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Chitinozoans are a group of biostratigraphically valuable microfossils which appeared in the Tremadocian and diversified during the rest of the Great Ordovician Biodiversification Event, to become extinct in the Devonian. The early diversification of chitinozoans on the Baltica palaeocontinent has remained poorly known due to preservation and collecting bias. In this study we document the distribution of chitinozoans through the upper Tremadocian to lower Darriwilian strata in the Jagala waterfall section from northern Estonia, in order to better understand the regional diversity patterns and biogeographic links. Fifty samples were studied from the 7.5 m succession of sandstones, marls and carbonates revealing 47 chitinozoan species and 11 genera. The lowermost productive samples, attributed to the late Tremadocian, represent the earliest rich chitinozoan fauna from Baltica. Combined with previous reports this shows a diversity peak in the Tremadocian with balanced total diversity reaching 17, possibly followed by a decline in the Floian and then a rather gradual increase through the Dapingian and early Darriwilian. The new diversity curve shows more than three times higher values for the Tremadocian and Dapingian than indicated in previous studies, and thus less abrupt radiation in the Darriwilian. The Jagala section provided also new data on the regional biozonal species Conochitina cucumis and Cyathochitina regnelli, which characterize the middle and upper Volkhov and lower Kunda regional stages, respectively. Several other biostratigraphically significant taxa typical of other regions were recovered from the Jagala section, including Conochitina decipiens and Desmochitina bulla. A new short-ranging species Laufeldochitina toilaensis sp. nov. is introduced, having potential to become an index species for middle Dapingian strata. The early Darriwilian assemblage from Jagala includes Rhabdochitina sp. A with up to 2.7 mm long vesicle, making it the largest chitinozoan ever reported.

The causes of the Great Ordovician Biodiversification Event (GOBE) remain a topic of debate. Recent studies indicate that global and regional oceanic oxygenation was a plausible trigger of the GOBE. The Ce anomaly of rare earth and yttrium (REY) elements in shallow marine carbonates is thought to be a powerful indicator of marine redox conditions of local waters. In this study, we analyzed REY, major and trace element compositions, and C and O isotopes of the Lower Ordovician microbial-dominant reefs on the Yangtze Platform, South China. We evaluate which carbonate components within the reefs most reliably record the seawater REY composition. The elemental correlations show that the REY elements of oncoids, stromatoporoids, sponges and carbonate infillings were affected by clastic influence, diagenetic processes, and the vital effects of carbonate secreting organisms; whereas the microbialites REY were partially modified by synsedimentary terrigenous and little diagenetic modification. Such introduction of synsedimentary terrigenous REY increased ΣREY and flattened the REY patterns of the microbialites, but the redox signals (Ce anomalies) of input source of the water column might still preserved in microbialites formed in situ. Although the magnitude of Ce anomalies is small, variation might still reflect authentic redox history of primary local seawater. In the middle Tremadocian, microbialites have positive Ce anomalies which were coeval to positive δ13Ccarb shifts. The positive δ13Ccarb shifts were documented from other sections of central Yangtze Platform, North American and Argentina, which might indicate global anoxic event. In the late Tremadocian to early Floian, microbialites have more visible negative Ce anomalies, which might indicate regional oceanic oxygenation event. The negative Ce anomalies provide direct local-geochemical evidence of oceanic oxygenation in the late Tremadocian-early Floian on the Yangtze Platform of South China, which might facilitate the first acme of biodiversification of local faunas predating the main pulse of the GOBE on the other palaeoplates.

New stable carbon isotope data (δ13Ccarb and δ18Ocarb) from the Lower–Middle Ordovician is presented from petroleum exploration well Olympic 1 in the Canning Basin, Western Australia. Olympic 1 continuously cored 319.53 m of sedimentary strata, from the basal part of the Willara Formation through the majority of the Nambeet Formation. This cored section comprises carbonate and siliciclastic deposits that constitute the primary fill sequence of the Canning Basin. Previous studies report CA-IDTIMS dates between 479.37 ± 0.16 Ma and 470.18 ± 0.13 Ma from bentonite beds within the Nambeet Formation and lower Willara Formation in Olympic 1. Four conodont biozones ranging from the Paroistodus proteus conodont Biozone (upper Tremadocian) to the Jumudontus gananda Biozone (upper Floian to lower Dapingian) have been identified from the same interval. The new carbon isotope data from the Olympic 1 core are superimposed on this robust framework to provide context for Tremadocian to Dapingian stable isotope trends for Australia. Three positive δ13Ccarb excursions are identified in the Olympic 1 δ13Ccarb isotope curve, which are correlated to the global generalized δ13C curve. The highest of these occurs in the basal Willara Formation, immediately above a CA-IDTIMS radioisotopic date of 470.18 ± 0.13 Ma within the upper Floian to lower Dapingian J. gananda conodont Biozone. The second falls in the upper Nambeet Formation, immediately above a middle Floian CA-IDTIMS radioisotopic date of 472.82 ± 0.13 Ma, and within the Oepikodus communis conodont Biozone. The oldest positive excursion occurs in the lower Nambeet Formation, immediately above a CA-IDTIMS radioisotopic date of 479.37 ± 0.16 Ma, and within the upper Tremadocian P. proteus conodont Biozone. Correlation of the Olympic 1 δ13Ccarb curve to Lower-Middle Ordovician δ13Ccarb curves from the Great Basin USA, Argentine Precordillera, Western Newfoundland and South China confirms the global distribution of dissolved inorganic carbon preserved in the geological record. This integrated record may be a useful reference for constraining the timing of major oceanographic events, and a key to establishing better links between the sedimentary successions of Western Australia and the larger Ordovician world.

