Pattern of some Silurian bioevents

The best‐known Silurian bioevent occurred at the end of the Wenlock: the lundgreni event, together with the nassa‐ludensis crisis, was established among planktic graptolites. The East Baltic data show several peaks of high diversity (especially triangulatus, turriculatus, scanicus zones) and three levels of low diversity of graptolites (antennularius, radians, ludensis zones). These are more or less coincident with sea‐level changes. Energetic innovation of the shallow shelf corals started in the early Silurian. The late Wenlock regression seems not to affect them seriously but the late Silurian decline coincides with the aridization of the climate and a regression of the shelf seas. The Agnatha had strong radiations in the Wenlock (ludensis Zone) and Ludlow (leintwardinensis Zone), the fishes in the Pridoli. Many Silurian vertebrates were long‐ranging and extinction rate was relatively low; only at the late leintwardinensis level and in the latest Ludlow did considerable extinctions occur.     

The position of graptolites within Lower Palaeozoic planktic ecosystems

An integrated approach has been used to assess the palaeoecology of graptolites, both as a discrete group and as a part of the biota present within Ordovician and Silurian planktic realms. Study of the functional morphology of graptolites and comparisons with Recent ecological analogues demonstrates that graptolites most probably filled a variety of niches as primary consumers, with modes of life related to the colony morphotype. Graptolite coloniality was extremely ordered, lacking any close morphological analogues in Recent faunas. To obtain maximum functional efficiency, graptolites would have needed varying degrees of coordinated automobility. A change in lifestyle related to ontogenetic changes was prevalent within many graptolite groups. Differing lifestyle was reflected by differing reproductive strategies, with synrhabdosomes most likely being a method for rapid asexual reproduction. Direct evidence in the form of graptolithophage 'coprolitic' bodies, as well as indirect evidence in the form of probable defensive adaptations, indicate that graptolites comprised a food item for a variety of predators. Graptolites were also hosts to a variety of parasitic organisms and provided an important nutrient source for scavenging organisms. □ Feeding, graptolites, hydrodynamics, ontogeny, parasitism, palaeoecology, plankton, predation.     

Sheinwoodian (Silurian) conodonts and graptolites from NE Anti‐Atlas,Morocco

Männik, P., Loydell, D.K. & Lubeseder, S. 2010: Sheinwoodian (Silurian) conodonts and graptolites from NE Anti‐Atlas, Morocco. Lethaia, Vol. 44, pp. 410–416. Conodonts and graptolites from a limestone at the base of the Tamaghrout Formation indicate that this is of early Wenlock age. This is the first unequivocal dating of a Sheinwoodian limestone from Morocco. All of the conodonts are known also from several other regions and suggest that there was no major difference in conodont faunas in northern Gondwana, Baltica and Laurentia at this time. Biostratigraphy, conodonts, graptolites, Morocco, Silurian, Sheinwoodian.     

Retiolitid graptolites from the collection of Hermann Jaeger II: <Emphasis Type="Italic">Cometograptus</Emphasis>, <Emphasis Type="Italic">Spinograptus</Emphasis> and <Emphasis Type="Italic">Plectograptus</Emphasis>

Graptolites from the Jaeger collection at the Museum für Naturkunde (Berlin, Germany) provide important information on structural details of Silurian (Wenlock–Ludlow) retiolitids as well as for the biostratigraphic and biogeographic distribution of these magnificent graptolites. Species of the genera Cometograptus, Spinograptus and Plectograptus are described from isolated glacial boulder material, collected in northern Germany and from shale specimens found in the Lower Graptolite Shale of Thuringia. The biostratigraphic placement of material derived from glacial erratic boulders, however, is far from being precise. The fauna associated with the neotype of Plectograptus macilentus in the ‘Unterer Graptolithenschiefer’ of Thuringia is discussed and illustrated. Cometograptus alfeisenacki from the Cyrtograptus lundgreni Biozone is recognized as a new species. The genus is discovered for the first time in North German glacial erratic boulders.     

