High diversity dinoflagellate cyst assemblages from the Late Triassic of southern England: new information on early dinoflagellate evolution and palaeogeography

Abundant and diverse dinoflagellate cyst assemblages from the Rhaetian of southern England are characterized by the occurrence of a new species of Rhaetogonyaulax, some undescribed taxa and numerous forms with Arctic and Australasian affinities. The dinoflagellate cyst assemblages permit a discussion of the palaeogeographic distribution of dinoflagellate cysts in the Late Triassic. The hypothesis on a Late Triassic migration event of organic walled microplankton from higher latitudes to the Boreal domain is presented.     

Lunnomidinium scaniense Lindström, gen. et sp. nov., a new suessiacean dinoflagellate cyst from the Rhaetian of Scania, southern Sweden

The dinoflagellate cyst Lunnomidinium scaniense gen. et sp. nov. is present in the lower part of a thin sequence of Rhaetian sedimentary rocks exposed in the Lunnom Coal and Clay Pit in NW Scania, southern Sweden. It occurs in diverse, Rhaetian palynomorph assemblages, dominated by spores and pollen, but with rare specimens of the dinoflagellate cysts Rhaetogonyaulax rhaetica (Sarjeant) Loeblich and Loeblich 1968, Shublikodinium sp. and Beaumontella? caminuspina (Wall) Below 1987. Lunnomidinium scaniense is characterized by an epicystal {tAtItP} archeopyle, a large number of paraplates arranged in seven or eight latitudinal series, and intratabular ornamentation in some but not all of the paraplate series. Thus, L. scaniense is assignable to the family Suessiaceae. Lunnomidinium scaniense can be subdivided into two different morphological varieties, based on the autophragm ornamentation and cyst size.     

A latest Cretaceous to earliest Paleogene dinoflagellate cyst zonation from Antarctica,and implications for phytoprovincialism in the high southern latitudes

The thickest uppermost Cretaceous to lowermost Paleogene (Maastrichtian to Danian) sedimentary succession in the world is exposed on southern Seymour Island (65° South) in the James Ross Basin, Antarctic Peninsula. This fossiliferous shallow marine sequence, which spans the Cretaceous–Paleogene boundary, has allowed a high-resolution analysis of well-preserved marine palynomorphs. Previous correlation of Cretaceous–Paleogene marine palynomorph assemblages in the south polar region relied on dinoflagellate cyst biozonations from New Zealand and southern Australia. The age model of the southern Seymour Island succession is refined and placed within the stratigraphical context of the mid to high southern palaeolatitudes. Quantitative palynological analysis of a new 1102 m continuous stratigraphical section comprising the uppermost Snow Hill Island Formation and the López de Bertodano Formation (Marambio Group) across southern Seymour Island was undertaken. We propose the first formal late Maastrichtian to early Danian dinoflagellate cyst zonation scheme for the Antarctic based on this exceptional succession. Two new late Maastrichtian zones, including three subzones, and one new early Danian zone are defined. The oldest beds correlate well with the late Maastrichtian of New Zealand. In a wider context, a new South Polar Province based on Maastrichtian to Danian dinoflagellate cysts is proposed, which excludes most southern South American marine palynofloras. This interpretation is supported by models of ocean currents around Antarctica and implies an unrestricted oceanic connection across Antarctica between southern South America and the Tasman Sea.     

Middle-Upper Jurassic Foraminifera from Jumara Hills, Kutch, India

A prolific foraminiferal assemblage comprising 51 species is reported from Jurassic sediments of the Chari Formation, Jumara Hills, Kutch, India. The assemblage is dominated by the families Vaginulinidae and Nodosariidae. Sixteen species are reported for the first time from the Indian region including one new species. The foraminiferal assemblage suggests a Callovian to Oxfordian age for the studied sequence. The depositional environment of the studied sequence is interpreted based on foraminiferal evidence combined with lithology and megafauna which indicates that the sediments of the Chari formation exposed at Jumara Hills were deposited in a shallow-water, near shore, environment with fluctuating shoreline in a tectonically unstable shelf zone. The Jumara Hills foraminiferal assemblage exhibits close affinity with certain other Jurassic assemblages of the Tethyan Realm and helped in drawing palaeogeographic conclusions, suggesting that during the Middle and Late Jurassic epochs Kutch had close sea connections with Central Arabia, Afghanistan and Rajasthan on the one hand and Somalia and Malagasy on the other.     

