Search

The Great Ordovician Biodiversification Event (GOBE) encompasses a series of rapid and sustained diversification pulses unparalleled in Earth history. Despite the uniqueness and magnitude of the GOBE, a singular driving force for this critical interval has not been identified. Multiple hypotheses have been presented, some pairing the GOBE as cause and effect with extrinsic factors such as changing global temperatures, ocean chemistry, or paleogeography, while others have pointed to factors intrinsic to biology itself, such as the proliferation of skeletonization, predator-prey escalation, or the development of complex tiered seafloor communities. Sufficient data are not available at present to ascertain what combination of these factors was the key to generating the GOBE. Here, we present new paired carbon and sulfur isotope data (δ13C carb , δ13C org, and δ34S CAS), along with iodine concentration data from a Darriwilian carbonate sequence exposed at Meiklejohn Peak in southwestern Nevada, USA. Within a detailed lithologic and biostratigraphic framework, these new data provide a robust correlation to chemostratigraphic zonation established in Baltica and eastern Laurentia along with insights into the evolution of Middle Ordovician biogeochemical cycling. Carbon isotope data indicate a slight (2 ‰) increase in the fractionation between carbonate carbon and organic matter (Δ13C) during the onset of the middle Darriwilian carbon isotope excursion (MDICE), and new carbonate-associated sulfate (CAS) sulfur isotope data are consistent with a decrease in global pyrite burial during this time. Both of these observations support previous work arguing for a synchronous increase in the [O 2 ] of the global oceans. New iodine concentration data from Meiklejohn Peak also suggest a progressive and sustained increase in the [O 2 ] of regional waters throughout the Darriwilian during a period of relative sea-level highstand. This expansion of stable and well‑oxygenated ecospace could have laid the foundation for rapid diversification during one of the largest pulses of the GOBE. • New C and S isotopic data characterize the Middle Ordovician MDICE event. • MDICE event is globally correlative using d13C values, suggesting minimal alteration. • MDICE event records oxygenation based on new I/Ca values from bulk carbonate. • Oxygenation during Darriwilian Stage coincides with the GOBE diversification. [ABSTRACT FROM AUTHOR]

The Great Ordovician Biodiversification Event (GOBE) was considered to be primarily triggered by climate cooling. However, the precise timing of the greenhouse-icehouse transition during the Ordovician Period remains largely unconstrained. In order to reconstruct palaeotemperatures during the Middle−Late Ordovician transition in the Tarim Basin, northwestern China, a high-resolution SIMS-based oxygen isotope (δ18O apatite) profile was obtained from 65 conodont elements from the Nanyigou section. The oxygen isotope record based on mono-generic samples reveals an increase in δ18O apatite by 1.6‰ (VSMOW) during the late Darriwilian to Sandbian, translating to tropical sea-surface water cooling of ∼7 °C, probably marking the initiation of polar ice sheet growth and the prelude of the Hirnantian maximum. This δ18O apatite shift agrees well with the time-equivalent δ18O apatite records from other continents, thus suggestiing a global signal. Furthermore, the positive shift of δ18O apatite overall coincides with inorganic carbon isotope (δ13C carb) positive excursion, indicating that enhanced organic carbon burial may have led to the atmospheric p CO 2 drawdown. Additionally, the temporal coincidence between the δ18O apatite positive excursion and development of the purplish-red nodular limestone of the Tumuxiuke Formation suggests that global cooling may have contributed to the formation of coeval marine red beds. • A + 1.6‰ δ18O apatite shift characterized the Middle–Late Ordovician transition. • This major climate cooling probably marks the initiation of polar ice sheet growth. • This major climate cooling was controlled by the global carbon cycle. • Climate cooling may have contributed to the formation of coeval marine red bed. [ABSTRACT FROM AUTHOR]