Graptolite extinction at the Llandovery—Wenlock boundary

Combined graptolite range data through the late Llandovery and early Wenlock of the Canadian Arctic Islands, the British Isles, and Lithuania reveal that survivorship rates from one zone to the next are between 56 and 85%, except at the Llandovery—Wenlock boundary, where only 19% (8 of 42) of the taxa of the upper sakmaricus—crenulata Zone survive into the centrifugus Zone. The high rate of graptolite extinction appears to coincide with glacioeustatic sea-level fall and may be attributed to changing oceanic temperatures, salinity, circulation patterns, oxygenation, and/or productivity associated with the onset of a glacial maximum event. □ Graptolite, extinction, Silurian, Llandovery, Wenlock.     

Ecology of a Late Silurian fauna of graptolites and associated organisms

The Ludlow Series of Wales and the Welsh Borderland represents an ocean-bordering basin and shelf which received a shoaling sequence of sediments in the late Silurian. Graptolites grade from highest diversity and density in basin areas to lowest diversity and density on the nearshore shelf. By analogy with modern zooplankton, this distribution is attributed to differentiation of surface water masses over basin and shelf. During Ludlow time, the center of graptolite abundance shifted northward, indicating a restriction and seaward movement of the basin water mass. Small nektic cephalopods and phyllocarids parallel distribution trends of the graptolites, as does a fauna of small bivalves and brachiopods which may have lived on floating plants. The water mass model accounts for most distributional patterns of pelagic organisms in this local area, and has broader application to the distribution and evolution of graptolites.     

Population structure of graptolite assemblages

Graptolite rhabdosomes display a diverse suite of morphologies. The range of morphotypes present within most moderate- to high-diversity assemblages from the Ordovician and Silurian is similar, despite the different taxonomic composition of the faunas at different times. Survivorship analyses of graptolite faunas from the Ordovician and Silurian demonstrate strong similarities in the mortality rates of unrelated graptolites of similar functional morphology. They also show a strong correlation between decreasing mortality rates among more mature colonies with increasing rhabdosome complexity. This similarity in both functional morphology and life history of graptolites suggests that they lived within a very stable planktic community structure.     

Graptolites from glacial erratics of the Laerheide area,northern Germany

Ordovician and Silurian glacial erratics of the Laerheide area (Lower Saxony, north-western Germany) bear well-preserved graptolites. The faunas provide important information on the origin and transport direction of the sediments preserved in a kame, representing the Drenthe stadial of the Saalian glaciation. The faunas even include species not commonly encountered in the successions of mainland Sweden, from where the erratics presumably originated. The most common graptolites are from Upper Ordovician (Sandbian to Katian) limestones and from Katian black shales. More common, however, are greenish limestones, sand- and siltstones, often combined in the term ‘Grünlich-Graues Graptolithengestein’, in which upper Wenlock to Ludlow (upper Silurian) graptolites are common.     

Sea level and faunal change during the late Wenlock and earliest Ludlow (Silurian): A point of view from the Algerian Sahara

A late Wenlock transgression (C. rigidus zone) affecting a large area of the Algerian Sahara has been known for the last thirty years; in this case there is no break at the Wenlock‐Ludlow boundary and the sedimentation is continuous to the earliest Ludlow. But in many other areas of Sahara a late Wenlock regression is observed. This regression may or may not be followed by a latest Wenlock or an early Ludlow transgression. A sedimentary model is proposed in despite of several problems. In order to compare what happens in each region a schematic diagram is used showing the tendencies to shallowing and deepening. Differences observed between the North Sahara basin and the other parts of the Algerian Sahara and comparison between local sea‐level curves and the global sea‐level curve proposed by Johnson et al. (1991) lead to reconsider the evidence of an eustatic early Ludlow event. To end the problem of the disappearance of nearly all graptolites at the end of Wenlock is discussed. The influence of land emergence in the evolution of marine fossil groups in the disturbance that it can bring to the development of the microplankton and the chemistry of the sea seems to have been too neglected.     