Chitinozoan biostratigraphy of the Upper Ordovician Fågelsång GSSP, Scania, southern Sweden

In the Fågelsång section, the new Global Stratotype Section and Point (GSSP) for the base of the Upper Ordovician Series, 22 samples from the E14 (a, b and c) and E15 outcrops have been studied for chitinozoans. They yield rich and rather diverse species assemblages of this microfossil group. The approximately 16-m-thick sampled part of the section can be subdivided, from bottom to top, into two biozones and a subbiozone: the Laufeldochitina stentor zone, the Eisenackitina rhenana subzone and the Lagenochitina dalbyensis zone. The boundary between the lowest biozone and the subzone is situated 1.7 m below the marker “Fågelsång Phosphorite” bed, thus only slightly lower (0.3 m) than the base of the Nemagraptus gracilis graptolite biozone, which defines the base of the Upper Ordovician Series. The base of the L. dalbyensis zone is located just above the Fågelsång Phosphorite, remarkably low compared to the graptolite biostratigraphy. It is evident that the chitinozoan biozonation provides an additional tool to recognise the base of the Upper Ordovician in this section. Furthermore, a distinct faunal change is shown across the phosphorite bed, possibly indicating a hiatus.     

ISLANDINIUM BREVISPINOSUM SP. NOV. (DINOFLAGELLATA), A NEW ORGANIC‐WALLED DINOFLAGELLATE CYST FROM MODERN ESTUARINE SEDIMENTS OF NEW ENGLAND (USA)1

Modern estuarine environments remain underexplored for dinoflagellate cysts, despite a rapidly increasing knowledge of cyst distributions in open marine sediments. A study of modern estuarine sediments in New England has revealed the presence of Islandinium brevispinosum sp. nov., a new organic‐walled dinoflagellate cyst that is locally common and probably of heterotrophic affinity. Resistance of this cyst to standard palynological processing indicates its geological preservability, although fossils are not yet known. Previously assigned species of the genus Islandinium are characteristic of polar and subpolar environments today and cold paleoenvironments in the Quaternary. The present record of I. brevispinosum extends the ecological and geographical range of this genus into the warm temperate zone, where I. brevispinosum occupies specific environments with reduced salinities and elevated nutrient levels.     

Protoperidiniacean dinoflagellate cyst taxa from the Upper Miocene of ODP Leg 178, Antarctic Peninsula

Protoperidiniacean dinoflagellate cysts have been recovered from Upper Miocene sediments of Hole 1095, ODP Leg 178, drilled to the west of the Antarctic Peninsula. These cysts make up virtually the entire dinoflagellate cyst assemblages, and three new species are formally described herein as Selenopemphix bothrion sp. nov., Selenopemphix kepion sp. nov. and Selenopemphix minys sp. nov., together with one informal taxon and two species attributed to published taxa. The occurrence of Late Miocene protoperidiniacean-dominated assemblages in the vicinity of the Antarctic continent is of special interest. Their presence may indicate the initiation of the Antarctic Circumpolar Current and increased nutrient supply within the Southern Ocean as the Antarctic Divergence became established. A comparison with several other protoperidiniacean dinoflagellate assemblages of similar age in other parts of the world provides some preliminary evidence for possible concomitant oceanographic change at this time.     