Lower Ordovician stromatolite-like columns and thrombolite-like mounds, composed of fossilised keratose sponges (keratolites) and microbial carbonates, are reported from the Tremadocian Mungok Formation, Yeongwol, Korea. The stromatolite-like columns, which are up to 10 cm wide and high, consist of an inner core with low-angled (10–45°) layers that are covered by high-angled (>45°) layers. The inner core is made up of millimetric layers of alternating keratolite and microbial carbonate, and microbial carbonate dominantly comprises the outer cover. The entire columns are surrounded by bioclastic packstone to grainstone. The thrombolite-like mounds are domes a maximum of 100 cm high and 40–60 cm wide embedded within lime mud and shale. These mounds consist of keratolite–microbial carbonate clots and minor lithistid sponge–microbial carbonate clots. The stromatolite-like columns were formed in a high-energy subtidal setting, in which laminoid keratolite and microbial carbonate formed the tight laminar frame columns. Continued growth of the column narrowed the intercolumnar space, resulting in higher-energy hydrodynamic conditions that limited the growth of sponges but promoted growth of microbial organisms. In contrast, thrombolite-like mounds developed in a low-energy environment below fair-weather wave base, where irregular to bulbose keratose sponges with minor lithistid sponge–microstromatolite associations formed cluster reefs. There appear to have been ecological and/or environmental factors that affected the distribution of these sponges; keratose and lithistid sponges rarely occur together in the Mungok reefs, whereas lithistids are pervasive within coeval intermediate-energy microbial reefs elsewhere. These results demonstrate the importance of hydrodynamic controls on overall reef morphology and configurations during the Early Ordovician, and suggest that keratosan–microbial consortia may have been an integral component of the Great Ordovician Biodiversification Event, together with the lithistid sponge–microbial consortium.

Made available in DSpace on 2018-11-26T17:06:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-10-15 Hanover College Faculty Development Committee The fossil record of conulariids (Cnidaria, Scyphozoa) extends downward into the topmost part of the Ediacaran System, but the first appearance of diverse, widespread conulariids is in siliciclastic rock units of Early Ordovician age, which collectively host at least six conulariid genera. Some of these same units also contain Sphenothallus, a probable medusozoan that frequently co-occurs with conulariids in Ordovician and younger deposits. Lower Ordovician conulariid localities are distributed among five (originally) Southern Hemisphere terranes, namely Core Gondwana (Archaeoconularia, Eoconularia and Teresconularia), Armorica (Conularia azaisi), Avalonia (Archaeoconularia, Eoconularia and Exoconularia), Perunica (Archaeoconularia, Conularia and Conulariella) and South China (Conulariella). C azaisi, currently known from the Southern Montagne Noire (France), probably represents a new genus. Sphenothallus occurs in South China, North China (Korea), Armorica (Southern Montagne Noire) and Core Gondwana (Morocco). In southeastern Morocco, Burgess Shale-type Konservat-Lagerstatten in the Fezouata Shale (Tremadocian Floian) yield Archaeoconularia sp., Eoconularia sp. and at least one species of Sphenothallus. This low-diversity conulariid assemblage is most similar to the Tremadocian assemblage of Wales (Avalonia), which likewise consists of a single species each of Archaeoconularia and Eoconularia. In the Fezouata Shale, Archaeoconularia sp. and Eoconularia sp. frequently occur in monospecific mass associations. Such associations probably represent an original clumped distribution on the seafloor. Additionally, some Eoconularia sp. occur in V-like pairs or radial clusters, and also some specimens were attached at the apical end to a phosphatic brachiopod or to a corner sulcus of a larger specimen of Eoconularia sp. Similar conulariid/brachiopod associations, consisting of Conularia trentonensis and Onniella sp., occur in the Upper Ordovician (Katian) Collingwood Shale of southern Ontario, Canada. (C) 2016 Elsevier B.V. All rights reserved. Hanover Coll, Dept Geol, Hanover, IN 47243 USA Cincinnati Museum Ctr, Dept Invertebrate Paleontol, 1301 Western Ave, Cincinnati, OH 45203 USA Amgueddfa Cymru Natl Museum Wales, Dept Geol, Cathays Pk, Cardiff CF10 3NP, S Glam, Wales Sao Paulo State Univ, Dept Zool, BR-18618000 Sao Paulo, Brazil Univ Sao Paulo, Geosci Inst, R Logo 562, BR-05508080 Sao Paulo, Brazil Univ Sao Paulo, Biosci Inst, R Matao 101, BR-05508090 Sao Paulo, Brazil Univ Sao Paulo, Ctr Marine Biol, Sao Sebastiao, Brazil Univ Southern Mississippi, Dept Marine Sci, Stennis Space Ctr, MS 39529 USA Sao Paulo State Univ, Dept Zool, BR-18618000 Sao Paulo, Brazil

