The Ordovician acritarch genera Tranvikium and Ampullula: their relationship and taxonomy

A restudy of the Ordovician (Arenig-Llanvirn) acritarch taxa Tranvikium polygonale Tynni, 1982, and Ampullula suetica Righi, 1991, indicates that they represent extremes in a single morphological plexus. At one extreme are forms with a polar 'excystment' aperture (closed by an operculum or two opercular pieces) and a smaller opening (plugged or open) at the opposite pole; at the other are forms lacking a polar aperture but having, at the opposite pole, a tube open distally and plugged or open basally. New morphological terms for these structures are proposed. The genera and species are treated as synonyms and an emended diagnosis is given for Tranvikium polygonale, incorporating this whole morphological plexus. The possible purposes of the structures exhibited and the likely affinity of T. polygonale to various groups of algae are discussed. The emendation of Ampullula by Brocke (1997) is rejected and the genus Stelomorpha Yin, 1994 retained, with an emended diagnosis. The new combination Stelomorpha princeps (Brocke, 1997) Uutela and Sarjeant is proposed.     

Ordovician and Silurian acritarch assemblages from the west Leinster and Slievenamon areas of southeast Ireland

The Lower Palaeozoic sequences west of the Leinster Granite and in the Slievenamon Inlier of southeast Ireland have been palynologically re-investigated. Most of the productive samples yielded sufficient identifiable acritarchs for positive stratigraphical age determinations for several of the formations. The samples also include rare cryptospores, scolecodonts and tubular structures. Previous work in the area west of the Leinster Granite proposed an unbroken succession from Early Ordovician Ribband Group turbidites and volcanics passing up conformably to Early Ordovician to Late Silurian Kilcullen Group. The new palynological data clearly show that the Kilcullen Group in this area is entirely Silurian (Llandovery-early Wenlock) in age, also results obtained from the same group at Slievenamon confirm the previously reported Silurian age. Ordovician acritarchs found in the Kilcullen Group of both study areas are reworked and range in age from late Tremadoc to Llanvirn. The new data reveal a major stratigraphic break between the Ribband Group dated as Early and Middle Ordovician and the Silurian Kilcullen Group. This major break extends some hundreds of kms southwest to the Dingle Peninsula and possibly equates with a similar discontinuity in the Isle of Man to the northeast. This break would thus appear to be a major feature within the northwestern Avalonian margin sequence.     

The messaoudensis-trifidum acritarch assemblage (Ordovician: late Tremadoc-early Arenig) of the Barriga Shale Formation, Sierra Morena (SW-Spain)

The "messaoudensis-trifidum acritarch assemblage" is currently considered to be characteristic of latest Tremadoc-earliest Arenig cold-water environments on the periphery of Gondwana, at high latitudes in the southern hemisphere. An integrated biostratigraphical study on both acritarchs and graptolites was until now only available for localities in northwest England. Reinvestigation of the "messaoudensis-trifidum acritarch assemblage" from the Barriga Formation (Sierra Morena, southwestern Spain), which contains some graptolite horizons that can be attributed to the latest Tremadoc (pre-phyllograptoides and pre-approximatus graptolite biozones), strengthens the biostratigraphical potential of the "messaoudensis-trifidum assemblage", and the importance of some of the acritarch taxa recovered from these levels. It is concluded that the acritarch genera Coryphidium Vavrdová, 1972, Peteinosphaeridium Staplin et al., 1965 emend. Playford et al., 1995, Striatotheca, Burmann, 1970, and the Veryhachium lairdii group (rectangular veryhachiids) appear in the latest Tremadoc, and should not be considered as indicators of the base of the Arenig, as previously suggested.     

A nearshore–offshore trend in acritarch distribution from the Early–Middle Ordovician of the Yangtze Platform, South China

