[Show abstract][Hide abstract] ABSTRACT: The Late Ordovician is a period of major global environmental changes. Its understanding requires an integrated stratigraphical approach. Here, chitinozoan assemblages are described from the Borenshult-1 drillcore from Motala (Sweden) and related to stable carbon isotope event stratigraphy. Fifty-six rock samples, corresponding to a stratigraphic interval ranging from the upper Sandbian to Llandovery, were dissolved and analysed for chitinozoans. This stratigraphic interval covers the Sandbian to Llandovery and includes, in ascending order, the Freberga, Slandrom, Fjäcka, Jonstorp, Loka and Motala formations.
This study resulted in identification of two biozones the Spinachitina cervicornis Biozone (lower part of the Freberga Formation), as well as its Angochitina multiplex Subzone, and the Fungochitina spinifera Biozone (upper part of the Freberga Formation). The spinifera Biozone is characterized by the presence of Lagenochitina baltica, whereas the cervicornis Biozone is defined by the presence of its index species.
Despite a low recovery of chitinozoans from Borenshult-1, we suggest that the Guttenberg Isotopic Carbon Excursion (GICE), as observed in the Borenshult-1 drillcore, correlates with the S. cervicornis Biozone. This fits well with chitinozoan and carbon isotope data from the Fjäcka section in central Sweden (Dalarna, Siljan region) and from the Smedsby Gård drillcore (Östergötland). The A. multiplex Subzone which is recognised in the Borenshult-1 and nearby Smedsby Gård drillcores, predates the GICE at both localities.
Review of Palaeobotany and Palynology 01/2013; 198:134-144. · 1.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Late Ordovician and Silurian are characterised by several strong, global positive δ 13 C carb excursions. Some of them exceed + 5‰ and thus belong to the strongest perturbations of the carbon cycle in the Phanerozoic. The onset of the excursions is characterised by extinction and/or turnover events of several groups of marine invertebrates. The causal mechanisms of the carbon cycle perturbations, however, are still unknown and current-ly a matter of vigorous scientific debate. Our own investigations in the Hirnantian (latest Ordovician) have shown that the onset of the major δ 13 C excursion (HICE) is characterised by very high abundances of acritarchs showing abnormal, teratological forms. A critical review of published reports of abnormal acritarchs from the Late Ordovician to Early Devonian, and a correlation of their occurrences with the global stable carbon isotope curve, show that high abundances of teratological forms of acritarchs are often coeval to the run-up of δ 13 C ex-cursions. High abundances of teratological forms in modern marine protists are commonly observed in environ-ments with a high degree of environmental stress. In the fossil record, the challenge is to attribute abnormal forms of organisms to specific environmental circumstances. Our study implies that they are somehow related to the global carbon cycle, i.e., to carbon isotopic composition of the ambient sea water, and that they share a common extrinsic cause with the contemporaneous extinction and/or turnover events in other fossil groups.
[Show abstract][Hide abstract] ABSTRACT: The Late Ordovician culminated in a major glacial period that has been related to one of the strongest mass extinctions recorded during the Phanerozoic. During this interval, Anticosti Island (Québec, eastern Canada) was located at low to intermediate palaeolatitudes (15–30° S) on the eastern margin of Laurentia. It displays a relatively complete section across the Ordovician–Silurian (O/S) boundary. Upper Ordovician to Lower Silurian strata of Anticosti studied here comprise the Vauréal (Katian), Ellis Bay (Hirnantian) and Becscie (latest Hirnantian–Rhuddanian) formations. Phytoplankton dynamics inferred from the acritarch content of these strata are locally compared with new geochemical environmental proxies (δ 13 C), with recent paly-nological data from Baltica and also with phytoplankton dynamics as described from near-polar, high latitude localities of the Gondwana region. Two positive excursions of the carbon isotope record have been identied in the lower and upper Ellis Bay Formation. Overall, phytoplankton trends are as follows: (1) as with chitinozoans, conodonts, brachiopods and corals, acri-tarchs from Anticosti display a major turn-over during the early Hirnantian (uppermost Vauréal Formation– lower Ellis Bay Formation), with the appearance of taxa that exhibit Silurian afnities (e.g., Tylotopalla sp., Ammo-nidium sp., Oppilatala sp., Evittia sp., Dilatisphaera spp.). High polymorphism for some common taxa is observed in the interval corresponding to the rst δ 13 C positive excursion. Using sequence stratigraphy, this period may be related to a rst glacially driven sea-level drawdown corresponding to terrestrial ice-sheet growth on Gondwa-na, such as recorded in Morocco. (2) Subsequently, during the late Hirnantian of Anticosti (uppermost Ellis Bay Formation), phytoplanktic communities suffered a crisis, exemplied by the disappearance of several typical, large Late Ordovician taxa (i.e., Orthosphaeridium spp., Peteinosphaeridium spp., large Baltisphaeridum spp., Sacculidium sp.). The lower Becscie