Acritarch Species Research Articles

Palynology, sequence stratigraphy, and palaeoenvironments of Middle to Upper Jurassic strata, Bayu-Undan Field, Timor Sea Region - Part One

Detailed palynological analysis of 230 core and sidewall core samples from the uppermost Plover, Elang, and lower Frigate formations, Bayu-Undan Field, Timor Sea, reveals the presence of diverse and moderately well preserved palynomorph assemblages (predominantly dinoflagellate cysts, together with acritarchs, spores, and pollen grains) that facilitate biostratigraphic, sequence stratigraphic, and palaeoenvironmental interpretations. The palynomorph assemblages, detailed systematically herein, comprise 96 spore and pollen species, 84 dinoflagellate species, and 15 acritarch species. Of these, the following 14 dinoflagellate cyst species are newly instituted: Ampulladinium aiax, Batiacasphaera macbethiae, Broomea fusticulum, Cauca bayuiana, Egmontodinium elongatum, Ellipsoidictyum fenestellum, Evansia? lacryma, Leberidocysta? strigosus, Leptodinium? ancoralium, Meiourogonyaulax baculata, Nannoceratopsis reticulata, Paragonyaulacysta? feneseptata, P. helbyi, and Yalkalpodinium playfordii. A further five species are not formally named due to a lack of adequately preserved specimens. One genus, Broomea COOKSONE viz., microphytoplankton diversity increases through transgressive systems tracts to a peak diversity at the maximum flooding surface; atypically brown wood frequencies may also peak at maximum flooding surfaces; and black debris frequencies increase higher in the highstand systems tracts. Palaeoenvironmental interpretations based primarily on palynological assemblages (especially microphytoplankton groupings and palynodebris distribution) provide a means to identify depositional settings in non-cored intervals. For instance, the occurrences of Ternia balmei, the Meiourogonyaulax group, the Ctenidodinium group, and the Rigaudella group represent an approximate gradient from very nearshore to offshore environments or possibly an increase in salinities from euryhaline to stenohaline conditions.

DESCRIPTION AND MICROSCALE ANALYSIS OF SOME ENIGMATIC PALYNOMORPHS FROM THE MIDDLE DEVONIAN (GIVETIAN) OF LIBYA

Large acritarchs were recovered from the Awaynat Wanin II Formation in the A1-69 borehole, western Libya at 394.4 m. This sample has been dated by acritarchs and spores as Givetian (Middle Devonian). Two species of acanthomorph acritarchs were recovered that are unusually large for the Paleozoic. The first type, represented by a single specimen, is a new occurrence of a previously described but unnamed form from the Middle Devonian of the Sahara. The second type, which is more abundant, is named as Vanguestainidium cucurbitulum gen. et sp. nov. Biogeochemical analysis using infrared microspectroscopy shows that the wall composition of this new taxon is close to algaenans isolated from the green alga Botryococcus braunii Kutzing 1849 although these forms are not morphologically similar. The presence of well-preserved fragile palynomorphs, abundant terrestrially-derived palynomorphs including large megaspores, and rare marine palynomorphs suggests deposition in a low-energy nearshore environment such as a coastal swamp.

Palynostratigraphy and Paleoecology of the Upper Part of Akkas Formation in Borehole KH5/1in the Western Iraqi Desert

Fourteen sample from the stratigraphic interval (1330-1600m.) in the borehole KH5/1 were studied palynoogically. The samples yielded well preserved and diversified palynomorphs except one sample at depth (1330m.) was barren. Identification of (54) species of acritarchs belong to (29) genera and (16) species of Chitinozoans belong to (9) genera, (16) species of miospores belong to (13) genera and (6) species of Scolecodont belong to (4) genera was the conclusion of this study. Comparision with similar palynomorphs in different parts of the worlds indicates clearly Upper Silurian (Ludlow) age for the samples studied. The Palynomorphs and especially the acritarchs reveal that the studied section was deposited in near

Biostratigraphic and paleogeographic significance of a palynological assemblage from the Middle Devonian Ulusubasite Formation, eastern Junggar Basin, Xinjiang, China

