Marco Vecoli

The 2.1 Ga old Francevillian biota: biogenicity, taphonomy and biodiversity.

The Paleoproterozoic Era witnessed crucial steps in the evolution of Earths surface environments following the first appreciable rise of free atmospheric oxygen concentrations ∼2.3 to 2.1 Ga ago, and concomitant shallow ocean oxygenation. While most sedimentary successions deposited during this time interval have experienced thermal overprinting from burial diagenesis and metamorphism, the ca. 2.1 Ga black shales of the Francevillian B Formation (FB2) cropping out in southeastern Gabon have not. The Francevillian Formation contains centimeter-sized structures interpreted as organized and spatially discrete populations of colonial organisms living in an oxygenated marine ecosystem. Here, new material from the FB2 black shales is presented and analyzed to further explore its biogenicity and taphonomy. Our extended record comprises variably sized, shaped, and structured pyritized macrofossils of lobate, elongated, and rod-shaped morphologies as well as abundant non-pyritized disk-shaped macrofossils and organic-walled acritarchs. Combined microtomography, geochemistry, and sedimentary analysis suggest a biota fossilized during early diagenesis. The emergence of this biota follows a rise in atmospheric oxygen, which is consistent with the idea that surface oxygenation allowed the evolution and ecological expansion of complex megascopic life.

THE ACRITARCH GENUS VERYHACHIUM DEUNFF 1954: TAXONOMIC EVALUATION AND FIRST APPEARANCE

Abstract Veryhachium Deunff 1954, originally described from the Ordovician of western France, is one of the most frequently recorded acritarch genera. Over 250 species and subspecies, from the Cambrian to the Neogene, have been attributed to the genus. This genus has a simple morphology; it displays a triangular, rectangular, or polygonal central vesicle, with a few, simple processes drawn out from the angles of the vesicle in a single plane, and sometimes with supplementary or auxillary processes arising from the vesicle body. Veryhachium has been emended and revised numerous times. The number of valid species is excessive: most are probably synonyms. To facilitate effective classification, only a few morphological categories should be retained. For the Lower Paleozoic, the use of two informal groups is proposed. These are the Veryhachium trispinosum group for triangular specimens, and the Veryhachium lairdii group for rectangular forms. Although generally abundant and widespread throughout the Phanerozoic, Veryhachium is of limited biostratigraphic, paleoecologic, or paleogeo-graphic value. However, its First Appearance Datum (FAD) is of great importance for Ordovician stratigraphy; the first Veryhachium morphotypes appear in the Tremadocian Stage, making the genus an important biostratigraphic marker.

Timing of the Avalonia-Baltica plate convergence as inferred from palaeogeographic and stratigraphic data of chitinozoan assemblages in West Pomerania, northern Poland

Abstract Tectonically disturbed Ordovician rocks penetrated by deep drillholes in Pomerania, NW Poland (Koszalin-Chojnice Zone) belong to the Heligoland-Pomerania Deformation Belt. Earlier data demonstrate that the Avalonia-Baltica collision occurred in Late Ordovician times, but in Pomerania, the timing of convergence has not been ascertained, and it is uncertain if the rocks underneath the Koszalin-Chojnice Zone belong to Avalonia or Baltica. Data from chitinozoans, organic-walled Palaeozoic microfossils with applications in biostratigraphy and palaeobiogeography, were assessed from ten boreholes (Brda 2; Brda 3; Chojnice 5; Karsina 1; Kościernica 1; Nowa Wieś 1; Okunino 1; Sarbinowo 1; Skibno 1; Wyszebórz 1) to address these problems. The results improve the biostratigraphy of the cores and demonstrate that the youngest Ordovician rocks are of a Burrellian (early mid Caradoc) to Cheneyan (late mid Caradoc) age. Because these rocks are interpreted as forming part of the deformation belt, the obtained ages put a lower age limit on the initiation of foreland basin sedimentation on the foreland of the orogeny, i.e. the Baltic platform. Quantitative comparison of chitinozoan assemblages demonstrates a high level of similarity between Pomerania and Avalonia. Together, Pomerania and Avalonia show greater similarity to Baltoscandia than to North Gondwana, supporting the idea that the Tornquist Ocean had narrowed significantly in early Caradoc times.