The last phase of the end-Ordovician extinction event involved substantial sea-level changes. The Oslo/Asker District in Norway is a rare place where the deeper-water early Hirnantian fauna is succeeded by the equivalent deeper-water Edgewood-Cathay Fauna. Both faunas are highly diverse, with the same small-shelled brachiopods, Onniella, Leangella, and Eoplectodonta, dominating. They also share a large number of long-ranging and eurytopic genera. Taxa from contemporary shallower-water environments are rare. Near a third of the brachiopod genera in the Norwegian deeper-water early Hirnantian Fauna went extinct, including seven genera that survived into the Ordovician/Silurian boundary strata. Deeper-water early Hirnatian and Edgewood-Cathay collections are not well-known worldwide. Global quantitative samples from low latitudes, including the Norwegian samples, were compared against each other using NMDS (Non-metric Multidimensional Scaling) with the Bray-Curtis index. Qualitative samples used NMDS with the Raup-Crick index and Network Analysis. The Raup-Crick index sharply differentiated the early Hirnantian and Edgewood-Cathay faunas, possibly due to sensitivity to extinction and origination data. In contrast, the paleogeographic affinity between collections of the two faunas is pronounced using the Bray-Curtis index. Network Analysis also demonstrates regionality; the Edgewood-Cathay Fauna is especially heterogeneous. This contrasts with the shallower-water Hirnantia Fauna, which is more cosmopolitan. Quiet waters below the storm-wave base possibly hindered the spread of larvae, and anoxic plumes of water may have caused further barriers to the lateral spreading of the Edgewood-Cathay Fauna. As in Norway, long-ranging, eurytopic taxa were shared between the two faunas with few typical shallow-water taxa. • The end Ordovician Extinction event was two-phased and coincided with a glaciation in Gondwana • The shallow-water Hirnantian brachiopod fauna occurred globally with limited regional differentiation during the glaciation • The offshore, deeper, and diverse Hirnantian fauna are more heterogenic than previously thought and differ considerably geographically • This is even more pronounced in the post-glacial, offshore Edgewood-Cathay Fauna • Regionality played a prominent role in contrast to the more cosmopolitan shallow-water faunas. [ABSTRACT FROM AUTHOR]

Flexible crinoids (subclass Flexibilia) likely originated from the cladid crinoid genus Cupulocrinus or one of its immediate ancestors in the Middle to Late Ordovician. A remarkably constant and clade-defining character of flexible crinoids is the possession of three plates in the infrabasal circlet of the calyx. This character was a significant morphological modification for the origin of the flexible clade, resulting from the reduction of the number of infrabasal plates from five plates in the ancestral cupulocrinid. Of 615 museum specimens of Cupulocrinus sp. for which the number of infrabasal plates could be determined, 21 specimens, or 3.4%, displayed a deviation from the normal five infrabasals. Of the aberrant specimens, fifteen have four infrabasal plates, and six have six infrabasal plates. An additional aberrant specimen has five infrabasals, with one significantly reduced in size. Although the number of infrabasals is typically thought to be constant within a species, this trait appears to have been variable in the immediate ancestor of flexible crinoids, in the time period just before the number of infrabasals became fixed at three for the Flexibilia. This paper documents the range of aberrations within the infrabasal circlet of Cupulocrinus and considers the implications for the origin of the flexible crinoids. Unlabelled Image • Flexible crinoids originated during the GOBE from the cladid crinoid Cupulocrinus. • A major flexible synapomorphy may have originated in aberrations of Cupulocrinus. • Cupulocrinus crinoids have a high incidence of aberrations in the infrabasal circlet. • The most common aberration was fusion of two adjacent infrabasal plates. • Other specimens possessed two infrabasal plates within the same ray. [ABSTRACT FROM AUTHOR]