Gastropoda, Tergomya, and Paragastropoda (GTP) are a small but recognizable part of the collective Fezouata biota from the Lower Ordovician (Tremadocian–Floian) Fezouata Formation in Morocco. GTP range through the sequence but become more abundant and diverse in the stratigraphically higher and shallower marine parts of the succession. About 200 rock samples in existing collections have GTP but usually each slab contains several specimens so the number of individual is many times higher. A total of seven species are recognized of which four were known earlier. Gastropods are represented by the planispiral bellerophontoid Sinuites sp., recognized for the first time in the Tremadocian part of the succession, and the anisostrophic, nearly planispiral Lesueurilla prima (Barrande in Perner). Tergomyans are most abundant, dominated by the genus Carcassonnella with Carcassonnella courtessolei Horný and Peel, Carcassonnella vizcainoi Horný and Peel, and Carcassonnella sp. The latter encompass several specimens from different localities and stratigraphical levels, and may represent one of the named species or new varieties. Carcassonnella is for the first time recorded in the Tremadocian part of the succession. A second tergomyan is Thoralispira laevis (Thoral), while paragastropoda are represented by Pelecyogyra fezouataensis Ebbestad and Lefebvre. In the peri-Gondwana area Carcassonnella, Thoralispira, and Lesueurilla are considered signature taxa, and the Fezouata GTP compare closely with those of Montagne Noire, France, both in composition and distribution. The Bohemian fauna is slightly younger (Floian–Darriwilian) with different species, except for Lesueurilla prima. The latter may have a wider distribution, being tentatively recognized in the Lower Ordovician of Argentina and Spain.

The present study documents new palynological investigations of the Fezouata Shale from the Anti-Atlas (Morocco). Palynomorphs were extracted from samples collected from both outcrops and drill cuttings. Outcrop samples were taken near Zagora, and include some that were collected during excavation of stratigraphic horizons where exceptionally well-preserved fossils of the Fezouata Lagerstatte occur. Subsurface samples were taken from the AZ-1 (Adrar Zouggar Mountain) borehole, which was extracted some 300 km to the southwest of Zagora. The palynological samples yielded acritarchs, chitinozoans, scolecodonts, conodonts and fragments of graptolites. The abundance and quality of preservation of palynomorphs varies greatly, but rich and diverse assemblages were recovered from several samples. The diversity and composition of the assemblages points to an open shelf environment. Generally mixed preservation states suggest resedimentation or differential transport histories. The acritarchs can be assigned to the diagnostic messaoudensis-trifidum acritarch assemblage, which is typical of the Tremadocian/Floian boundary interval of the Gondwanan margin in high southern palaeolatitudes. The acritarch taxa present in some of the lower parts of the Fezouata Shale including levels of exceptional preservation can be attributed to sub-assemblages 1-2 of the messaoudensis-trifidum assemblage and thus point to a late Tremadocian age of the Fezouata Lagerstatte, confirming biostratigraphic data provided by graptolites of the Araneograptus murrayi graptolite biozone. Chitinozoans from the Fezouata Shale are from the E. symmetrica and E. brevis biozones and include various species that are well-known from several localities on the Gondwanan margin and from other palaeocontinents (Baltica, Laurentia, and South China), demarcating broad links between those regions. The coincidence between index fossils of these three groups, hitherto not all found in the same level, suggests that the age assignments of chitinozoan biozones may be in need of revision. (C) 2016 Elsevier B.V. All rights reserved.

Sponges form a significant component of the diversity of the Tremadocian (Early Ordovician) Fezouata Biota in the Anti-Atlas of Morocco, but are distributed intermittently and have so far received only limited attention. New material reveals a high diversity of undescribed taxa (a total of at least 27 species), including numerous representatives of protomonaxonid groups such as the Leptomitidae, Piraniidae and “Choiidae”. Some of these taxa show unusually complex skeletal architecture, and represent derived variations of their lineages relative to those in the Cambrian Burgess Shale-type faunas, although most species are assignable to described families. Reticulosan sponges are rare in the Fezouata Biota, and usually fragmentary. The palaeoecology of the sponge fauna is unusual, with most species known only from single sites (frequently single bedding planes), but are often abundant where they occur. It is very rare for two species of sponge to be found on the same slab, or at precisely the same horizon. With some species of protomonaxonid, particular bedding planes are crowded with individuals of species that are rarely, if ever, found isolated. Only two species (Pirania auraeum Botting, 2007 and an undescribed hazeliid) are found at numerous levels, and these are not known from crowded assemblages; this may relate to differences in reproductive strategy. The dense, usually monospecific populations can best be explained through repeated colonisation events in a frequently hostile environment, rather than representing a range of different stable communities.