Acritarchs from the Ordovician–Silurian boundary beds of the Valga-10 drill core, southern Estonia (Baltica) and their stratigraphical and palaeobiogeographical implications

Fourteen samples of the Valga-10 drill core, south Estonia, from the lower Jelgava Formation (middle Pirgu Regional Stage, Upper Katian) to the lowermost Õhne Formation (lowermost Juuru Regional Stage, Lower Rhuddanian) were investigated for acritarchs. The section is biostratigraphically and chemostratigraphically well constrained; it comprises the rugata, taugourdeaui and scabra chitinozoan zones, the ordovicicus and giradeauensis conodont zones and the Hirnantian Isotopic Carbon Excursion (HICE). The good preservation allowed the identification of three prasinophyte phycomata and 52 acritarch species including the four new species Evittia porkuniensis, Helosphaeridium tongiorgii, Nexosarium leherissei and ?Veryhachium bulliferum. One new combination is proposed: Poikilofusa obliquipunctata (Uutela & Tynni 1991) comb. nov. Comparison with contemporaneaous palynofloras shows that eastern Laurentia and Baltica share a high number of species during the latest Katian—Hirnantian. Some of these species show a potential for future improvement of biostratigraphical correlation between the uppermost Katian— Hirnantian strata of low to mid-latitude carbonate platforms in eastern Laurentia and Baltica. Conversely, significant taxonomic differences exist between the assemblage studied and typical Gondwanan palynofloras. These results suggest that the Laurentian/Baltic and Gonwanan phytoplanktonic palaeoprovinces existed during latest Ordovician times. Published data reveal similar distribution pattern for chitinozoans and graptolites during the Hirnantian. A bathymetric ridge rise associated with the opening of the Rheic Ocean, coupled with the Hirnantian glacially-driven sea-level fall might have prevented water mass exchange and mixing/migration of phytoplankton between Gondwana and Laurentia/Baltica, thus being at the origin of the observed acritarch bioprovincialism. Additionally, distribution and diversity patterns of acritarchs are compared to those of other microfossils in the Valga-10 section. Near the base of the Hirnantian (Porkuni Regional Stage), benthic organisms (ostracods and scolecodonts) and phytoplankton (acritarchs) show increasing diversity with appearances of new taxa and new morphologies. Planktonic (chitinozoans) and nektonic (conodonts) organisms show a different pattern, with a decline in diversity during the earliest Hirnantian and a marked increase in the later part of the stage. Two alternative hyptotheses are proposed to explain these differences in diversification patterns: (1) the development of a shallower, proximal environment in the locality studied during the Hirnantian glaciation may have been more favourable to the diversification of benthonic organisms; (2) the planktonic and nektonic organisms suffered the effects of glaciation more than benthonic ones.

Billingen (Lower Arenig/Lower Ordovician) acritarchs from the East European Platform and their palaeobiogeographic significance

Billingen (Lower Arenig/Lower Ordovician) sediments of the St. Petersburg region, northwest Russia and the Leba area, northern Poland of the East European Craton yield acritarch assemblages, which are largely homogenous though displaying minor compositional differences that probably reflect a gradient from inner to outer shelf environments. Comparison with coeval acritarch microflora from the Yangtze Platform, South China, shows an overall similarity between Baltoscandian and South Chinese phytoplankton. The widespread uniformity in the fossil microphytoplankton may be related to the extensive global ‘evae’ sea-level transgression, which characterized the Billingen time. This suggests that during the Tremadoc through early Arenig times, acritarch assemblages displayed essentially an undifferentiated cold-water and oceanic character along the whole margin of Perigondwana in the South, as well as on the South Chinese and Baltic platforms, at middle latitudes (Mediterranean oceanic Realm). Despite this overall similarity, however, some typical taxa of the high-latitude Mediterranean Province (Arbusculidium, Coryphidium and Striatotheca) occur in South China, but are absent in Baltica. This discrepancy is explained as caused by differences in climatic and physiographic conditions that prevailed at the two palaeocontinents at this time. The inferred pattern of oceanic circulation during the Lower Ordovician is consistent with the palynological evidence of a prevailing warmer climate in Baltica than in South China, although the two palaeocontinents occupied the same palaeolatitudinal position.

The effects of terrestrialization on marine ecosystems: the fall of CO2

Abstract The rise of land plants during the early Palaeozoic had profound effects upon subsequent Earth history and evolution. The sequestration of standing biomass and carbon burial caused a primary shift in the distribution of active carbon within the biosphere and surficial Earth systems. This manifested itself in a dynamic decline in pCO2 during Silurian–Devonian time, affecting both terrestrial and marine ecosystems. We examined first-order correlations between terrestrialization and pCO2 by comparing the GEOCARB III data with time-constrained fossil events in the early evolution of land plants. We compared the same GEOCARB III data with the species/genus richness of lower Palaeozoic acritarchs. The correlation between the rise of woody plants and pCO2 is built into the GEOCARB model for the Late Devonian and later, but pCO2 begins to decline in the Cambrian long before the origin of woody trees (lignophytes). The influence of early phases in plant evolution may be seen in a two-stage pCO2 decline corresponding to fossil evidence for the origin of thalloid bryophytes in the Middle Cambrian and the origin of tracheophytes near the Ordovician–Silurian boundary. The decline of the acritarchs shows a highly correlated lag of about 10 Ma with respect to the pCO2 decline. The relation between pCO2 and acritarch species richness suggests a tight coupling between the evolution of the marine phytoplankton and atmospheric CO2, supporting previous suggestions that pCO2 was a significant causal factor in the near extinction of acritarchs by the end of the Devonian.