In order to improve our understanding of the biodiversification of marine life that took place during the early Palaeozoic, we present this special issue focused on the Ordovician radiations. The Great Ordovician Biodiversification Event (GOBE) is a conceptual term that is today largely used to refer to the most significant increase of marine biodiversity of the Phanerozoic that occurred during the Ordovician Period between 485 and 444 Ma. Some authors, focusing on taxonomic diversity counts of selected groups, understand the GOBE to be related to a single dramatic biodiversification event of short duration in the Darriwilian Stage of the Middle Ordovician Series between 470 and 455 Ma, whereas others follow the more traditional view and consider the Ordovician biodiversification as an aggregation of radiation events capturing a large and complex increase of taxonomic diversity but also ecological complexity of marine organisms covering the entire Ordovician. This special issue features 16 selected papers that provide different perspectives on these Ordovician biodiversification events, illustrating a variety of radiations occurring during the Ordovician Period. Several papers focus on the available biodiversity datasets and their biases, and the difficulty to distinguish and interpret the various regional and global scales. It becomes clear that the Cambrian and Ordovician radiations are artificially separated by a late Cambrian 'Furongian Biodiversity Gap' and that a single long-term early Palaeozoic radiation is more and more visible. A few papers provide additional data on this crucial interval across the Cambrian-Ordovician boundary displaying a much higher ecological complexity than previously assumed. Both the diversification of the plankton and the evolution of nektonic groups during the Ordovician are investigated, in papers including Computational Fluid Dynamic simulations. The virtual special issue closes with papers documenting the development of reefs during the Late Ordovician. • This is the editorial preface for the VSI on Ordovician radiations. • 16 papers are included and shortly introduced. • One review paper is included. • The 16 papers provide new views on the Ordovician radiations. • They cover intervals from the late Cambrian to early Silurian. [ABSTRACT FROM AUTHOR]

The Cambrian–Ordovician interval was an intriguing period of geologic time, when reef transitions occurred along with important biodiversification events. This study examines the secular reef transitions in North and South China during the Cambrian and Ordovician in terms of changes in reef construction and related geobiological changes. Microbial reefs in the Terreneuvian have not been well documented in South China. The Cambrian Series 2 interval saw the almost simultaneous appearance of reef-building skeletal archaeocyaths and calcimicrobes. The widely developed archaeocyath–calcimicrobe reefs had totally disappeared by the middle of Series 2 in South China and were followed by purely calcimicrobial reefs. During the Miaolingian and Furongian, skeletal-reef builders were commonly thought to have been inhibited owing to elevated sea-water temperature, which caused lowered oxygen. However, microbial reefs containing lithistid sponges and keratosan sponges were more common in this interval in North China than previously thought. The first skeletal-dominated reefs were established during the Early Ordovician, coincident with a decline in stromatolite reefs in the central Yangtze Platform. During the Middle and Late Ordovician interval, a variety of skeletal reef-builders with diversified calcimicrobes contributed markedly to reef construction in limited areas of both North and South China. The appearance and increase of skeletal-dominated reefs are considered to have been related to lowering global sea-water temperature. Recently reported Late Ordovician cement-rich reefs in both North and South China may contain further evidence of global cooling; however, from an ecological perspective, the novel niches and biological interactions created by early skeletal-dominated reefs may have promoted the further development of reef-building organisms. Cambrian and Ordovician reef successions in North and South China are distributed discontinuously; however, integrating information from both North and South China could yield improved understanding of reef evolution and background environments through these intervals. • Four intervals of Cambrian–Ordovician reef transitions in China are identified. • Calcimicrobial diversity changed dramatically throughout the intervals. • Stromatolite decline coincided with increasing diversity of metazoans. • Data from North and South China will improve understanding of reef succession. [ABSTRACT FROM AUTHOR]

Ordovician epeiric sea carbonates in intracratonic basins of Laurentia are enigmatic due to their unique depositional settings and the absence of modern analogs. Integrated ichnologic and sedimentologic analysis of the Upper Ordovician Stony Mountain Formation in the Williston Basin of Canada allows recognition of neritic-marine, nearshore-marine, open-lagoon, restricted-lagoon, peritidal sand-shoal and peritidal-flat subenvironments. The Cruziana ichnofacies occurs in neritic (between fair-weather and storm wave bases) and nearshore (around fair-weather wave base) marine environments. The depauperate Cruziana ichnofacies is present in open- and restricted-lagoon environments, indicating a shift from fully marine to stressed conditions. In the open lagoon, composite ichnofabrics related to omission surfaces illustrate the low rates of background sedimentation interrupted by event deposition and early cementation, illustrating the Glossifungites and Trypanites ichnofacies. The decreased size of discrete burrows in the restricted lagoon is attributed to reduced oxygenation under stagnation rather than hypersalinity. The peritidal complex includes high-energy sand shoals and low-energy tidal flats. Only monospecific colonization took place sporadically in associated subtidal environments within the peritidal complex deposits. [ABSTRACT FROM AUTHOR]