The onset of major Northern Hemisphere glaciations and their impact on dinoflagellate cysts and acritarchs from the Singa section, Calabria (southern Italy) and DSDP Holes 607/607A (North Atlantic)

The climatic deterioration related to the onset of Northern Hemisphere glaciations (circa 2.52 Ma B.P.) must have lead to reorganization and relocation of species associations and may have enhanced species turnover. The present study investigates how this deterioration affects the dinoflagellate cyst and acritarch assemblages from two locations, DSDP Site 607 (North Atlantic) and the Singa section (southern Italy). The records from these locations cover the interval from 2.8 to 2.2 Ma with at least a 5 ka resolution and they have been correlated to the Milankovitch periodicities on a cycle to cycle basis by means of integrated high resolution stable isotope, calcium carbonate, foraminiferal, palynological and magnetostratigraphical datasets. In the present study this high resolution stratigraphic framework is used for a detailed correlation of events occurring in each of the depositional sequences. It also enables further assessment of the palaeoenvironmental preferences of some dinoflagellate cyst forms. Comparison of the two palynological records reveals a close correspondence in the timing of major assemblage changes and extinction events, confirming their Milankovitch cycle based correlation. A close link between periods of Northern Hemisphere cooling (at oxygen isotope stages 110, 104 and 100-96) and increased dinoflagellate cyst turnover appears to be present for both DSDP Site 607 and the Singa section. The turnover events can also be recognized in the records of planktic foraminifera and calcareous nannoplankton. Comparison of the Singa section with Site 607 and with other time equivalent marine palynological data sets, shows that some oceanic taxa respond similarly over a large area. The biostratigraphical implications are discussed. Notably the last occurrence of Invertocysta lacrymosa appears to be a valuable marker for isotope stage 110 in the Mediterranean and North Atlantic.     

Age and implications of the Forest Bed, Falkland Islands, southwest Atlantic Ocean: Evidence from fossil pollen and spores

Diverse pollen and spore assemblages (microfloras) are preserved in organic clays and lignites associated with fossil tree remains in the Forest Bed. At present, these microfloras provide the only reliable basis for determining the age, composition, structure and affinity of the Forest Bed community. The Bed is suggested to be Middle? Miocene to Early Pliocene (age range Oligocene–Early Pliocene) based on the time distribution of a number of rare pollen types and the present-day ecology of their nearest living relatives in central–southern America and the southwest Pacific. The wet forest community whose remains form the Forest Bed was a form of broadleaf-gymnosperm temperate rainforest in which Dacrydium, Lagarostrobos and Podocarpus are likely to have formed an overstorey above a closed canopy dominated by Nothofagus (Nothofagus) spp. The latter subgenus is now endemic to ±uniformly wet regions in southern South America whilst two of the tall gymnosperms (Dacrydium, Lagarostrobos) are now endemic to the southwest Pacific region. An extinct, probable shrub-sized, conifer (Podosporites microsaccatus) appears to have formed the ground cover. Ferns and fern allies were very rare, in contrast to most living Nothofagus-gymnosperm communities. Fossil pollen and spore types produced by uncommon subcanopy tree, shrub and fern species potentially provide a means for more accurately dating the deposit once their time of first occurrence and extinction at subantarctic latitudes in the southwest Atlantic region is established (nearest living equivalents given in parentheses) — Clavatipollenites cf glarius (Hedyosmum), Thymelaepollis sp. (Ovidia-type) and Cyatheacidites annulatus (Lophosoria). This will require palynostratigraphic analysis of Late Palaeogene–Neogene sediments recovered from basins surrounding the Falkland Islands, for example from DSDP Site 329 drilled on the Maurice Ewing Bank at the eastern margin of the Falkland Plateau.     

Palynofacies and TOC analysis of marine and non-marine sediments across the Middle–Upper Jurassic boundary in the central-northern Lusitanian Basin (Portugal)