Abundant palynomorphs are here reported for the first time from tuffaceous siltstones of the Middle Devonian Ulusubasite Formation at the Zhifang Section, eastern Junggar Basin, Xinjiang, Northwest China. Thirty-four miospore species assigned to 23 genera (four established species, two cf., 19 sp., and nine spp.), 19 acritarch species assigned to 19 genera (10 established species, three cf., five sp., and one spp.), and scolecodonts are identified. The biostratigraphic ranges of the acritarchs and miospores indicate a Mid Devonian age, corroborating the age assignment previously based on associated corals, brachiopods, and plant fossils. Paleogeographic reconstruction for the Middle Devonian indicates that the eastern Junggar Basin was part of the Kazakhstan Plate and situated in the low-mid latitudes of the Northern hemisphere between the Euramerican landmass to the west, and the North and South China, and Gondwana landmasses to the east and south. Although there is some miospore compositional similarity between the eastern Junggar Basin and the landmasses of Euramerica, South and North China, and Gondwana, it is not particularly high, and is mainly at the generic level. The acritarch assemblage, however, consists mostly of cosmopolitan species and provides new evidence for extending the marine linkage between North America, Baltica, the Junggar Basin of Kazakhstan, and Gondwana. Paleontologic and lithologic evidence indicates that the Ulusubasite Formation was deposited in a near-shore marine environment.

Microfossils from the siltstones and muddy slates: Constraint on the age of the Taowan Group in the Northern Qinling Orogenic Belt, Central China

The age of the Taowan Group is still a hot topic in the Northern Qinling Orogenic Belt, because of limited fossil evidence from these strata and different ideals on the Cambrian fossils of the conglomerate beds and limestone blocks. Early Ordovician acritarchs, chitinozoa and scolecodonts occur in the siltstone and muddy slate beds of the Guoling, Sanchakou, Fengmaimiao and Goushenmiao Formations of the Taowan Group, indicating the age of the Taowan Group should be the Ordovician. These fossil assemblages consist of 7 genera and 13 species of acritarchs, 7 genera and 9 species of chitinozoa, and 2 genera and 2 species of scolecodonts.

A Middle and Late Cambrian age for the Booley Bay Formation, County Wexford, Ireland: New acritarch data and its implications

A well preserved Ediacara-type fauna was recorded for the first time a few years ago from the bases of seven sandstone beds in the type section of the Lower Palaeozoic Booley Bay Formation in County Wexford, Ireland. Four microfossil samples from mudstones interbedded with the sandstones yielded an acritarch microflora considered indicative of a late Late Cambrian age for the Ediacara fauna, thus suggesting extension of its range from the Neoproterozoic to the Late Cambrian, rather than to the Middle Cambrian as had been previously suggested. The current study examines the microfossils in more detail, with 18 samples, all of which were productive. One additional Ediacara bed approximately 100 m above those recorded earlier was also observed. The exceptionally well preserved acritarch assemblages indicate a clear Middle Cambrian age for the Ediacara beds, thus predating the previously suggested upper limit for Ediacara faunas. The uppermost part of the formation, above known occurrences of Ediacara beds, is younger; that is, early Late Cambrian, probably coeval with the Olenus trilobite zone. In addition, the age now proposed for the Booley Bay Formation shows that it is part of the Lower Palaeozoic Ribband Group of southeast Ireland, and not part of the Cahore Group as has recently been suggested by the Irish Geological Survey. Thirty-seven acritarch species (or species groups) are distinguished and illustrated. One new species, Stellechinatum ? mariatheresae Vanguestaine, is established.