Paired measurements of bulk carbonate (δ13Ccarb), organic matter (δ13Corg), and their difference (Δ13C) can be used to estimate changes in isotopic fractionation through time as a function of O2/CO2 in the atmosphere. However, because local scale processes can also affect Δ13C, it is essential to compare sections from widely separated water masses. Here we present new δ13Corg data from Ordovician carbonate rocks from the Great Basin, Oklahoma, and Appalachian Basin and compare with published δ13Ccarb records from these sections and paired δ13C values from other carbonate successions around North America. These new data complement previous studies that focused on Upper Ordovician δ13Ccarb excursions and now provide a composite Ordovician δ13Corg record. New Lower Ordovician (Tremadocian Stage) δ13Corg data range from ca. − 26 to − 28‰, decreasing throughout the Lower–Middle Ordovician (Floian–Dapingian Stages) to ca. − 29 to − 31‰. δ13Corg values remain at their lowest throughout the Sandbian and are similar to other published Upper Ordovician (Sandbian–Katian) δ13Corg data from North America. Δ13C values from well-preserved intervals generally vary between + 26 to + 28‰ throughout the Lower to Middle Ordovician (Tremadocian to early Darriwilian), but increase to + 31‰ during the mid–late Darriwilian and mid Sandbian, similar to published data from younger Late Ordovician positive δ13C excursions known as the Guttenberg (GICE) and Hirnantian (HICE) events. The overall Δ13C trend shows a ~ 3‰ increase throughout the Early–Middle Ordovician and coincides with a previously interpreted period of ocean cooling and some of the earliest pulses of global biodiversity of marine invertebrates and planktonic organisms. Modeling studies predict that pCO2 decreased during this time, suggesting that the effect of pCO2 on Δ13C may have been overwhelmed by other controls, such as an in increase in pO2 or a higher O2/CO2 ratio during this biodiversification event.

The Cambrian and Ordovician radiations were the most important biodiversification events in the history of animal life, yet they were separated by intervals of recurrent anoxic sedimentation and regional biomere extinction events in the Late Cambrian and Early Ordovician. Recent work linked biomere extinction events to positive carbon isotopic excursions interpreted to represent enhanced organic matter burial under anoxic conditions. This led to the hypothesis that an increase in oceanic oxygenation following the last of the biomere extinctions paved the way for the rapid diversification of the Ordovician Radiation. Here we test the hypothesis of increasing ocean oxygenation using Th/U preserved in carbonate rocks from the Lower–Middle Ordovician succession near Ibex, Utah, USA. The global seawater uranium inventory is expected to vary inversely with the amount of deep-water anoxic deposition due to uranium sequestration in black shales. Therefore in shallow water carbonates higher Th/U ratios may reflect increased global ocean anoxia. Our data show that the highest Th/U values are coincident with the heaviest carbon isotopic values reported from the base of this succession, which is latest Tremadocian in age. Th/U then decreases and remains low through the Floian up through the contact with the Kanosh Formation, a shale-dominated unit with interspersed limestone units. The geochemistry of the carbonates within the Kanosh Formation likely represents local rather than global conditions in the marine environment. We interpret the overall decrease in Th/U ratios throughout the Tremadocian to Floian interval to represent an increase in global seawater uranium concentration. These data are consistent with the hypothesis that an increase in ocean oxygenation occurred prior to the major pulses of diversification associated with the Ordovician Radiation.

Ordovician conodont studies have revealed a depositional hiatus extending from the late Floian to early Darriwilian on the North China Platform. Recognition of this widespread gap entails revision of the original concept of the Huaiyuan Epeirogeny, with definition of two distinct regional tectonic events: Event 1, which initiated this hiatus, and a subsequent Event 2 that was responsible for terminating Early Palaeozoic sedimentation in the region from the late Katian. The timing of these two events partly coincides with widely recognized eustatic sea-level falls, and separates Ordovician sedimentation into two episodes that can be broadly correlated with eustatic sea-level rises. In combination with these sea-level changes, the Huaiyuan Epeirogeny played a decisive role in shaping and controlling Ordovician sedimentation and sequence stratigraphic architecture on the North China Platform. Lower Ordovician carbonates were deposited during an apparent regression (decreasing accommodation space), resulting from rapid sediment accumulation exceeding the overall rate of basement subsidence and eustatic sea-level rise. Sedimentation ceased in the middle to late Floian when basement uplift commenced in the south and extended northward to affect the entire platform. The diachronous top surface of the Lower Ordovician succession reflects extensive erosion that is most pronounced in the south and southwest parts of the platform where the disconformity surface cut down into Tremadocian (or even upper Cambrian) strata. Deposition of the younger sequence (Darriwilian to Katian) was the result of the interplay between rejuvenated basement subsidence and the late Middle Ordovician eustatic sea-level rise. Event 2, which was initially coupled with eustatic sea-level fall induced by the end-Ordovician glaciations, terminated Ordovician deposition in the region with the top of the Ordovician marked by an unconformity, representing a hiatus of some 122 Ma extending from latest Ordovician to latest Mississippian.