The Middle–Upper Jurassic transition is a geodynamic benchmark in the evolutionary history of several peri-Atlantic basins. Contrary to the vast Tethyan and peri-Tethyan areas, in the Lusitanian Basin (Portugal), this interval corresponds to a major basin-wide disconformity preceded by a complex forced regression that induced sharp facies variations across the depositional systems. The Middle Jurassic units below are fully marine, whereas the Upper Jurassic sediments (Lower?-Middle Oxfordian), correspond to freshwater/brackish lacustrine, grading into punctuated brackish-restricted lagoonal and shallow-marine paleoenvironments. This study presents total organic carbon and palynofacies data of 34 samples (total analyzed thickness about 85 m) collected from three key sections encompassing the Middle–Upper Jurassic transition in the central-northern sector of the Lusitanian Basin. The palynofacies of the analyzed part of Middle Jurassic units (Cabo Mondego Formation) are characteristic of marine environments at their base, evidencing upwards a regressive trend of the depositional systems. Total organic carbon content is generally low, reaching up to 1.94 wt%. The Upper Jurassic Cabaços Formation presents kerogen assemblages mostly of continental origin, although punctuated by minor intervals of marine influence. Total organic carbon content is more variable, reaching up to 30.56 wt%. Intraclasts of re-deposited fragments of microbial mats were found incorporated in the kerogen assemblages, which point to highly dynamic erosional and depositional processes. Diversity of Botryococcus sp. occurrences was confirmed as an indicator of the degree of paleoenvironmental stability. The vertical distribution and comparison of the kerogen assemblages of the different sections indicate major changes of these parameters among relatively close settings and along narrow vertical intervals, attesting to the high sedimentary dynamics observed in the Lusitanian Basin.     

Palynostratigraphie du Callovien-Oxfordien (Jurassique) dans les Terres noires, sud-est France

The Terre noires are monotonous sedimentary deposits comprising thick, dark marls and thin carbonaceous layers. They constitute a lithostratigraphical group known in the major part of the SE French Basin. In the East of this basin this group, dated from late Bajocian to mid Oxfordian, is well developed; on the other hand on the West, it is reduced until it disappears locally on the border of the Massif Central. In the East, the succession is precisely dated by ammonites, whereas in the west, ammonites are not consistently present, and do not permit the same precision. Analysis of dinoflagellate cyst assemblages, at the Callovian-Oxfordian boundary, show that it is possible to correlate the eastern to the western succession. Two marker species allow the correlation of four outcrop sections. The first index, Wanaea fimbriata, is well known; its first appearance datum is exactly at the Callovian-Oxfordian limit in the Boreal and Mediterranean realms. It is the index taxon of the palynological parazone Wfi. The second index, Stephanelytron ceto, an endemic taxon in the SE French, has its last appearance datum within the Scarburghense horizon, the second subzone of the oldest zone of the Oxfordian (Mariae zone). The first section studied was sampled in the east part of the basin (Hautes Alpes department). It corresponds to the maxima of marine deposits and is used as the palynostratigraphical reference. The three other sections analyzed, sampled in the west part of the basin (Ardèche department) where marine deposits are more condensed, are correlated and dated due to dinoflagellate cyst markers observed in the reference section. These results indicate that dinoflagellate cysts are a reliable correlation tool. Batiacasphaera rugosa (Courtinat, 1980) nov. comb is proposed.     

Distribution and habitat preferences of species within the genus Elaphoidella Chappuis, 1929 (Crustacea: Copepoda: Harpacticoida) in Slovenia

Twelve species of the harpacticoid genus Elaphoidella, most of them exclusively groundwater species, have been recorded in Slovenia (SE Europe). Their distribution and ecology are reviewed with the aim of evaluating distribution patterns, species preferences for groundwater habitats, ecological preferences and interactions with other copepod species at regional scale. Data on Elaphoidella species were obtained partly from the existing literature and partly from the author's (AB) own sampling campaigns carried out together with his co-workers. During the rich history of collecting copepods in Slovenia (from the 1920s to present), Elaphoidella species were recorded at 78 sampling sites altogether. The majority of collecting was conducted in the southern (Dinaric region) and north-western (Alpine region) Slovenia. The most intensively sampled habitats were porous aquifers of alluvial plains, springs in the karstic unsaturated zone and percolation water in caves. The highest species richness of Elaphoidella was recorded in the southern Slovenia, where 10 species were found. The strictly “Dinaric” species are E. charon, E. franci, E. karstica, E. stammeri, E. sp. 1 and E. sp. 2, while E. phreatica and E. bidens were found exclusively in the north-western Slovenia. From the latest data on the copepod distributions (2002–2004), where the environmental characteristics of sampling sites were also measured, the relationship between selected environmental characteristics of the habitats and the presence of Elaphoidella species was analysed. The distribution of Elaphoidella species in Slovenia was found to be related with the region, habitat type, altitude, conductivity and pH of the water.     