Ediacaran acritarch biota from the Giles 1 drillhole, Officer Basin, Australia, and its potential for biostratigraphic correlation

The remarkable diversification of single-celled photosynthesising biota of algal and other as yet unknown affinities (acritarchs), followed by the diversification of metazoans, occurred during the Ediacaran Period, which is marked by extreme climatic and environmental changes. Here we describe a taxonomically diverse acritarch association from the Ediacaran part of the Giles 1 drillcore in the Officer Basin, South Australia, which documents further the Ediacaran phytoplankton radiation. The studied palynoflora comprises 21 known acritarch species belonging to 15 genera. One new monospecific genus is described ( Calyxia xandaros sp. nov.) as well as one new species of Tanarium ( Tanarium anozos sp. nov.). We also propose the genus Knollisphaeridium to replace the preoccupied genus Echinosphaeridium. Three stratigraphically successive assemblages that match previously observed patterns of acritarch replacement are distinguished. The present record is from slightly older strata than in previous records, thus extending the ranges of certain acanthomorphic species. The lower boundaries of three stratigraphically higher assemblage zones among the four formerly established zones, are identified by the occurrence of the index species Tanarium conoideum, Tanarium irregulare and Apodastoides verobturatus, respectively. The substantial morphological disparity of acritarchs in the Giles 1 succession suggests that they may represent a great diversity of microorganisms, not only as biological species but also representing perhaps vegetative and encysted stages in their life cycle. The reconstructed palaeogeographic distribution of several species between Australia, Siberia, Baltica (the East European Platform), and to South China, shows that acritarchs are suitable for both intra- and inter-regional correlation.

Fossil microphytoplankton dynamics across the Ordovician–Silurian boundary

A critical review of all available data on acritarch biostratigraphy and diversity dynamics across the Late Ordovician through the early Silurian, permits a better appreciation of the potential of acritarchs for the recognition of the systemic boundary. This analysis also reveals the response of marine microphytoplankton populations to the Late Ordovician palaeoenvironmental crisis (Hirnantian glaciation). Previous zonal schemes are improved, and an updated acritarch biostratigraphic chart is proposed, plotted against the most recent chronostratigraphic subdivisions. Sections from Anticosti Island (Québec, Canada), Algeria, Morocco, and Estonia preserve the best palynological record for the investigated interval. The present analysis shows that no true mass-extinction event occurred in latest Ordovician times in connection with the well known glacial event. “Pre-glacial” Ashgill acritarch suites are dominated by species of Baltisphaeridium, Multiplicisphaeridium, Ordovicidium, Orthosphaeridium, and netromorph acritarchs. An important proportion of these taxa (excluding Ordovicidium and Orthosphaeridium) survive the onset of glacial conditions in Hirnantian (latest Ordovician) times and continues through the early Silurian. The development of morphological polymorphism appears as a response (a survival strategy?) to the establishment of glacial conditions. In glacial-related sediments of Hirnantian age in North Africa, acritarch assemblages display a burst of relative abundance and intra-specific morphological variability (polymorphism) of long-ranging taxa such as Veryhachium, Multiplicisphaeridium, Dactylofusa, Poikilofusa, and Evittia. The extinction of several species characteristic of Upper to uppermost Ordovician strata occurs near the boundary, in “post-glacial” Ashgill (uppermost Hirnantian). This extinction event is counterbalanced by the almost contemporaneous (within the limits of stratigraphic resolution) appearance of several new taxa showing already a clear “Silurian affinity”, e.g., Tylotopalla, Cymbosphaeridium, and Visbysphaera. This origination event is observable in, and correlatable between the North African, the Bohemian and the Anticosti sections, making it global in extent. The completion of the palynological turnover and the establishment of a diverse Silurian acritarch suite occurs well above the base of the Silurian, during Aeronian times. The strong survival capability of the oceanic plankton through periods of palaeoenvironmental crisis in latest Ordovician times (but also throughout the Phanerozoic) could have played an important role in the post-extinction rebounds of metazoan clades, by assuring the continuity of marine trophic resources to consumers and avoiding irreversible disruptions of the trophic chains.