strata, corresponding to the time of deglaciation at the O/S boundary in-terval, only contain a low diversity assemblage dominated by large acritarchs of the genus Hoegklintia and other morphologically related forms, and a few tolerant and long-ranging taxa (e.g., Evittia remota, Micrhystridium spp., leiospheres). Despite a possible taphonomic bias in the uppermost Ellis Bay Formation, the major crisis in the late Hirnantian coincides with the second δ 13 C excursion and a second, stronger glacially-driven sea-level fall corre-sponding to the time of the maximum extend of the ice sheets on Gondwana. (3) Such phytoplankton events have also been detected in Gondwanan ice-center area, but with "typical Ordovician taxa crises" occurring appar-ently later when plotted against positive carbon isotope excursions, as also observed in Baltica. This time-lag may be linked to a basin conguration of the studied areas, where environmental stress consequently differed or linked to the presence of omission surfaces coupled with preservation bias of acritarch assemblages in the upper-most Ordovician strata of Anticosti. In addition to their biostratigraphic and palaeobiogeographic utility, acri-tarchs turn out to be more specic in highlighting palaeoenvironmental conditions. (A. Delabroye), email@example.com (A. Munnecke), firstname.lastname@example.org (M. Vecoli), email@example.com (P. Copper), firstname.lastname@example.org (N. Tribovillard), email@example.com (M.M. Joachimski), firstname.lastname@example.org (A. Desrochers), email@example.com (T. Servais).
[Show abstract][Hide abstract] ABSTRACT: 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 bathymétrie 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.
[Show abstract][Hide abstract] ABSTRACT: Based on the extensive literature on Ordovician acritarchs, biodiversity curves of marine microphytoplankton of the palaeocontinent Baltica have been compiled. The dataset is derived from more than 100 publications and includes over 600 species whose ranges can be used in diversity analysis. Stratigraphically well-constrained data from the Rapla and Männamaa boreholes, northern Estonia, are analysed separately in order to provide additional information on the Middle to Late Ordovician microphytoplankton evolution on shallow shelf settings.The total species diversity and normalised diversity curves based on range-through data show a slight decrease from the Tremadocian to Floian, which may partly be attributed to limited data available. A continuous increase in diversity from the base of the Dapingian to late Darriwilian — early Katian can be observed, with highest total diversity approaching 250 species in the Keila Regional Stage. The highest appearance rates are recorded in the Dapingian and Darriwilian. Diversity progressively diminished after the early Katian with a more pronounced decrease in the upper Ordovician Porkuni Regional Stage, corresponding to the Hirnantian. A significant faunal turnover took place in the uppermost Ordovician Pirgu and Porkuni stages, with high extinction rates as well as the appearance of many acritarch taxa that are typical of the Silurian floras.The phytoplankton diversity curves match rather well with those of several other fossil groups in Baltica, notably brachiopods and ostracods. Comparison with other palynomorphs reveals both similarities and differences. The chitinozoans show the highest diversities in the upper Darriwilian, but their later decline predates that of acritarchs. A rapid diversification of scolecodonts (jawed polychaetes) can also be observed in the upper Darriwilian. However, unlike acritarchs or chitinozoans, their diversity continues to increase until the uppermost Katian.The acritarch diversity changes are analysed and discussed in the context of palaeogeographic (northwards drifting of Baltica) and palaeoclimatologic changes (rising sea levels up to the middle part of the Upper Ordovician). The increasing diversity of the phytoplankton not only roughly correlates with the Baltic and global sea-level changes, but also with the diversification of marine invertebrate groups.
[Show abstract][Hide abstract] ABSTRACT: A large number of well-preserved immature exoskeletons enable the first detailed quantitative description of the ontogeny of the trinucleid Marrolithus bureaui (Oehlert, 1895) from the Lower Caradoc of the Armorican Massif (Armorica, northwest France). For the accurate description of cephalic features, a landmark-based approach was adopted in order to recognize ontogenetic instars and to establish the patterns of size increase and shape change during ontogenetic development. The quantitative analysis permitted the demonstration of a progressive shape change in agreement with ontogenetic ordination and a comparison of the timing of size and shape changes. Changes that took place during the meraspid period included, in particular, the development of a more pronounced sub-quadrangular cephalic outline, a compression (sag.) of the glabella and a size decrease until disappearance of the alae in late stages. This quantitative ontogenetic reconstitution of Marrolithus bureaui might be considered as a reference for the better understanding of phylogenetic affinities between other trinucleid species.
Journal of Paleontology 01/2008; 82(4):800-810. · 1.10 Impact Factor