The stratigraphic distributions, taxonomy, diversity, evolutionary lineages and events of late Cambrian to Middle Ordovician euconodonts of Baltica are briefly reviewed and re-evaluated. The euconodonts of the late Furongian (Stage 10, late Cambrian) to Middle Ordovician successions of Baltica record innovation, immigration and two invasions of short duration and one major extinction. The innovation event with an origin on Baltica is the rise of the Proconodontus serratus lineage in the late Furongian (Stage 10). The immigration events comprise the Cordylodus immigration to Baltoscandia in latest Cambrian and this phylogenetic lineage persisted to mid-Tremadocian (Early Ordovician) and vanished at the global mid-Tremadocian extinction event. The late Floian – basal Dapingian transition is a significant event on Baltica. It is characterized by the immigration of the important genera Baltoniodus and Microzarkodina and upon their arrival these taxa remained on the Baltica palaeocontinent and stayed for a long period forming evolutionary lineages. The basal Darriwilian immigration event by the genus Lenodus and coeval prominent sea-level lowstand were caused by a short global icehouse stage related to the expansion of the polar ice sheets. The extinction event caused the disappearance of the characteristic Cordylodus fauna. Most events are related to changes in the palaeoclimate resulting from the movement of the Baltica palaecontinent and associated eustatic sea-level changes. However, the cause of the global early Tremadocian extinction event is not yet completely clear and remains unsolved. Perhaps abrupt and significant palaeoceanographic changes on a global scale (i.e., plate tectonic movements causing sudden change of ocean currents and climate from warm to cold) were the cause of this significant extinction in the Early Ordovician.

Post-mortem feeding activities along or within the interiors of shallowly buried shells can be preserved by sediment infilling and be visible on internal moulds of shelly fossils. Almost no such activities were recorded in three lowermost Ordovician formations (Třenice, Milina and Klabava; Tremadocian to Dapingian) of the Prague Basin in the Barrandian area (Czech Republic). In contrast, a distinct increase occurs in the Sarka Formation (early and middle Darriwilian). Sarka feeding traces are common in shells preserved within siliceous nodules. They are associated with almost all taxonomic groups composing a rich and diversified fossil assemblage within that formation; but there is no evidence of similar traces produced by feeding on soft-bodied fauna. Arachnostega, Pilichnus and Palaeophycus are the most abundant ichnogera observed. Their trace makers selectively oriented on decaying soft tissues. Palaeophycus producers apparently preferred an easily accessible and nourishing food that was easily consumed. These traces typically intersect a digestive tract, especially the gut. The Arachnostega and Pilichnus traces are oriented in a manner suggesting systematic feeding. These tracemakers spent more time in a carcass, reaching lower nutrient tissues, including various membranes, that degraded more slowly and were more difficult to access. The preferential distribution of feeding traces in each shell reflects the topology of the soft tissues serving as a food. All studied feeding traces were found in nodules, never in shales of the Sarka Formation. This reflects a taphonomical bias influencing occurrences of these traces. Therefore, a lack of the feeding traces in the units underlying the Sarka Formation could be the result of taphonomic differences related to depositional conditions and diagenetic processes. Nevertheless, the abundance of those traces in the Sarka Formation illustrates the prosperity of this feeding behavior during the Darriwilian. An increase in the abundance, diversity and complexity of communities that fed within the internal ecospace of shells while utilizing feeding strategies developed during previous periods is considered to be a primary effect related to increased ecosystem complexity during the Great Ordovician Biodiversification Event.

New stable carbon isotope data (δ13Ccarb) from Lower–Middle Ordovician (Tremadocian to Darriwilian) carbonate mudstone and wackestone rocks of the Pogonip Group are presented from two sections in the Great Basin region (USA) — Shingle Pass (east-central Nevada) and the Ibex area (western Utah). The Pogonip Group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the Late Cambrian (Furongian) to Middle Ordovician (Darriwilian). The Shingle Pass and Ibex area sections have been previously studied for their conodont biostratigraphy and contain a North American Midcontinent conodont fauna that range from the Cordylodus intermedius Zone (uppermost Cambrian) to the Phragmodus polonicus Zone (Darriwilian). The δ13C trend has four distinct characteristics recognized in both Great Basin sections: 1) a drop in δ13C from + 1‰ at the base of the Ordovician (Tremadocian) to − 0.7‰, 2) a 1 to 2‰ positive δ13C shift in the uppermost Rossodus manitouensis Zone during the late Tremadocian, 3) a gradual δ13C increase from − 2‰ to ca. 0‰ during the end of the Early Ordovician (Floian), and 4) a steady δ13C decrease from 0‰ to − 4 to − 5‰ during Middle Ordovician (Dapingian–Darriwilian). In the Lower Ordovician, δ13C trends reported here from the Great Basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies. Instead, these Lower Ordovician isotope data most likely reflect primary seawater chemistry and changes in δ13C on a global scale. This interpretation is supported by the excellent correlation of δ13C in the Lower Ordovician to other δ13C trends reported from the sections in the Argentine Precordillera (La Silla and San Juan formations) and in western Newfoundland (St. George and Table Head groups). These correlations using δ13C are consistent with published biostratigraphic data and provide an integrated and high-resolution chemo-biostratigraphic framework for the Lower Ordovician sedimentary record of the Laurentian margin. The Middle Ordovician portion of the δ13C curves in the Great Basin represented by the Kanosh and Lehman formations shows significant isotopic depletion relative to the section in Argentina. Thus, although there is some indication that minima and maxima in the Middle Ordovician curves can be correlated, the Great Basin sections show clear evidence of overprinting by local variables related to both diagenesis (dolomitization) and platform restriction.

