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Trabajo presentado en el International Geoscience Programme Project 653 Opening Meeting (2016), celebrado en Durham (Reino Unido), del 25 de septiembre al 1 de octubre de 2016, The Fezouata Biota from the Ordovician of Morocco is preserved in a Konservat-Lagerstätte of major importance, that is today considered as an Ordovician ‘Burgess Shale-type Lagerstätte.’ The exceptionally preserved biota occur at di erent intervals within the Fezouata Formation. This Early Ordovician (Tremadocian – Floian) formation is composed mostly of ne-grained siliciclastics and was deposited on the peri-Gondwanan shelf near the palaeo-South Pole. According to recent sedimentological studies (Martin et al., 2016; Vaucher et al., 2016), the benthic communities of the Fezouata Biota lived in an open-marine environment, in a relatively shallow setting (upper o shore to lower shoreface) above storm wave base. The Fezouata Lagerstätte-bearing beds were initially estimated as ranging from upper Tremadocian to upper Floian (Van Roy et al., 2010) and later restricted to the upper Tremadocian (Martin et al., 2016). Here we present new biostratigraphical results from several fossil groups, including graptolites, trilobites, acritarchs, chitinozoans and conodonts. These new data provide for the rst time a precise biostratigraphical calibration of the Fezouata Biota. Acritarchs of the sub-assemblages of the messaoudensis-tri dum assemblage con rm the late Tremadocian age of the oldest Lagerstätte-bearing beds and allow a correlation with other peri-Gondwanan localities (Nowak et al., 2016). New graptolite studies (Gutiérrez-Marco and Martin, 2016) allow a detailed view of the most complete Lower Ordovician graptolite succession known from the African continent. They also indicate that most Lagerstätte-bearing beds are present in the late Tremadocian, while another level of exceptional preservation occurs in the ?Baltograptus jacksoni graptolite Biozones of middle Floian age. The conodont assemblages are devoid of any warm/tropical and temperate water taxa and belong to a ‘subpolar faunal domain.’ The conodont associations of low diversity con rm the late Tremadocian to basal Floian ages of the Lagerstätte (Lehnert et al., 2016).

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The carbon isotope composition (delta C-13) of bulk organic matter and two palynomorph groups (scolecodonts and chitinozoans) from the Llandovery-Wenlock strata of Gotland (E Sweden) are compared to gain knowledge about carbon cycling in the Silurian (sub) tropical shelf environment. The delta C-13 values of the palynomorphs are mostly lower than the delta C-13 values of the bulk organic matter, and the delta C-13 values of the benthic scolecodonts are lower than those of the planktonic chitinozoans. While the difference between bulk and palynomorph delta C-13 may be in part a function of trophic state, the lower values of the scolecodonts relative to those of chitinozoans, which are assumed to live in the well-mixed water column, might imply an infaunal mode of life for the polychaetes that carried the scolecodonts. Lower delta C-13 for the scolecodonts in the middle of the section may represent variations in primary marine productivity (supported by acritarch abundance data), oxidation of organic matter in the bottom waters, or genera effects. In general, however, trends between the three data sets are parallel, indicating similarities in the low frequency, environmentally forced controls. The delta C-13 data show a decreasing trend from the base of the section, up to a horizon well below the base of the Upper Visby Formation. At this level, and therefore probably several 10 kyr before the delta C-13 increase in the carbonates, the delta C-13 organic values increase by similar to 1 parts per thousand. This perhaps is an expression of a changed composition of the bulk organic matter associated with the extinction events prior to the Llandovery-Wenlock boundary.

BackgroundDuring the Ordovician the global diversity increased dramatically at family, genus and species levels. Partially the diversification is explained by an increased nutrient, and phytoplankton availability in the open water. Cephalopods are among the top predators of today's open oceans. Their Ordovician occurrences, diversity evolution and abundance pattern potentially provides information on the evolution of the pelagic food chain.Methodology/principal findingsWe reconstructed the cephalopod departure from originally exclusively neritic habitats into the pelagic zone by the compilation of occurrence data in offshore paleoenvironments from the Paleobiology Database, and from own data, by evidence of the functional morphology, and the taphonomy of selected cephalopod faunas. The occurrence data show, that cephalopod associations in offshore depositional settings and black shales are characterized by a specific composition, often dominated by orthocerids and lituitids. The siphuncle and conch form of these cephalopods indicate a dominant lifestyle as pelagic, vertical migrants. The frequency distribution of conch sizes and the pattern of epibionts indicate an autochthonous origin of the majority of orthocerid and lituitid shells. The consistent concentration of these cephalopods in deep subtidal sediments, starting from the middle Tremadocian indicates the occupation of the pelagic zone early in the Early Ordovician and a subsequent diversification which peaked during the Darriwilian.Conclusions/significanceThe exploitation of the pelagic realm started synchronously in several independent invertebrate clades during the latest Cambrian to Middle Ordovician. The initial rise and diversification of pelagic cephalopods during the Early and Middle Ordovician indicates the establishment of a pelagic food chain sustainable enough for the development of a diverse fauna of large predators. The earliest pelagic cephalopods were slowly swimming vertical migrants. The appearance and early diversification of pelagic cephalopods is interpreted as a consequence of the increased food availability in the open water since the latest Cambrian.