The stratigraphical interval of the late Early Ordovician Didymograptus deflexus and the early Middle Ordovician Azygograptus suecicus graptolite Biozones was investigated from seven sections from the upper Yangtze Platform, southern China. These are located on different parts of the platform, between the nearshore environments of the Kunming area, Yunnan Province, and the offshore carbonate shelf of the Yichang area, Hubei Province. The assemblages recovered from the different parts of the platform vary both in terms of diversity and composition. The nearshore environments show low diversity assemblages with about 10 acritarch species, whereas the offshore shelf environments reflect higher diversities with about 40 species. The composition of the assemblages also changes from simple morphologies (micrhystrids, leiosphaerids, fusiform acritarchs) in nearshore environments to specimens with longer and more complexly branched processes on the shelf. The polygonomorph acritarchs are common over all the upper Yantze Platform, while the acanthomorph genera Baltisphaeridium and Peteinosphaeridium are the most abundant taxa on the offshore carbonate shelf area. Phylogenetic or sea-level changes are probably not responsible for the compositional and diversity changes that occurred during the investigated interval. This study confirms previous interpretations that poorly diversified Palaeozoic acritarch assemblages occur in neritic environments and more complex, highly diversified assemblages are found on the shelf.     

The Early Cambrian acritarch Skiagia and its significance for global correlation

Global records of Early Cambrian acritarchs from the Skiagia plexus reveal that they are stratigraphically significant and can be used as biostratigraphic markers. Skiagia ornata and Skiagia ciliosa have a cosmopolitan distribution and the levels of their first appearance datums are proposed to serve as biostratigraphic horizons for interregional correlation. These levels are close to the lower boundary of the undefined Cambrian Stage 3, and within the Stage 3, respectively. The acritarch assemblage diagnostic of the Heliosphaeridium dissimilare-S. ciliosa Zone of Baltica is recorded worldwide and some species are traced on four of the Cambrian continents that include South Australia, South China and Laurentia. This supports the intercontinental correlation of certain Lower Cambrian strata and establishes the time equivalence between the regional trilobite zones to which these strata are attributed. The regional zonation is briefly reviewed for the purpose of comparing and evaluating the acritarch ranges. The biostratigraphic correlation of the Lower Cambrian strata, belonging to different faunal provinces, is based for the first time on the same fossil species, which have a cosmopolitan distribution and well-defined first appearance datums, and on the entire diagnostic acritarch assemblages.     

Chinese Paleozoic acritarch research: review and perspectives

Paleozoic acritarchs from China have been reported in more than 100 articles, most of which are published in Chinese journals and in Chinese. One half of the papers concern the Ordovician, and the majority of the investigated sections are located in the provinces of southern China. This review includes the first complete bibliographical listing of all publications on Chinese Paleozoic acritarchs.     

The Late Cambrian acritarch Cristallinium randomense: morphology,taxonomy and stratigraphical extension

The holotype of Cristallinium randomense Martin and other figured specimens are re-examined. The processes are often grouped as those of typical Vulcanisphaera species. Probable connections between Cristallinium and Vulcanisphaera are indicated. These genera form a plexus in which Retisphaeridium is also possibly involved. A complete bibliographical list of C. randomense is provided. Few published specimens are demonstrated to belong to this species. As a consequence, some stratigraphical dates, based on the restricted Late Cambrian range of the species, are questioned. The new combination Retisphaeridium ovillense (Cramer and Diez de Cramer) Vanguestaine nov. comb. is proposed.     

The Cambro - Ordovician boundary

Henningsmoen, G.: The Cambro-Ordovician boundary.
The Cambrian and Ordovician Systems were established in Britain, and it seems appropriate to define the Cambro-Ordovician boundary in Britain or where there is a closely related faunal succession at the junction. When Lapworth erected the Ordovician System in 1879, his intention was that its lower boundary should separate the so-called primordial (or first) and second Palaeozoic faunas. He drew this boundary at the base of the Lower Arenig, but apparently included the Upper Tremadoc in the Lower Arenig. It seems problematic to base the Cambro-Ordovician boundary on grounds of historical priority, and we seem to be free to decide whether to have the boundary at the base of the Tremadocian, at the base of the Arenigian, or within the Tremadocian (e.g. at the base of the Upper Tremadoc). The alternatives are shortly discussed. Arguments are put forward for placing the boundary not at a break, but preferably in a sequence with cosmopolitan fossils, either in a sequence of uniform facies or perhaps rather in a sequence with frequently alternating facies.     