Stratigraphic distribution of selected acritarchs in the Ordovician subsurface, Canning Basin, Western Australia

Palynological investigation of 80 core samples from four boreholes, drilled through the Lower to Middle Ordovician marine sections of the central-northeastern Canning Basin, northwestern Australia, reveals diverse acritarch associations including a number of stratigraphically significant species. The stratigraphic succession studied, embracing the Willara Formation and conformably overlying Goldwyer Formation, has been independently dated as early Arenig to Llanvirn by conodont faunas ( Oepikodus communis through Phragmodus- Plectodina zonal interval). Nine morphologically distinctive acritarch species have relatively narrow vertical ranges within the study interval and likely constitute serviceable palynostratigraphic indices for the Ordovician in and possibly beyond the Western Australian study area. Three new species, Comasphaeridium setaricum, Polyancistrodorus kunzeanensis, and Stelomorpha calix, are established; and one new combination, Baltisphaeridium variocavatum (Playford and Martin, 1984), is proposed. Other stratigraphically significant species include Petaloferidium comptum Playford and Martin, 1984 and Peteinosphaeridium sp. cf. P. exornatum Tongiorgi et al., 1995; together with Dorsennidium symmetricum (Lu, 1987) Sarjeant and Stancliffe, 1994, Peteinosphaeridium coronula Yin et al., 1998, and Striatotheca rarirrugulata (Cramer et al., 1974) Eisenack et al., 1976, which are found for the first time in Australia. The vertical distribution, and hence the stratigraphic utility, of each of the above species is documented through the studied sections.

Environmental changes reflected by palynomorphs in the early Middle Ordovician Hanadir Member of the Qasim Formation, Saudi Arabia

Core samples from the QASIM-801 (QSIM-801) water well in the Qasim Region of central Saudi Arabia were investigated palynologically. The interval studied contains the transgressive succession from the Sajir Member of the Saq Formation to the Hanadir Member of the Qasim Formation, and yields well-preserved and abundant cryptospores, microphytoplankton (acritarchs and chlorophycean algae), and chitinozoans, indicative of a Darriwilian (Llanvirn) age. The lowermost portion of the cored interval from the upper Sajir Member consists of fine-grained sandstones deposited in tidal flat, and shallow-marine settings. It is characterized by abundant and diverse cryptospores (permanent tetrads, dyads, monads), cuticle-like phytoclasts, and a low diversity assemblage of marine palynomorphs. The upper part of the cored interval from the Hanadir Member is composed of hemipelagic to pelagic, organic-rich, graptolitic shales, and micaceous shales, of middle- to outer-shelf depositional settings, which yield abundant marine palynomorphs (acritarchs and chitinozoans). Cryptospore-bearing horizons are interspersed in the succession. Changes in the composition of palynomorph assemblages reflect higher frequency environmental changes within an overall transgressive succession. Four new acritarch species are described in open nomenclature, Tyrannus sp. A, Clypeolus sp. A,? Pulvinosphaeridium sp. A, and? Tinacula sp. A.

Palynostratigraphy and palaeogeography of the Padeha, Khoshyeilagh, and Mobarak formations in the eastern Alborz Range (Kopet-Dagh region), northeastern Iran

A total of 113 surface samples collected from the Padeha, Khoshyeilagh, and Mobarak formations of Kuh-e-Ozum, northeast of Jajarm town were processed for palynomorphs, in order to determine age relationships. Well-preserved and abundant palynomorphs dominated by organic-walled-marine microphytoplankton (acritarchs and prasinophyte phycomata), miospores and subordinate chitinozoans, and scolecodonts were recovered. Seven species of prasinophyte phycomata (four genera), 19 acritarch species (14 genera), one species of chitinozoa, and 26 miospore species (19 genera) were recorded and assigned to eight local Assemblage Zones. Assemblage Zones I–IV occur in the Padeha Formation and suggest an Early Late Devonian (Frasnian) age whilst assemblages zones V–VII are present in the Khoshyeilagh Formation and indicate Late Devonian (Famennian) ages. Assemblage zone VIII, which occurs in the basal part of Mobarak Formation, suggests a Lower Mississippian (Tournaisian) age for this formation. Many of the palynomorph groups encountered are closely comparable with coeval assemblages recorded from Western Australia, southwest Ireland, England, Turkey, Saudi Arabia, North Africa, and South America, indicating the close relationship of the Iranian Platform to other parts of the northern Gondwana Domain during the time interval represented by these strata. The presence of marine palynomorphs (acritarchs/prasinophyte phycomata, chitinozoans, and scolecodonts), and shelly macrofauna (brachiopods, gastropods, and corals) in Member c of the Padeha Formation (as well as the Khoshyeilagh and Mobarak formations), together with associated miospores, indicate an open marine (moderately nearshore) depositional environment for the Upper Devonian and Lower Carboniferous deposits in northeastern Alborz Range (Kopet-Dagh region) of Iran.