Metal limitation of cyanobacterial N2 fixation and implications for the Precambrian nitrogen cycle

Nitrogen fixation is a critical part of the global nitrogen cycle, replacing biologically available reduced nitrogen lost by denitrification. The redox‐sensitive trace metals Fe and Mo are key components of the primary nitrogenase enzyme used by cyanobacteria (and other prokaryotes) to fix atmospheric N₂ into bioessential compounds. Progressive oxygenation of the Earth's atmosphere has forced changes in the redox state of the oceans through geologic time, from anoxic Fe‐enriched waters in the Archean to partially sulfidic deep waters by the mid‐Proterozoic. This development of ocean redox chemistry during the Precambrian led to fluctuations in Fe and Mo availability that could have significantly impacted the ability of prokaryotes to fix nitrogen. It has been suggested that metal limitation of nitrogen fixation and nitrate assimilation, along with increased rates of denitrification, could have resulted in globally reduced rates of primary production and nitrogen‐starved oceans through much of the Proterozoic. To test the first part of this hypothesis, we grew N₂‐fixing cyanobacteria in cultures with metal concentrations reflecting an anoxic Archean ocean (high Fe, low Mo), a sulfidic Proterozoic ocean (low Fe, moderate Mo), and an oxic Phanerozoic ocean (low Fe, high Mo). We measured low rates of cellular N₂ fixation under [Fe] and [Mo] estimated for the Archean ocean. With decreased [Fe] and higher [Mo] representing sulfidic Proterozoic conditions, N₂ fixation, growth, and biomass C:N were similar to those observed with metal concentrations of the fully oxygenated oceans that likely developed in the Phanerozoic. Our results raise the possibility that an initial rise in atmospheric oxygen could actually have enhanced nitrogen fixation rates to near modern marine levels, providing that phosphate was available and rising O₂ levels did not markedly inhibit nitrogenase activity.     

Graptolithen aus ordovizischen Geschieben und die frühe Stammesgeschichte der Graptolithen

Primitive Ordovician Graptolites are compared with the recent PterobranchRhabdopleura. It is concluded that these tree-like primitive Ordovician Graptolites had creepingRhabdopleura-like ancestors. Probably all Graptolites developed fromRhabdopleura-like ancestors. — The soft parts of Graptolites had two arms likeRhabdopleura. Observations on the outer surface of Graptolites supportBeklemishev’s hypothesis, that Graptolite individuals were able to leave their tubes: moving outside on their tubes the individuals deposited material by their cephalic shields, forming the so called Cortex. A similar secondary deposition on the housing is known in recent Pterobranchs.     

Sea level and faunal changes during the latest Llandovery and earliest Ludlow (Silurian)

It is now recognized that the late Telychian and early Gorstian sea level changes are, like many others in the Silurian, of world‐wide extent. The 30–50 m deepening events at these times were between 1 and 2 Ma in duration, so melting continental ice caps appear to be the most probable cause. The faunal changes associated with the two events are respectively very close to the base and the top of the Wenlock Series and thus segregate many of the faunas diagnostic of the Llandovery, Wenlock, and Ludlow series. Permanent extinctions (often followed by radiations) are more prevalent in the graptolites, conodonts, and acritarchs, while benthic faunas are more affected by regional shifts in community distributions. This means that, in the benthos, slowly evolving lineages are the only reliable guides to time. Such phyletic evolution, however, appears to have been unaffected by sea‐level events.     