Most major phyla, and all skeletonized phyla have their first appearance in the Cambrian. The exception is the Phylum Bryozoa, which first appear in the Early Ordovician (Tremadocian 1b). Bryozoans have an excellent fossil record, due in part to their benthic marine habitat with skeletal colonies composed largely of stable, low-Magnesium calcite. These factors provide an unrivaled opportunity to observe patterns and rates of radiation of a new phylum into disparate morpho- ecospace through the Great Ordovician Biodiversification Event (GOBE). In this study, the colonial growth habits of all known skeletonized Bryozoa (181 species) from the Early and Middle Ordovician are characterized in a new classification scheme based on processes of growth, rather than their end geometry as most traditional classifications of bryozoan growth habits. These fundamental categories are: orientation, dimensions of primary growth, width of colony unit, layers of zooecia, substrate relationships, space utilization, skeleton mineralization, plus sub-categories of orientation based on their geometry. In the Early and Middle Ordovician there are 85 unique growth habits defined by these eight growth habit characters. By the end of Middle Ordovician, about 4.8% of the morpho-ecospace defined by two subsets of the characters had been occupied by five bryozoan orders (represented by 34 families, 77 genera). When plotted by Ordovician stage time-slices (ca. 2.2 my each), a sharp increase in taxonomic and growth habit occurrence is observed at the Early-Middle Ordovician transition, earlier than in other groups. Individual growth habit character states also show significant changes at this transition. In the Early Ordovician, low bryozoan taxonomic and growth habit richness is due in part to sampling bias and modified search methods are need, however, the overall patterns (Early-Mid. Ordovician transition) observed in this study are robust and not expected to change fundamentally with additional data.

The Cambrian–Ordovician boundary interval in Scandinavia is characterized by largely endemic trilobite species and fossil-meager intervals within the Alum Shale Formation. Previous investigations of this interval in Scandinavia, based on drill cores, are rather sketchy. In order to characterize the faunal signature in a largely dysoxic setting during this time interval, as well as providing biostratigraphic and chemostratigraphic data valuable for intercontinental correlation, a small strip in the outskirts of the village Sodra Sandby in Scania, southern Sweden, was excavated. Nearly 5 m of the Cambrian–Ordovician boundary strata, largely represented by alum shale, were exposed and the profile was meticulously investigated for fossil content and lithological characteristics and sampled for δ13Corg analyses. The uppermost Cambrian in Sweden has previously been described as barren of fossils; however, the present study reveals a rather diverse fauna, including lingulid brachiopods, trilobites, protoconodonts, paraconodonts and euconodonts in the uppermost 1.6 m of the Furongian. The first appearance datum of planktic graptolites is represented by a single shale surface covered by specimens of Rhabdinopora flabelliforme parabola at 1.74 m above the base of the section and roughly corresponds to the Cambrian–Ordovician boundary. The conodont fauna includes several cordylodids important for intercontinental correlation. The Sodra Sandby section δ13Corg data were coupled with isotope data from two Scanian drill cores, Haslov-1 and Tosterup-2, in order to compile a composite isotope curve spanning the uppermost Ctenopyge linnarssoni Trilobite Zone in the Furongian to the upper R. flabelliforme parabola Graptolite Zone in the Lower Ordovician (Lower Tremadocian). Two isotope shifts from baseline values, observed at the base of the Peltura paradoxa Trilobite Zone and the lower part of the Peltura transiens Trilobite Zone, respectively, can be correlated with contemporaneous shifts in other parts of the world. The former, a negative shift of approximately 0.4–0.7‰, corresponds to the widely documented Top of Cambrian carbon isotope Excursion (TOCE) and the latter, a positive shift of approximately 1‰, corresponds to an as-yet-unnamed excursion at the base of the Cordylodus proavus Conodont Zone. In terms of faunal content and isotopic signals, the present study represents the first detailed description of the Cambrian–Ordovician boundary interval in Baltica. The relatively diverse fauna recorded suggests that the dysoxic environment was not a serious inhibitor for marine life. Globally, no isotope values significantly different from background values have been reported at the Cambrian–Ordovician boundary. This is confirmed by the present study; however, as the base of the Ordovician GSSP lacks meaningful isotope data, global correlation of this important boundary is problematic.