Palynostratigraphy of Ordovician strata, Canning Basin, Western Australia. - Part One: acritarchs and prasinophytes

This paper is the initial part of a comprehensive bipartite monograph of palynomorphs (viz., acritarchs, prasinophyte phycomata, and chitinozoans) that are represented profusely in marine lower Palaeozoic strata of the Canning Basin, Western Australia. The prime aim is to establish a palynologically based zonal scheme for the Ordovician sequence as represented in five cored boreholes drilled through the Lower to Middle Ordovician strata of the central-northeastern Canning Basin. These strata embrace the Oepikodus communis through Phragmodus-Plectodina conodont zonal interval and comprise (in ascending order) the Willara, Goldwyer, and Nita formations, of inferred early Arenig to Llanvirn age. All three formations contain moderately diverse and variably preserved palynomorphs. The palynomorph taxa, detailed systematically in the current Part One of this monograph, comprise 66 species of acritarchs and six of prasinophytes. Of these, two species of prasinophytes and 11 of acritarchs are newly established: Cymatiosphaera meandrica and Pterospermella franciniae; Aremoricanium hyalinum, A. solaris, Baltisphaeridium tenuicomatum, Gorgonisphaeridium crebrum, Lophosphaeridium aequalium, L. aspersum, Micrhystridium infrequens, Pylantios hadrus, Sertulidium amplexum, Striatotheca indistincta, and Tribulidium globosum. Pylantios (typified by P. hadrus), Sertulidium (typified by S. amplexum), and Tribulidium (typified by T globosum); are defined as new acritarch genera. Three new combinations are instituted: Baltisphaeridium pugiatum (PLAYFORD & MARTIN 1984), Polygonium canningianum (COMRAZ & PENIGUEL 1972), and Sacculidium furtivum (PLAYFORD & MARTIN 1984); and Ammonidium macilentum PLAYFORD & MARTIN 1984 and Sacculidium furtivum (PLAYFORD & MARTIN 1984) are emended. An appreciable number of palynomorph species are not formally named owing to lack of sufficient or adequately preserved specimens; others are compared but not positively identified with previously instituted species. The ensuing Part Two of this study will complete the systematic-descriptive documentation, i.e., chitinozoans, and evaluate the Canning Basin palynoflora in terms of its chronological and stratigraphic-correlative significance.

Neoproterozoic (Ediacaran) diversification of acritarchs—A new record from the Murnaroo 1 drillcore, eastern Officer Basin, Australia

The Neoproterozoic is characterised by global environmental change and the diversification of many groups in the fossil record including animals and photosynthesising microflora. The cause–effect relationships between environmental and biotic changes are poorly understood but drastic events such as low-latitude glaciations, ocean oxidation and perhaps a bolide impact (on a more regional scale) may have had a decisive influence on the biota at the time. A new study on organic-walled microfossils (acritarchs) from the Murnaroo 1 drillcore, eastern Officer Basin, Australia, reveals the presence of assemblages with great morphological disparity, taxonomic diversity and abundance. This microbiota represents a part of the Ediacaran diversification event recognised previously from the Officer Basin, and other sedimentary basins in south-central Australia. Earlier studies included only a cursory analysis of Murnaroo 1. Our more extensive study of this drillhole supports the acritarch-based zonation established by Grey for the middle Ediacaran System and both the uppermost and the lowermost acanthomorph zones in the Ediacaran biostratigraphic scheme have now been recorded in Murnaroo 1. Some species appear cosmopolitan, being known from Siberia and China, providing good potential for interregional correlation and palaeobiogeographic reconstructions. The significance of the present acritarch palynoflora, in addition to being more abundant, diverse and covering a wider stratigraphic interval, lies in the fact that (in Australia at least) it tightens the constraints on the first appearance of acanthomorphic acritarch species, and indicates that it is much closer to the Acraman impact ejecta layer than previously inferred. In the aftermath of the global glaciations and the Acraman impact, the Ediacaran acritarch diversification may have had a profound influence on the evolution of other life forms, such as the Ediacara fauna.