Oceanic oxygenation events in the anoxic Ediacaran ocean

The ocean‐atmosphere system is typically envisioned to have gone through a unidirectional oxygenation with significant oxygen increases in the earliest (ca. 635 Ma), middle (ca. 580 Ma), or late (ca. 560 Ma) Ediacaran Period. However, temporally discontinuous geochemical data and the patchy metazoan fossil record have been inadequate to chart the details of Ediacaran ocean oxygenation, raising fundamental debates about the timing of ocean oxygenation, its purported unidirectional rise, and its causal relationship, if any, with the evolution of early animal life. To better understand the Ediacaran ocean redox evolution, we have conducted a multi‐proxy paleoredox study of a relatively continuous, deep‐water section in South China that was paleogeographically connected with the open ocean. Iron speciation and pyrite morphology indicate locally euxinic (anoxic and sulfidic) environments throughout the Ediacaran in this section. In the same rocks, redox sensitive element enrichments and sulfur isotope data provide evidence for multiple oceanic oxygenation events (OOEs) in a predominantly anoxic global Ediacaran–early Cambrian ocean. This dynamic redox landscape contrasts with a recent view of a redox‐static Ediacaran ocean without significant change in oxygen content. The duration of the Ediacaran OOEs may be comparable to those of the oceanic anoxic events (OAEs) in otherwise well‐oxygenated Phanerozoic oceans. Anoxic events caused mass extinctions followed by fast recovery in biologically diversified Phanerozoic oceans. In contrast, oxygenation events in otherwise ecologically monotonous anoxic Ediacaran–early Cambrian oceans may have stimulated biotic innovations followed by prolonged evolutionary stasis.     

Pyrite in Silurian graptolites from Bornholm, Denmark

Pyrite is present in many horizons in dark grey graptolite-rich shales of Llandovery-early Wenlock age on the island of Bornholm, Denmark. The pyrite usually occurs as internal moulds of the graptolites, but rarely on the outside of the periderm. Occasionally the internal pyrite protrudes from the apertures of the rhabdosomes. Investigations using reflected light and scanning electron microscopy have shown that the internal pyrite is texturally variable. The abundance of the framboids is variable, ranging from thin linings on the inner side of the periderm to nearly complete infills of the graptolite colony. The pyrite also occurs as densely packed microcrystals and as larger euhedral crystals in all transitions from individual crystals, often developed on the surfaces of the framboids. to compact aggregations. Some of the pyritized graptolites have central cavities in the rhabdosomal linings. Within the cavities there are striking stalactite-like forms of pyrite suspended from the roof of the periderm. Some specimens have late infillings by calcite in the central part of the tubes. Most of the internal pyrite must have been formed very early during the taphonomic processes operating on the graptolites, pre-dating the compaction of the sediment. The framboids were first precipitated, followed by the aggregated euhedral pyrite. It is debatable whether the tubes of the graptolites were fluid-filled or partially gas-filled when the stalactites were formed; the latter is the more likely. The beds with fully pyritized graptolites might represent an episodic increase of sedimentation rate and oxygen content of bottom waters. □ Pyrite, Silurian, graptolites, Bornholm, stalactites, early diagenesis.     

Silurian (Wenlock–ludlow) Graptolites from Bolivia

Well preserved middle to upper Silurian (Wenlock–Ludlow) graptolites from Bolivia are described for the first time. Generally monospecific graptolite faunas, of species largely endemic to South America, are found in a few levels in the lower part of the Kirusillas, Rio Carrasco and Uncía formations. The oldest identified level yields specimens of Pristiograptus praedeubeli (Jaeger) and is referred to the upper Wenlock. Younger faunas belong to the Ludlow and include Saetograptus , Monograptus and Neodiversograptus specimens. These may be referred to the Gorstian (lower Ludlow). The fauna includes Saetograptus argentinus robustus ssp. nov. and Monograptus bolivianus sp. nov.     

Occurrence of Giant Borings of Osprioneides kampto in the Lower Silurian (Sheinwoodian) Stromatoporoids of Saaremaa,Estonia

The distribution of Osprioneides is more environmentally limited than that of Trypanites in the Silurian of Baltica. Osprioneides probably occurred only in large hard substrates of relatively deepwater muddy bottom open shelf environments. Osprioneides were relatively rare, occurring in 4.7% of all stromatoporoid specimens in that environment, in contrast to small Trypanites-Palaeosabella borings, which occur in 88.4% of stromatoporoids and 88.9% of heliolitid corals. Osprioneides is reported only from the lower Sheinwoodian stromatoporoids of the exposed Silurian of Saaremaa (Wenlock to Pridoli). Osprioneides borings probably played a minor role in the general bioerosion in the Silurian of Baltica.     