International audience; Here were present a synthesis of the published literature on in situ Carboniferous acritarchs, prasinophyte algae and associated microfossils. The Early Carboniferous had a rich acritarch fossil record, with assemblages documented in both equatorial and high latitude regions. However, the data suggest that there was no relationship between acritarch diversity and palaeolatitude in the Early Carboniferous. Our synthesis reveals a trend of declining acritarch diversity from the Tournaisian to the Visean, and generally very low diversity in the latest Early Carboniferous Serpukhovian and Late Carboniferous. However, fairly diverse acritarch assemblages of up to 13 taxa have been described from the Late Carboniferous of England and South America. They imply that the very low diversity so far documented could result from a lack of research input. However, these assemblages need to be re-examined as they can be based on a limited number of individuals and taxonomic over-splitting is a possibility. Furthermore, the reworking of older assemblages into these Late Carboniferous strata needs to be thoroughly discounted. The data also suggests a trend of declining prasinophyte diversity through the Carboniferous. However, the very low number of taxa typically reported and their grouping under open nomenclature (e.g. Cymatiosphaera spp.) makes this trend of questionable robustness. No clear pattern was observed in the diversity of the Zygnemataceae, although more diverse assemblages could be related to increased terrestrial influence or continental deposition. The drop in acritarch diversity in the Early Carboniferous represents a continuation of a decline that began in the Late Devonian. Although a number of hypotheses have been invoked to account for this Devonian-Carboniferous diversity decline, our synthesis has shown that the phytoplankton from this interval are generally poorly studied, with only a handful of investigations having been completed in stratigraphically continuous sequences. Further work is urgently needed if we are to evaluate these hypotheses and understand this fundamental change in the nature of the ocean's primary producers.

4 fig.; In order to investigate whether or not equivalents to modern calcareous plankton existed in Palaeozoic times, extremely well-preserved successions have to be investigated. The Silurian strata on Gotland (Sweden) are exceptionally well preserved because they have not experienced deep burial conditions and tectonic stress, due to their position on the stable Baltic Shield. Scanning electron microscope investigations of polished, slightly etched rock surfaces revealed the presence of a variety of calcareous micro- and nannofossils. Among these organisms, many can be termed "calcispheres" '(60-100 µm in diameter), whereas others due to their size range (nannofossils) are informally termed herein as "nannospheres" (10-25 µm in diameter). The systematic attribution of these fossils is unknown (incertae sedis). Mesozoic calcispheres are usually attributed to calcareous cysts of dinoflagellates following comparisons of ultrastructure with modern species. The abundance of different calcispheres in the Silurian sediments of Gotland and the observation that many of the calcareous microfossils occur in distinctly different facies as well as their spherical shape indicate that they probably belong to calcareous micro- and nannoplankton. We therefore conclude that calcareous plankton most probably existed already during the Palaeozoic, but can only be observed under conditions of exceptional preservation.

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Although acritarchs have been studied mostly for stratigraphic purposes (often in sediments devoid of other fossils), their biostratigraphic potential has not yet been fully exploited in the Ordovician. While another group of palynomorphs, the chitinozoans, have become increasingly important in international correlations, the acritarchs continue to play a minor role in Ordovician biostratigraphy. Today, about 1000 papers deal with Ordovician acritarchs, most of which are published by scientists working in Europe. The geographical areas studied in detail are in Europe and North Africa (Servais et al., in press). Investigations of Laurentian acritarchs are generally limited to the upper Middle Ordovician and the Upper Ordovician. Apart from two publications with detailed descriptions, studies from Australia are almost non–existant. In recent years, an increasing number of investigations have been carried out in China (Li et al., 2002) and South America (e.g., Rubinstein and Toro, 2001). However, at a global scale, many areas remain almost unstudied. Palaeobiogeographically, two distinct geographic acritarch assemblages can be recognized in the Lower–Middle Ordovician. A first assemblage (also named "province"), in the upper Tremadocian in low latitude areas, was attributed to warm–water environments, and a second assemblage, successively named "Mediterranean" or "peri–Gondwanan province", can easily be recognized around the southern border of Gondwana, from Argentina through north Africa and peri–Gondwana to Iran, Pakistan and southern China (Vavrdová, 1974; Li and Servais, 2002). Over 250 genera and over 1500 species of Ordovician acritarchs have been described so far (Servais et al., in press). Among the more than 500 acritarch species described from peri–Gondwana, some may be used as stratigraphical index taxa and may be helpful in the recognition and correlation of the Global Stage boundaries in the Lower and Middle Ordovician. The first appearance datum (FAD) of some 17 taxa, the taxonomy of which has been thoroughly reviewed in recent years, are plotted in this study against the British and Chinese graptolite zonations (Figure 1), following similar treatment of the FADs of 8 taxa by Brocke et al. (1995). The last appearance datum (LAD) of these taxa is more problematical, as palynomorphs of the size of the acritarchs may be reworked into younger sediments. LADs may be suggested with confidence in some instances, but only after a consistent pattern of occurrence has been determined from large data sets. Fil: Li, Jun. Nanjing Institute Of Geology And Palaeontology; China Fil: Molyneux, Stewart G.. British Geological Survey; Reino Unido Fil: Rubinstein, Claudia Viviana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Servais, Thomas. Université de Sciences Et Technologies de Lille; Francia

Servais Thomas, Li Jun, Raevskaya Elena. Acritarchs and the definition of the bases of the global stages in the Lower and Middle Ordovician. In: Documents des Laboratoires de Géologie, Lyon, n°156, 2002. STRATI 2002. 3ème congrès français de stratigraphie. Lyon, 8-10 juillet 2002. p. 215.