Phylloceratina ammonoids in the Iberian Basin during the Middle Jurassic: a model of biogeographical and taphonomic dispersal related to relative sea-level changes

Phylloceratina and Lytoceratina ammonoids in the Middle Jurassic of the Iberian Chain (E. Spain) represent less than 1% of the whole of Ammonoidea. There are two intervals, however, in which their occurrence is remarkably constant: within the Upper Bajocian and at the Lower/Middle Callovian boundary. These two dispersal episodes of Phylloceratina and Lytoceratina into the Iberian Basin are regarded to reflect changes in their palaeoecological and taphonomical behaviour, as a consequence of regional sea-level changes during the Middle Jurassic. A relative rise during the Late Bajocian favoured the immigration of juvenile phyllocerataceans. Phylloceras and Adabofoloceras immigrations gave rise to monospecific assemblages, where they soon died. They did not breed or ontogenically develop in this basin. In contrast, phyllocerataceans recorded at the Lower/Middle Callovian boundary constitute polyspecific assemblages, dominated by adult individuals. These Callovian assemblages were formed by nekroplanktonic drift, related to a relative sea-level fall and general homogenization of the shelf bottom, hence favouring the concentration of remains of organisms from more open marine and oceanic areas.     

Marine microplankton and calcareous nannofossil palaeoecology of the Toarcian stratotype

In the Bifrons to Aalensis Ammonite Zones of the Vrines section (Toarcian stratotype), woody phytoclasts, nonsaccate pollen grains, and marine assemblages (dinoflagellate cysts, acritarchs, and foraminiferal linings) dominate the palynofacies. The dinoflagellate cyst assemblage is cosmopolitan with minor Boreal influences, characterized by relatively high quantities of Micrhystridium, Baltisphaeridium, Mendicodinium spinosum, Nannoceratopsis, and the Parvocysta suite, dominating in turn the marine assemblages. Marine assemblage compositions, both dinoflagellate cysts and acritarchs, and calcareous nannofossil abundances are different in marl and limestone lithotypes of the Vrines section. Calcareous nannofossils are generally more abundant in marls than in limestones, they display however a cyclic pattern of semiquantitative abundances in phase with lithological cycles. Although a diagenetic overprint cannot be completely excluded to explain such a difference, it seems likely that these dissimilarities are in part primary, the results of variations in terms of proximality-distality, and climatic fluctuations. A mean duration of 117.6 Kyr per marl-limestone alternation, and the stacking of four marl-limestone alternations for 470.6 Kyr, suggest a control by the Earth's two orbital eccentricity cycles. It is likely that the palaeoenvironmental conditions, which influenced the formation of marl-limestone alternations, also controlled the variations in marine phytoplankton assemblages.     

Biogeography of dinoflagellate cysts in northwest Atlantic estuaries

Few biogeographic studies of dinoflagellate cysts include the near‐shore estuarine environment. We determine the effect of estuary type, biogeography, and water quality on the spatial distribution of organic‐walled dinoflagellate cysts from the Northeast USA (Maine to Delaware) and Canada (Prince Edward Island). A total of 69 surface sediment samples were collected from 27 estuaries, from sites with surface salinities >20. Dinoflagellate cysts were examined microscopically and compared to environmental parameters using multivariate ordination techniques. The spatial distribution of cyst taxa reflects biogeographic provinces established by other marine organisms, with Cape Cod separating the northern Acadian Province from the southern Virginian Province. Species such as Lingulodinium machaerophorum and Polysphaeridinium zoharyi were found almost exclusively in the Virginian Province, while others such as Dubridinium spp. and Islandinium? cezare were more abundant in the Acadian Province. Tidal range, sea surface temperature (SST), and sea surface salinity (SSS) are statistically significant parameters influencing cyst assemblages. Samples from the same type of estuary cluster together in canonical correspondence analysis when the estuaries are within the same biogeographic province. The large geographic extent of this study, encompassing four main estuary types (riverine, lagoon, coastal embayment, and fjord), allowed us to determine that the type of estuary has an important influence on cyst assemblages. Due to greater seasonal variations in SSTs and SSSs in estuaries compared to the open ocean, cyst assemblages show distinct latitudinal trends. The estuarine context is important for understanding present‐day species distribution, the factors controlling them, and to better predict how they may change in the future.     