Global marine redox changes drove the rise and fall of the Ediacara biota

The role of O₂ in the evolution of early animals, as represented by some members of the Ediacara biota, has been heavily debated because current geochemical evidence paints a conflicting picture regarding global marine O₂ levels during key intervals of the rise and fall of the Ediacara biota. Fossil evidence indicates that the diversification the Ediacara biota occurred during or shortly after the Ediacaran Shuram negative C‐isotope Excursion (SE), which is often interpreted to reflect ocean oxygenation. However, there is conflicting evidence regarding ocean oxygen levels during the SE and the middle Ediacaran Period. To help resolve this debate, we examined U isotope variations (δ²³⁸U) in three carbonate sections from South China, Siberia, and USA that record the SE. The δ²³⁸U data from all three sections are in excellent agreement and reveal the largest positive shift in δ²³⁸U ever reported in the geologic record (from ~ ?0.74‰ to ~ ?0.26‰). Quantitative modeling of these data suggests that the global ocean switched from a largely anoxic state (26%–100% of the seafloor overlain by anoxic waters) to near‐modern levels of ocean oxygenation during the SE. This episode of ocean oxygenation is broadly coincident with the rise of the Ediacara biota. Following this initial radiation, the Ediacara biota persisted until the terminal Ediacaran period, when recently published U isotope data indicate a return to more widespread ocean anoxia. Taken together, it appears that global marine redox changes drove the rise and fall of the Ediacara biota.     

A novel, mat-forming Thiomargarita population associated with a sulfidic fluid flow from a deep-sea mud volcano

A mat‐forming population of the giant sulfur bacterium Thiomargarita was discovered at the flank of the mud volcano Amon on the Nile Deep Sea Fan in the Eastern Mediterranean Sea. All cells were of a spherical and vacuolated phenotype and internally stored globules of elemental sulfur. With a diameter of 24–65 µm, Thiomargarita cells from the Eastern Mediterranean were substantially smaller than cells of previously described populations. A 16S rRNA gene fragment was amplified and could be assigned to the Thiomargarita‐resembling cells by fluorescence in situ hybridization. This sequence is monophyletic with published Thiomargarita sequences but sequence similarities are only about 94%, indicating a distinct diversification. In the investigated habitat, highly dynamic conditions favour Thiomargarita species over other sulfur‐oxidizing bacteria. In contrast to Thiomargarita namibiensis populations, which rely on periodic resuspension from sulfidic sediment into the oxygenated water column, Thiomargarita cells at the Amon mud volcano seem to remain stationary at the sediment surface while environmental conditions change around them due to periodic brine flow.     

Microbes,mud and methane: cause and consequence of recurrent Early Jurassic anoxia following the end‐Triassic mass extinction

The end‐Triassic mass extinction (c. 201.6 Ma) was one of the five largest mass‐extinction events in the history of animal life. It was also associated with a dramatic, long‐lasting change in sedimentation style along the margins of the Tethys Ocean, from generally organic‐matter‐poor sediments during the Triassic to generally organic‐matter‐rich black shales during the Jurassic. New core material from Germany provides biomarker evidence of persistent photic‐zone euxinia during the Hettangian, the onset of which is associated with a series of both negative and positive carbon isotope excursions. Combined inorganic and organic geochemical and micropalaeontological analyses reveal strong similarities between the Hettangian and the better‐known Toarcian anoxic event. These events appear to be the most clearly expressed events within a series of anoxic episodes that also include poorly studied black shale intervals during the Sinemurian and Pliensbachian. Both the Hettangian and Toarcian events are marked by important changes in phytoplankton assemblages from chromophyte‐ to chlorophyte‐dominated assemblages within the European Epicontinental Seaway. Phytoplankton changes occurred in association with the establishment of photic‐zone euxinia, driven by a general increase in salinity stratification and warming of surface waters. For both events, the causes of large negative carbon isotope excursions remain incompletely understood; evidence exists for both variation in the δ¹³C of atmospheric CO₂ and variation in the sources of organic carbon. Regardless of the causes of δ¹³C variability, long‐term ocean anoxia during the Early Jurassic can be attributed to greenhouse warming and increased nutrient delivery to the oceans triggered by flood basalt volcanism.