Although documented from Estonia, Latvia, Sweden, eastern North America, and China, the Middle Ordovician (Darriwilian) positive δ13C excursion known as the MDICE has previously not been recognized with certainty in South America. The most promising region in South America for detailed Middle Ordovician carbon isotope research is the Precordillera of western Argentina, where there are many excellent exposures of biostratigraphically well-dated carbonate successions spaning the Tremadocian through Sandbian stratigraphic interval. For this project, we collected numerous isotope and conodont samples from the middle Darriwilian Las Chacritas and Aguaditas formations at their type localities, which yielded important biostratigraphic data as well as informative δ13Ccarb values. In the E. pseudoplanus Zone in the upper half of the Las Chacritas Formation, there is a relatively modest but distinct δ13Ccarb excursion. Because its stratigraphic position and magnitude closely agree with the MDICE in other parts of the world, we recognize it as the first firm record of this excursion in South America. The fact that the δ13Ccarb curve from the Las Aguaditas Formation shows no such excursion is due to the existence of a stratigraphic gap between the Lower and Middle Members of this formation that cuts out the excursion interval. The Precordilleran MDICE is used for detailed long-range correlations with successions in Baltoscandia, Newfoundland, and China illustrating the usefulness of also this δ13Ccarb excursion as a global stratigraphic tool. A recent proposal of a greatly extended chronostratigraphic range of the Table Head Group on Newfoundland is rejected based on well-established biostratigraphic evidence. Fil: Albanesi, Guillermo Luis. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Museo de Paleontología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Bergström, Stig M.. Ohio State University; Estados Unidos Fil: Schmitz, Birger. Lund University; Suecia Fil: Serra, Fernanda. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Museo de Paleontología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Feltes, Nicolás Alexis. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Museo de Paleontología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Voldman, Gustavo Gabriel. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Museo de Paleontología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Ortega, Gladys del Carmen. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Museo de Paleontología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

The Baltica terrane is well known as the Asaphus Province of the Ordovician World. Being situated in intermediate latitudes and surrounded by fairly wide oceans, Baltica had a benthic fauna that developed in a relatively endemic direction. The most common trilobites were the asaphids. The present study discusses how they appeared, evolved in various environments around Baltoscandia and finally disappeared. The total range of the asaphids based on data from 381 species is estimated as extending from the Furongian, latest Cambrian to the end Ordovician. The remnant Cambrian Olenid Fauna, which was adapted to black bituminous shale facies, survived into the Tremadocian. This Cambrian fauna includes the first asaphids (Promegalaspides, Niobella, ?Eoasaphus). With a shift to lighter-coloured and carbonatic sediments in the middle Tremadocian, this fauna was replaced by the immigrating Ceratopyge Fauna. The latter contains the first five widely distributed asaphid genera Promegalaspides, Niobe, Niobella, Niobina, and Asaphellus. As carbonate sedimentation became more widespread in many areas from the Floian onwards, the asaphids increased in number and formed a stabilised Asaphid Fauna during the Dapingian. In Baltoscandia a gradual development of endemism related to facies belts is observed during the Darriwilian. Records of the mud-related niobines, Megistaspis and Ptychopyge faunas, together with some other benthic macrofaunas are not found beyond the boundary of the Kunda/Aseri regional stages (middle Darriwilian). Their disappearance may be due to a catastrophic event. In contrast Asaphus s.l. survived this interval. Drift of Baltica towards lower latitudes and fluctuations in sea-level influenced the Asaphid Fauna leading to its gradual collapse by late Sandbian time. Influx of the islotelines during the early Katian gave way to a set of illaenimorph species found together with corals that are related to a reefal environment. This morph disappears prior to the Hirnantian glaciation.

Early to Middle Ordovician cherts and cherty siltstones associated with distal turbidite deposition in back-arc basins, are widespread in the Hermidale and Albury–Bega Terranes of the Lachlan Orogen in New South Wales. Study of more than 2500 bedding plane-parallel thin sections prepared to a thickness of 50 μm from these cherts enables recognition of four conodont zones that range in age from the late Tremadocian to latest Darriwilian. Comparable cherts are present in two small remnants of oceanic derivation now exposed on the coast of New South Wales, in the Narooma Terrane (Furongian to Darriwilian), and in the New England Orogen at Port Macquarie in allochthonous blocks (Late Ordovician). Associated fauna include radiolaria, sponge spicules, lingulide and acrotretide brachiopods, fragmentary graptolites, and rare filaments attributed to cyanobacteria. Some of these organisms were pelagic, or may have been attached to floating material, and hence became entrapped in siliceous ooze on the sea floor when they settled under gravity. Others (e.g. the brachiopods) may have been attached to sponges growing on the sea floor. The presence of burrows and bioturbation demonstrates that the deep-sea environment in the Middle Ordovician was well-oxygenated, though this contrasts with Lower Ordovician environments where evidence for infauna is lacking. Predominant colouration of the cherts examined in thin section ranges from honey and yellow-brown (typical of semitransparent cherts) through cream-coloured translucent lithologies to opaque varieties. Dark brown cherty rocks that show evidence of burrowing or bioturbation tend to have a higher silt component. The Ordovician is also a time of extensive chert deposition elsewhere, including terranes in Kazakhstan (commencing in the Late Cambrian); these siliceous sediments display many of the features described from eastern Australia.