Lower and Middle Callovian gastropod assemblages from central European Russia

Two gastropod assemblages from the Lower Callovian of the Kostroma Region and the Middle Callovian of the Yaroslavl’ Region are studied and compared with synchronous assemblages of the Kostroma, Moscow, Bryansk, and Kursk regions. It is suggested that gastropods from the Yaroslavl’ and Kostroma regions inhabited colder water basins than gastropods from more southern regions. A new genus and species Znamenkiella ovalis gen. et sp. nov. (family Pseudomelaniidae) are described from the Lower Callovian of the Kostroma Region.     

Dinoflagellate cyst distribution in marine surface sediments off West Africa (17–6°N) in relation to sea-surface conditions, freshwater input and seasonal coastal upwelling

An organic-walled dinoflagellate cyst analysis was carried out on 53 surface sediment samples from West Africa (17–6°N) to obtain insight in the relationship between their spatial distribution and hydrological conditions in the upper water column as well as marine productivity in the study area.Multivariate analysis of the dinoflagellate cyst relative abundances and environmental parameters of the water column shows that sea-surface temperature, salinity, marine productivity and bottom water oxygen are the factors that relate significantly to the distribution patterns of individual species in the region.The composition of cyst assemblages and dinoflagellate cyst concentrations allows the identification of four hydrographic regimes; 1) the northern regime between 17 and 14°N characterized by high productivity associated with seasonal coastal upwelling, 2) the southern regime between 12 and 6°N associated with high-nutrient waters influenced by river discharge 3) the intermediate regime between 14 and 12°N influenced mainly by seasonal coastal upwelling additionally associated with fluvial input of terrestrial nutrients and 4) the offshore regime characterized by low chlorophyll-a concentrations in upper waters and high bottom water oxygen concentrations.Our data show that cysts of Polykrikos kofoidii, Selenopemphix quanta, Dubridinium spp., Echinidinium species, cysts of Protoperidinium monospinum and Spiniferites pachydermus are the best proxies to reconstruct the boundary between the NE trade winds and the monsoon winds in the subtropical eastern Atlantic Ocean. The association of Bitectatodinium spongium, Lejeunecysta oliva, Quinquecuspis concreta, Selenopemphix nephroides, Trinovantedinium applanatum can be used to reconstruct past river outflow variations within this region.     

Répartition stratigraphique et biozones des kystes de dinoflagellés au passage Jurassique moyen–Jurassique supérieur (Bathonien supérieur–Oxfordien inférieur) dans le Bassin de Guercif,Maroc nord-oriental

Despite the stratigraphical significance of dinoflagellate cysts as reliable markers for correlating and dating Jurassic–Cretaceous strata, investigations into this palynomorph group in the southern Tethyan Realm, specifically northwest Africa, are sparse and somewhat parochial. Most research on Jurassic dinoflagellate cysts is focussed on European depocentres in the Boreal and Sub-Boreal realms. This study is on biostratigraphical data from two petroleum boreholes (MSD1 and KDH1) drilled during 1985 in the Guercif Basin, northeast Morocco by ONAREP (Office National de Recherche et d’Exploitation Pétrolier), now ONHYM (Office National des Hydrocarbures et des Mines). These boreholes penetrated a thick siliciclastic succession, attributed to the Middle and Upper Jurassic, below Miocene marls. Over sixty dinoflagellate cyst taxa were identified. Four dinoflagellate cyst biozones, named GI to GIV, are established for the late Bathonian to early Oxfordian interval. These biozones are defined on the basis of the first appearance datum (FAD) and/or the last appearance datum (LAD) of some biomarker taxa which have wide geographical distributions. These are: Ctenidodinium combazii and Ctenidodinium sellwoodii for the GI Biozone (late Bathonian–early Callovian); Ctenidodinium continuum and Meiourogonyaulax caytonensis for the GII Biozone (middle Callovian); Gonyaulacysta centriconnata and Wanaea thysanota for the GIII Biozone (late Callovian–earliest Oxfordian); and Liesbergia liesbergensis and Systematophora penicillata for the GIV Biozone (early Oxfordian). These biozones are correlated to those already established for the respective intervals in other palaeogeographic regions, such as the Boreal, Sub-boreal and Tethyan realms.     