Reworked acritarchs as provenance indicators in the Lower Palaeozoic of Denmark

Profuse and well-preserved acritarchs were recovered from subsurface Lower Palaeozoic successions cored by the boreholes Slagelse-1 and Pernille-1 (Danish-North German Basin). Together with Llandovery in situ microphytoplankton, reworked Cam- brian and Ordovician species occur. The reworked Ordovician acritarchs show a clear Perigondwanan palaeobiogeographic affinity and indicate clastic sedimentary input from a Perigondwanan-related terrane located south of the East European Platform. Microflo- ral similarity enables identification of the detrital source area with the Avalonia Terrane. The present data also suggest that development of a foreland basin marginal to the Cale- donian Deformation Front in the Danish-North German Basin started in Early Silurian times.  2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

Laser Raman micro-spectroscopy of Proterozoic and Palaeozoic organic-walled microfossils (acritarchs and prasinophytes) from the Ghadamis Basin, Libya and Volta Basin, Ghana

Laser Raman microspectroscopy was used as a microchemical analysis technique to characterize the wall chem- istry of organic-walled microfossils (acritarchs and prasinophytes) extracted from Proterozoic (Tonian: ca. 900 Myr) and early Palaeozoic (Silurian: ca. 420 Myr) marine sediments in the Volta Basin of Ghana, and the Ghadamis Basin of Libya, re- spectively. Raman spectra of Proterozoic acritarchs show spectral features characteristic of kerogenous compounds at ∼1350 and ∼1600 cm −1 , consistently with previously published reports. In addition, spectra from prasinophyte algae from the Silurian sample also show an interesting spectral feature at ∼1707 cm −1 indicative of carbonyl moieties. Broadly speaking, shape and position of Raman bands appear to depend on the nature of the specimen considered, suggesting that laser micro-Raman analysis can potentially be used to establish phylogenetic relationships (high-rank taxonomy) among the main groups of pre-Cambrian to Palaeozoic palynomorphs.

Rhopaliophora? asymmetrica, a new acritarch species from the lowermost ‘Arenig’ of Baltica and its biostratigraphic potential

Abstract A shallow water analogue of the basal part of the stratotype for the ‘Second stage’ of the Lower Ordovician at Diabasbrottet, Mt. Hunneberg, Sweden, occurs in the Baltic Basin, in the St. Petersburg region, northwestern Russia, where it is represented by the Lakity beds of the lower Leetse Formation, Hunneberg Substage. Palynological investigations of these beds revealed a diverse acritarch association comprising several distinctive species with high biostratigraphic and palaeobiogeographic potential, ranging throughout the upper Tremadoc–lower Arenig interval. Rhopaliophora ? asymmetrica sp. nov., which characterises the Leetse Formation, is described from levels independently dated by co-occurring graptolites ( Tetragraptus phyllograptoides Zone) and conodonts ( Paroistodus proteus – Prioniodus elegans Zones). The possible significance of this species for the recognition in Baltica of the base of the ‘Arenig’ (base of the ‘Second stage’ of the Lower Ordovician Series) is discussed.

Ordovician spore ‘thalli’ and the evolution of the plant sporophyte

ABSTRACT Cryptospores from the Dapingian–Darriwilian Kanosh Shale at Fossil Mountain, Utah, USA, occur as tetrads, dyads, irregular clusters, or planar sheets of spore dyads. These spore ‘thalli’ are placed into the new taxon Grododowon orthogonalis gen. et sp. nov. based on the nature of division patterning and gross overall shape. The antithetic hypothesis of embryophyte origins dictates that spores evolved first and that the vegetative sporophyte evolved later via mitotic cell divisions that preceded meiosis and spore formation. We interpret the planar spore sheets of Grododowon gen. nov. to have formed via the co-option of a prior vegetative gametophytic developmental pattern which was expressed in the zygote, resulting in a two-dimensional, thalloid bauplan. However, the ploidy of the resultant ‘spores’ as haploid is necessarily conjectural, and this pattern of growth is clearly not the ancestral condition in the streptophyte lineage that gave rise to the first axial plant sporophyte.