About 500 samples from 11 Ordovician sections in South China have been analysed for their δ 13 C carb composition. Four sections are located in the Yangtze platform region, the remaining seven sections in the Jiangnan slope region. The sections span more or less the entire Ordovician, but the sample resolution in its lower part (Tremadocian to Dapingian) is lower than in its upper part (Darriwilian to Hirnantian). Overall, the data show a good correlation with the published global isotope curve. At the Cambrian–Ordovician boundary positive values are observed. In the succeeding Tremadocian the values decrease and remain negative into the early Floian. In the late Floian, the values increase to slightly positive. The correlation of Dapingian rocks between the Yangtze and the Jiangnan regions is somewhat problematic which might be due to high siliciclastic input in the Yangtze section. In the Darriwilian a small positive excursion is observed in the Yangtze platform area, however, it is missing in the Jiangnan slope area. This excursion probably correlates with the mid-Darriwilian excursion (MDICE) reported from Baltoscandia. Both chemostratigraphy and conodont biostratigraphy indicate a possible gap between the Shihtzupu Formation and the Pagoda Formation, i.e. most of the Sandbian is probably not represented in the Upper Yangtze region. The earliest Katian is characterised by a pronounced positive excursion correlating with the well-known global Guttenberg excursion (GICE). In the late Katian at least one additional positive excursion was determined. The precise biostratigraphy, however, is not clear, and therefore it is not known which of the known four to five remaining Katian δ 13 C excursions identified in Baltica correlate(s) with the one(s) observed in the present study, and further biostratigraphic study is required. The lower part of the Hirnantian excursion (HICE) is probably not preserved, with only small remnants of the HICE observed near the base of the Kuanyinchiao Bed.

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 Mannamaa 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.

44 pages; International audience; Stylophorans (cornutes, mitrates) represent one of the most diverse classes of Cambro-Ordovician echinoderms. They were freeliving, benthic, non-radiate forms, closely related to asterozoans and crinoids. Taphonomic, sedimentological, and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology. Such a combined approach suggests that the rarity of stylophorans in proximal environments (above storm-wave base) was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test. Conversely, stylophorans were relatively abundant in deeper settings (below storm-wave base), where they frequently occur in low-diversity dense beds (several tens to hundreds of individuals per square metre). Such assemblages were either largely dominated by stylophorans only (Middle Cambrian–Darriwilian), or by the association of both stylophorans and ophiuroids (latest Darriwilian–Silurian). Comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well-oxygenated, oligotrophic waters with low predatory pressure, and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans. Seven main stylophoran biofacies are identified and described. The associated environments and tropical distribution of primitive, heavily-plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings (earliest Middle Cambrian–Late Cambrian). However, from this primitive stock, stylophorans radiated several times towards deeper and/or colder environments, possibly because of the lower predation pressure in such settings (“cothurnocystid” cornutes in the Middle Cambrian, mitrates in the Late Cambrian, hanusiids in the Tremadocian). As a consequence, the fact that most Cambro-Ordovician cornutes and mitrates were “psychrospheric” (i.e. cold-water adapted) probably explains their relatively wide palaeobiogeographic distribution. In high palaeolatitudes, Cambro-Ordovician stylophorans were abundant and diverse in shallow to deep environments, but they were generally restricted to deep settings in low palaeolatitudes. The great Caradocian transgression favoured the extension of stylophorans from circum-polar regions (e.g. western margin of Gondwana) towards external settings at lower palaeolatitudes (Avalonia, Baltica, and eastern margin of Laurentia), where one group of mitrates underwent a major radiation (anomalocystitids), and extended towards shallower (and warmer) palaeoenvironments. In contrast, all Silurian stylophoran assemblages known so far are rare, poorly diversified, and restricted to shallow, warm to temperate environments in tropical regions. This distribution partly results from (1) the probable eradication of psychrospheric forms (e.g. cornutes) because of the prolonged anoxia of deep environments in the late Hirnantian–early Silurian ofWest Gondwana, (2) the radiation of some mitrates (anomalocystitids) in shallow tropical settings in the late Ordovician, and (3) of strong taphonomic and/or sampling biases (numerous ghost lineages).

Ji, Z. and Barnes, C.R., 1993. A major conodont extinction event during the Early Ordovician within the Midcontinent Realm. Palaeogeogr., Palaeoclimatol., Pataeoecol., 104: 37-47. The major change in the Lower Ordovician conodont faunas of the Midcontinent Realm was recognized by earlier workers. Our recent intense study of Midcontinent conodonts from the St. George Group, Port au Port Peninsula, western Newfoundland, has shown that seven major conodont lineages, involving some 33 multielement species, completely terminate in the late Tremadoc Series, within an interval of a few metres, at a level between the Lower Ordovician conodont Fauna C and Fauna D. The extinction event occurs during the peak of a regressive phase within the deposition of the St. George Group and at a time of apparent global eustatic fall at or slightly below the Tremadoc-Arenig series boundary. This important extinction bioevent has been recognized throughout the Midcontinent Realm in North America.