New acritarch species from Holocene sediments in central West Greenland

Anther development, microsporogenesis, and microgametogenesis were studied using both light and TEM microscopy in the six accessible subdioecious/cryptically dioecious species of Consolea (Cactaceae). Anther wall development, microsporogenesis, and microgametogenesis are uniform in staminate flowers of all six species, and are typical for Cactaceae. Breakdown of microsporogenesis in male‐sterile anthers occurs early, at the onset of meiosis, and results in anthers bearing no pollen grains. The abortive process follows a common pattern in all investigated species. The tapetum is the first layer to deviate from normal male‐fertile anther development. Tapetal cells in male‐sterile anthers elongate at an early stage and have abundant rER with atypical configurations. Ultimately, the tapetum becomes hypertrophied and non‐functional. Male‐sterility in pistillate flowers appears to be directly related to these anomalies. In addition, other anther layers and tissues are affected, and normal patterns of programmed cell death (PCD) are disrupted. The relationship between these patterns and the pattern of PCD in normal male‐fertile anthers is discussed. We hypothesize a single origin for the cryptically dioecious/subdioecious breeding system of Consolea based on the uniformity of the anther's abortive processes in pistillate flowers.     

The genus Argemone

Summary Crosses between three diploid (A. mexicana; A. subfusiformis; A. albiflora) and one tetraploid (A. ochroleuca) Argemone species were made. The F₁'s were cytogenetically analysed. All the triploid hybrids were sterile and did not set any seed. In the species there was predominantly bivalent pairing (14II; 28II) and high pollen and seed fertility. The F₁'s displayed different configurations, e.g. I, II and III, and pollen fertility was low; the capsules were shrunken and did not contain any seed.In the two combinations mexicana X ochroleuca and subfusiformis X ochroleuca, pairing was identical and both auto- and allosyndesis were observed. The number of univalents, bivalents and trivalents varied in the three combinations but the number of associations did not differ significantly. In the albiflora X ochroleuca combination as many as 13 trivalents were observed.In general a negative correlation was observed between univalents and chiasmata per cell. However, chiasma frequency and paired associations displayed a positive correlation.It is deduced that sufficient similarities existed between one of the ochroleuca and the three diploid species genomes; the remainder of the ochroleuca genome had homologous chromosomes. Apparently A. ochroleuca carried enough cryptic intergenomal homologies which ordinarily remained unexposed. In the hemizygous state however, as in the F₁'s, there was intergenomal pairing. In an attempt to resolve the conflict between homology and bivalent pairing in the species, a diploidizing genetic mechanism is envisaged. Alternatively an acute propensity to preferential pairing caused bivalent formation. Such a system or systems caused meiotic isolation of various genomes and instituted normal fertility. Furthermore, the segmental allotetraploid nature of A. ochroleuca is concluded. The cytogenetic relationship between mexicana and ochroleuca is appraised.     

The Desulfitobacterium genus

Desulfitobacterium spp. are strictly anaerobic bacteria that were first isolated from environments contaminated by halogenated organic compounds. They are very versatile microorganisms that can use a wide variety of electron acceptors, such as nitrate, sulfite, metals, humic acids, and man-made or naturally occurring halogenated organic compounds. Most of the Desulfitobacterium strains can dehalogenate halogenated organic compounds by mechanisms of reductive dehalogenation, although the substrate spectrum of halogenated organic compounds varies substantially from one strain to another, even with strains belonging to the same species. A number of reductive dehalogenases and their corresponding gene loci have been isolated from these strains. Some of these loci are flanked by transposition sequences, suggesting that they can be transmitted by horizontal transfer via a catabolic transposon. Desulfitobacterium spp. can use H2 as electron donor below the threshold concentration that would allow sulfate reduction and methanogenesis. Furthermore, there is some evidence that syntrophic relationships occur between Desulfitobacterium spp. and sulfate-reducing bacteria, from which the Desulfitobacterium cells acquire their electrons by interspecies hydrogen transfer, and it is believed that this relationship also occurs in a methanogenic consortium. Because of their versatility, desulfitobacteria can be excellent candidates for the development of anaerobic bioremediation processes. The release of the complete genome of Desulfitobacterium hafniense strain Y51 and information from the partial genome sequence of D. hafniense strain DCB-2 will certainly help in predicting how desulfitobacteria interact with their environments and other microorganisms, and the mechanisms of actions related to reductive dehalogenation.     

The genus Amauroascus

Summary The genusAmauroascus, with two species,A. verrucosus andA. niger, is reviewed historically and authentic herbarium material is cited. The characteristics of pure cultures for the two species are given for the first time. Data are given in support of placingAmauroascus in synonymy withArachniotus.