Spiniferites cruciformis: a fresh water dinoflagellate cyst?

Palynological studies of cored lacustrine sediments from the late Quaternary of Lake Kastoria, northern Greece, revealed a Late Glacial interval with abundant dinoflagellate cysts. Cyst assemblages include two identifiable species, Spiniferites cruciformis and Gonyaulax apiculata. The presence of the fresh water species G. apiculata is consistent with the lacustrine setting of these deposits, but that of S. cruciformis is anomalous. Previously, this species has only been recorded in abundance from presumed brackish marine sediments from the Black Sea and Marmara Sea sediments where geochemical data clearly record brackish salinities. Therefore, it has been regarded as a low salinity cyst type with a wide range of morphological variation that some workers have suggested to reflect salinity fluctuations. Specimens from Greece display only part of the range of morphological variability previously described from these (brackish) marine settings. Encountered morphological variation includes ellipsoidal/pentameral and cruciform endocyst shapes with rare intermediate shapes, and highly variable septa development. Specimens characterized by extremely reduced ornamentation known from (brackish) marine environments have not been recorded. Our records of S. cruciformis indicate that: (1) it could thrive in fresh water conditions; and (2) that apparently most of the strong morphological variations of the cysts are an intrinsic phenomenon for this taxon, and may only partly be linked to salinity variations as suggested earlier. We suggest that S. cruciformis essentially is a fresh water taxon, and that its records in (brackish) marine environments, with the exception of specimens with strongly reduced ornamentation, may be due to transportation, to short-lived fresh water surface conditions in such environments, or to tolerance of the species to brackish conditions.     

Palaeoenvironmental interpretation of microphytoplankton diversity trends in the Cambrian–Ordovician of the northern Sahara Platform

Analysis of vertical fluctuations in assemblage diversity and relative abundances of microphytoplankton morphological classes permits palaeoenvironmental reconstructions of the Cambrian–Ordovician stratal sequences in the Hassi-R'Mel area (north-central Algeria) and northern Rhadames Basin (southern Tunisia). Abundance fluctuations of acritarch morphological classes and assemblage diversity appear to be related to changes in depositional facies. The present palaeoenvironmental interpretation is primarily based on a comparison with microphytoplankton distributional trends in modern depositional environments. Where possible, the observations are tested against palaeoenvironmental information derived independently from previous lithostratigraphical, macropalaeontological, and petrographical studies of the formations investigated. The present data support the following palaeoenvironmental scenario: during Late Cambrian times, relatively shallow water, neritic conditions prevailed throughout the study area, with evidence for a more proximal environment in southern Tunisia. In the early Tremadoc, open marine, offshore conditions characterized the central Algerian region, but a shallower water setting is indicated for southern Tunisia. During the late Arenig–Llanvirn interval, an open marine, offshore setting is envisaged for the southern Tunisian region, with more proximal conditions prevailing in the Algerian Hassi-R'Mel area. During the entire Ashgill, an essentially restricted marine environment was established throughout the study area. Some stratigraphically important acritarch species (e.g. Acanthodiacrodium angustum) appear to be facies-sensitive; the recognition of palaeoecological control on microphytoplankton distribution has important consequences on the biostratigraphic application of these microfossils.