An Upper Ordovician acritarch and prasinophyte assemblage from Dawangou, Xinjiang, northwestern China: Biostratigraphic and paleogeographic implications

A moderately well-preserved and moderately diverse acritarch and prasinophyte assemblage was recovered from four Upper Ordovician formations of the Dawangou section, Xinjiang, northwestern China (Tarim Block). The palynomorph assemblage consists of 27 species assigned to 17 genera (24 species and 16 genera of acritarchs and three species and one genus of prasinophyte). One new acritarch species, Navifusa tarimensis is proposed in addition to one new combination, Leiosphaeridia caradocensis (= Dichotisphaera caradocensis Turner, 1984). The palynomorph assemblage is dominated by acanthomorphic acritarchs (particularly species of Baltisphaeridium), followed by netromorphs, represented by Navifusa. Leiosphaeridia is the only prasinophyte present. Palynologic and sedimentologic evidence indicates a transgressive and regressive sequence occurred during deposition of the Dawangou section sediments. Comparison of the palynomorph assemblage with correlative age assemblages elsewhere supports the observation that there was little faunal or floral provincialism during the Late Ordovician. In addition, 13 species recovered from the Dawangou section are restricted to the Caradoc–Ashgill, and have a wide geographic range making them useful biostratigraphic taxa.

Upper Devonian biostratigraphy of the Iberian Pyrite Belt, southwest Spain - Part Two: Organic-walled microphytoplankton

Upper Devonian rocks of the Iberian Pyrite Belt (IPB) in southwest Spain, comprising the Phyllite-Quartzite Group (PQ) and the lower part of the overlying Volcano-Sedimentary Complex (VSC), contain a diversity of terrestrial and marine palynomorphs (miospores and organic-walled microphytoplankton, respectively), which constitute the basis of this biostratigraphically oriented research project. Part One of the report has previously detailed the miospore content of the constituent 117 palyniferous samples. In the present paper (i.e., the concluding Part Two), the organic-walled microphytoplankton (acritarchs and prasinophyte phycomata) are systematically described and illustrated, and their occurrence in the study material is fully documented. The acritarchs are represented by 23 species (including one species complex) allocated among 14 genera (one of which, Dupliciradiatum, is newly established), together with a very rare and novel category (informally termed Gen. nov. A). The following new acritarch species are formally instituted: Dupliciradiatum crassum (type species), D. tenue, Histopalla languida, and Winwaloeusia repagulata. Five genera allied with the prasinophycean algae are identified; these accommodate a total of 15 species of which two - Cymatiosphaera tenuimembrana and Maranhites multioculus - are formally proposed as new. In addition, representatives of the prasinophyte genera Leiosphaeridia and Tasmanites are recorded but are not discriminated at species level. The microphytoplankton suite is clearly consonant, from previously published occurrences in other regions, with a Late Devonian dating. However, most of the species are known to be relatively long ranging through (and in some cases beyond) that epoch and hence are not amenable to detailed biozonal subdivision of the IPB succession. Moreover, the distribution of the species therein tends to be erratic in comparison with the more consistently occurring miospores, possibly due to stress factors induced by fluctuating conditions in the IPBs Upper Devonian marine environment. By contrast, the land-derived (miospore) assemblages are readily applicable in a blostratigraphic context: they can be correlated precisely with the Devonian miospore biozonation scheme for Western Europe. In those terms, the sampled PQ strata are assignable to the Diducites versabilis-Grandispora cornuta (VCo) Biozone of late Famennian age; while the samples from the anoxic sequence at the base of the VSC belong to the Retispora lepidophyta-Verrucosisporites nitidus (LN) Biozone (latest Famennian = latest Devonian). The biochronostratigraphic data, in conjunction with the findings from earlier IPB studies, imply two appreciable palynostratigraphic breaks within the PQ. These are representative, respectively, of the lower Frasnian-middle Famennian interval and of part of the Strunian/upper Famennian. Speculation currently remains as to whether the inferred gaps are more apparent than real; i.e., whether one or both represent actual hiatuses in IPB sedimentation or are simply a manifestation of hitherto unsampled and/or non-palyniferous PQ strata.

