refs. 3, 48 to only poorly characteristic or badly identifiable taxa 52) published monographs are based on samples ‘with (e.g., in Austria; Bachman and Schmid 1964), or those little accurate age control’ (Cramer and Diez 1977, without adequate precision of provenance or list ofp. 290) from the Wenlock onwards. In Brittany and determinations (e.g., northern Canada and Saudi northern France (Refs. 41 to 45) strata have been attri­ Arabia; Cramer 1970, text-fig. 1A) are omitted; these buted generally to the Ludlow or the Pridoli on the basis localities may be checked in Cramer and Diez (1979, fig. of palynological criteria; at Klonk, Czechoslovakia 6.3). Publications on acritarchs from the eastern part of (Ref. 35), the Pridoli is very poor in acritarchs. In North America (Refs. 1 to 8) concern essentially the Podolia (Refs. 33, 34), authors’ taxonomic concepts 208 F. MARTIN 65 26 ;T íp24 r 2 5 U ^ > 2 8 - 3 2 1 6 - 1 8 —f , "38-40 3 3 6 6 - ' 4 1 -4 6 *'3 5 * - 3 4 3-7 3 6-* 37 47 3 - 4 ' 3, 4 8 - 5 2 - r ' * ’ •5 4 55> 56 5 7 - 6 0 62 63 64 ,-12 f ig . 148. Generalised geographical map of Silurian acritarch localities. 1, Duffield and Legault (1981, 1983). 2, Achab (1976). 3, Cramer (1970). 4, Cramer and Diez (1972¿>). 5, Thusu (1973a, 1973¿>). 6, M. A. Miller and Eames (1982). 7, Loeblich (1970). 8, Tappan and Loeblich (1971). 9, Lange (1967). 10, Brito (1967). 11, Cramer et al. (1974). 12, Bultynck and Martin (1982). 13, 14, Pöthe de Baldis (1974, 1981). 15, Doming (1982), 16, Claytonetal. (1980). 17, Smith (1981). 18, Holland and Smith (1979). 19, Downie (1963). 20, Lister (1970). 21, P. J. Hill (1974). 22, P. J. Hill and Doming in Cocks etal. (1984). 23, Doming (1981a). 24, J. B. Richardson et al. (1981). 25, Downie (1984). 26, Doming and Aldridge (1982). 27, Schultz (1967). 28, Cramer et al. (1979). 29, Le Hérissé (1984). 30, 31, Eisenack (1970, 1974). 32, H. N. Umnova (1975). 33, Kirjanov (1978). 34, Sheshegova (1974). 35, Deunff (1980). 36, Martin (1978). 37, Kalvacheva (1978). 38, 39, Martin (1969, 1974). 40, Vanguestaine (1979). 41, Deunff and Paris (1972). 42, Deunff et al. (1971). 43, Rauscher and Robardet (1976). 44, Deunff and Chateauneuf (1976). 45, Rauscher (1974). 46, Moreau-Benoit (1974). 47, Deflandre (1944-1945). 48, Cramer (1964), 49, Eisenack etal. (1976). 50, Diez and Cramer (1976). 51, Cramer et al. (1976). 52, Rodriguez (1983). 53, del Rio et al. (1980). 54, Erkmen and Bozdogan (1979). 55, Rahmani (1983). 56, J. B. Richardson and Ioannides (1973). 57, Magloire (1967). 58, Jardiné and Yapaudjian (1968), 59, 60, Jardiné etal. (1972, 1974). 61, Schrank (1984). 62, Cramer and Diez (1972a). 63, Baudelot et al. (1980). 64, Bär and Riegel (1980). 65, 66, Sheshegova (1984, 1975). limit the understanding of palynological results relating the list of the former group suggests that assemblages of to the Silurian, from the end of the Llandovery or thedifferent ages, or containing reworked material, have beginning of the Wenlock. In North Africa the been considered together. principal works (Refs. 57 to 60) concern the Algerian Among several hundred acritarch taxa recorded from Sahara, from the Llandovery onwards, estimated as the Silurian, 33 with characters sufficiently distinct to equivalent to the Aeronian or the Telychian; most ofavoid ambiguous determination are listed here (Fig. the Silurian deposits are assigned to the Ludlow on the 149). This choice, which is not exhaustive and proposed basis of their correlation with other Saharan boreholes only by way of an example, is also influenced by per­ dated by means of graptolites, or by comparison with sonal rapid examination of samples from the Llan­ palynological data, principally from Spain. A synthesis dovery of Gaspé, Anticosti Island, and Norway, and (Ref. 60) of the combined sections has permitted the from the Llandovery to Downton of Great Britain establishment of six informal biozones for the Silurian. collected by the author during Silurian Subcommission In Turkey (Ref. 54), Guinea (Refs. 62, 63), and Ghana field meetings in 1979, 1981, and 1982; others were (Ref. 64) dating of beds is based principally on acri­ obtained with M. G. Bassett and W. T. Dean in the tarchs or chitinozoans, and in one publication (Ref. 63)Llandovery area, Wales, in 1984. The selection of ACRITARCHS 209 acritarchs could have been wider, or different, but it erae appears in the atavus Biozone. Eupoikilofusa aff. E. would still show the same deficiencies in knowledge, ampulliformis at the beginning of the Llandovery is documen­ notably the lack of information on the lowest ted only in Anticosti (Duffield and Legault 1981, 1983) and Gaspé (personal observation; sample BC-1-1). Llandovery and the unreliability of most datings of Homerian to Pridoli strata outside their type areas. All Group 2: Aeronian Stage (in part) the genera and species chosen, except for two indicated Marked by the appearance of Domasia, essentially D. limaci­ by an asterisk*, are present in at least two continents. formis, from the sedgwickii Biozone onwards, and of Dactylo­ No formal scheme based on standard successions and fusa estillis. of intercontinental application has yet been proposed. Ideally it should be based on first appearances of taxa in Group 3: Aeronian Stage (in part) and Telychian Stage (in order to avoid errors resulting from the presence ofpart) reworked material; the latter may only be detected with Appearances of several new species of Domasia and of Dilati­ confidence either on the basis of differences in state of sphaera willierae a little below the turriculatus Biozone. Appearance and development of species of Deunffia from the preservation from that of autochthonous material, or if crispus Biozone onwards. Appearance of Elektoriskos aurora, several cycles of redeposition sufficiently different inNeoveryhachium carminae, Solita, and Geron; the last three age can be recognised. In the British Isles, for example, extend into the Lower Devonian.