An Ordovician age for the Muggort's Bay Lower Palaeozoic inlier, County Waterford, Ireland—the southernmost exposure of the Irish Caledonides

The most southerly exposed Lower Palaeozoic strata in Ireland occur on the southwest coast of County Waterford along a 2.5 km long coastal section at Muggort's Bay where they are surrounded by Devonian rocks. Five formations can be distinguished which, in ascending order, are: the Ballycurreen, Carrickbrean, Rathnameenagh, Moanbrack and Killinoorin formations. The total thickness of the succession is over 1800 m. No macrofossils are present, but the lithologies are largely fine-grained turbidites and subordinate volcanic rocks which closely resemble the Ribband Group seen elsewhere in southeast Ireland and have previously therefore been classified with it. Palynological analysis was undertaken on 25 samples collected from Muggort's Bay, of which eight were productive. Diagnostic microfossils, comprising acritarchs, chitinozoans and scolecodonts, indicate an Early to Middle Ordovician age for both the Rathnameenagh and the Moanbrack formations. These ages confirm that the strata are part of the Ribband Group which elsewhere has been biostratigraphically dated as ranging from Mid-Cambrian to Mid-Ordovician. Reworked mid-Middle Cambrian acritarchs occur in the Moanbrack Formation and reworked late Middle to early Late Cambrian acritarchs in the Rathnameenagh Formation. Despite generally poor preservation of the organic matter, some 20 acritarch species have been distinguished. Among these, three species belong to the herein revised genus Retisphaeridium for which an emended diagnosis is proposed together with two new combinations, Retisphaeridium capsulatum (Jankauskas, 1976) Vanguestaine nov. comb. and Retisphaeridium pusillum (Moczydlowska, 1998) Vanguestaine nov. comb. Copyright © 2005 John Wiley & Sons, Ltd.

Acritarchs from the Cambrian‐Ordovician boundary interval on Kolguev Island, Arctic Russia

New evidence from taxonomically rich and age‐diagnostic acritarch assemblages is used to recognize Upper Cambrian and Tremadoc strata in the Bugrino 1 and North‐Western 202 boreholes on Kolguev Island, Arctic Russia. The position of the boundary between the Cambrian and Ordovician systems in the sedimentologically continuous successions is inferred, as are stratigraphic intervals equivalent to the Late Cambrian Peltura and Acerocare trilobite zones of Baltica. This is the first paleontological documentation of Cambrian strata on the island and, on a regional scale, within the Pechora Basin and northeastern Europe. The newly established biostratigraphy provides the maximum relative age for the basal portion of the sedimentary cover overlying the Timanian angular unconformity, which separates the sediments from deformed and metamorphosed basement. In turn, it constrains the time‐span of the hiatus as pre‐Late Cambrian and thus is significant for reconstructing the timing of the major tectonic events in this region of the Baltica palaeocontinent. Twenty‐seven acritarch species are described in detail and 10 taxa are left under open nomenclature. The taxonomic attribution of several species is revised. The diagnosis of the genus Acanthodiacrodium is restricted, and those of Dasydiacrodium and Polygonium are emended. New combinations are proposed for the species Actinotodissus crinitus, A. formosus and Solisphaeridium akrochordum, and new emendations and combinations for Actinotodissus polimorphus, A. secundarius, and A. spinutisus. A new species, Solisphaeridium Chinese, is recognized. The stratigraphic ranges of taxa are compiled from global records and extension of some species ranges is suggested.

New acritarch species from Holocene sediments in central West Greenland

Palynologic assemblages from late Quaternary sediments from Disko Bugt, central West Greenland, contain a new acritarch species described herein as Palaeostomocystis subtilitheca sp. nov. The type of the genus Beringiella, Beringiella fritilla Bujak 1984, is transferred to the genus Palaeostomocystis Deflandre 1937, and the new combination Palaeostomocystis fritilla (Bujak 1984) is proposed. Palaeostomocystis subtilitheca sp. nov. and P. fritilla (Bujak 1984) comb. nov. are characteristic of polar to subpolar environments and may be potential environmental indicators.

Dinoflagellate cyst stratigraphy and palaeoecology of the Pliocene in northern Belgium, southern North Sea Basin

Dinoflagellate cysts and other palynomorphs from the Pliocene Kattendijk and Lillo formations, exposed in two temporary outcrops in northern Belgium, provide new information on the biostratigraphic position and sequence stratigraphic interpretation of these units. Dinoflagellate cysts from the Kattendijk Formation indicate an age between about 5.0 Ma and 4.7–4.4 Ma (early Early Pliocene) in our sections, confirming a correlation with standard sequence 3.4 and implying a slightly greater age than the Ramsholt Member of the Coralline Crag Formation of eastern England. The unconformity at the base of the Kattendijk Formation was not seen, but presumably correlates with sequence boundary Me2 at 5.73 Ma. The overlying Lillo Formation is late Early Pliocene or early Late Pliocene (c. 4.2–2.6 Ma) in age, and the unconformity at its base may be correlated with sequence boundary Za2 at 4.04 Ma or Pia1 at 3.21 Ma. The Oorderen Sands and superjacent Kruisschans Sands members (Lillo Formation) are both part of the same depositional cycle. They were probably deposited before 2.74 Ma, and certainly before the onset of Northern Hemisphere cooling at c. 2.6 Ma. Evidence from dinoflagellate cysts indicates that both a shelly unit at the base of the Lillo Formation and the lower part of the overlying Oorderen Sands were deposited during a conspicuously cool climatic phase, with warmer temperatures returning during later deposition of the Oorderen Sands and Kruisschans Sands members. Many dinoflagellate cyst and acritarch species are reported here for the first time from the southern North Sea Basin. Selenopemphix conspicua (de Verteuil & Norris, 1992) stat. nov. is proposed.

Acritarchs from the Lower Palaeozoic succession on the south County Wexford coast, Ireland: new age constraints for the Cullenstown Formation and the Cahore and Ribband Groups

AbstractLower Palaeozoic sediments crop out on the southern coast of County Wexford, Ireland, comprising three distinctive lithostratigraphical units: from west to east the Cahore Group, the Blackhall Formation of the Ribband Group and the Cullenstown Formation. The three units are largely devoid of macrofossils and thus their ages have to date been uncertain and, in the case of the Cullenstown Formation, speculative. In the Cahore Group, a diverse assemblage of acritarchs composed of seventeen species has been recorded indicating a middle Early Cambrian age. This is similar to the age of the lithologically identical Bray Group to the north, in County Wicklow. In the Ribband Group, two very distinct assemblages have been noted. Both are poorly preserved, but diagnostic species have been determined giving a biostratigrapical range of early Mid‐Cambrian to Llanvirn. Palynomorphs have been recorded for the first time from the Cullenstown Formation allowing comparison with eastern Newfoundland where a similar, less diverse assemblage has been recorded. The age indicated is latest Mid‐Cambrian to early Late Cambrian. Overall, despite generally poor preservation of the organic matter, some 45 acritarch species have been distinguished, among which one new combination is proposed: Retisphaeridium postae instead of Cymatiosphaera postae. Copyright © 2004 John Wiley & Sons, Ltd.