Network and frequency distribution analyses of global brachiopod occurrences in the earliest Silurian (Rhuddanian–Aeronian) revealed that brachiopod recovery from the end-Ordovician mass extinction during the early–late Rhuddanian was represented mainly by the reestablishment of Late Ordovician cosmopolitan holdover taxa. Brachiopods nearly doubled their generic diversity from Rhuddanian to Aeronian, owing to the radiation of both endemic and cosmopolitan taxa, associated with an overall post-glacial amelioration of global environment and increased habitat heterogeneity as a result of marine transgressions and expansion of epeiric seas. A drastic turnover from the Ordovician-type to Silurian-type brachiopod faunas took place in the Aeronian, several million years after the terminal Ordovician mass extinction. Compared with the orders of orthides and strophomenides that radiated and predominated in the Ordovician, the atrypides and pentamerides displayed pulses of drastic diversification from the Rhuddanian to Aeronian, mainly in tropically located paleoplates, to become the most abundant and diverse brachiopod orders in the Silurian. Atrypides recovered in the early Rhuddanian, whereas diversification of pentamerides was delayed until the Aeronian and becoming widespread in South China, Kazakhstan terranes, Baltica, Avalonia, and Laurentia. [ABSTRACT FROM AUTHOR]

Influences of depositional setting and tectonics on recording Milankovitch (orbital) signals within evolving Ordovician foreland successions during arc-continent collision are poorly documented. To address this, we analyzed downhole gamma ray vs depth time series from three wells on the Central Appalachian foreland, eastern USA for evidence of Milankovitch forcing during the 25 Myr duration Middle and Upper Ordovician Darriwilian to Katian interval. Relatively slowly accumulating Darriwillian sabkha anhydritic dolomites (up to 450 m thick), are dominated by eccentricity and obliquity forcing whereas Darriwillian to Sandbian restricted inner ramp carbonates (350 m thick) show eccentricity forcing only as do the overlying basal Katian (80 m thick) mid-ramp units. Rapidly accumulating lower Katian deeper ramp, shale-prone sediments (400 to 500 m thick) and upper Katian peritidal siliciclastic units (∼300–550 m thick) show a full suite of short eccentricity, obliquity and precession forcing. Best recording of the astronomical signal occurred during Katian rapid tectonic subsidence whereas a much poorer record was preserved during the Darriwilian-Sandbian. Milankovitch forcing of climate influenced input of siliciclastics as well as controlling sea-level fluctuations and position of storm wave-base. Initially sea level changes may have been driven by greenhouse aquifer eustasy but following later Darriwilian cooling, glacio-eustasy was dominant. • Ordovician convergent foreland records Milankovitch signal under range of tectonic subsidence regimes. • Milankovitch record best recorded in rapidly subsiding peritidal and deeper subtidal settings. • Peritidal sabkha settings under moderate subsidence record all but precession. • Milankovitch record poorly recorded by slowly subsiding inner- and mid-ramp carbonates. [ABSTRACT FROM AUTHOR]

Dendroids have palaeoecological potential but are underrated in general. The present study is focused on the interval from the Floian to the earliest Sandbian (Klabava, Šárka and Dobrotivá formations) in the Ordovician of the Prague Basin (Czech Republic). It is based especially on the new datasets of all dendroid graptolites from the Šárka and Dobrotivá formations at our disposal combined with the data of the overall biodiversity in the studied interval. The analysis of diversity illustrates that there is a remarkable contrast in the studied period: dendroids declined when other groups flourished and the diversity in the Prague Basin reached the first of the two Ordovician maxima coeval with the GOBE. The patterns of those shifts seem to be primarily influenced by local environmental changes, i.e. ecologically controlled in the Prague Basin. The decline of dendroid graptolites was apparently caused by the disappearance of habitats, represented by proper substrate in the oxygenated environment, due to a gradual but continuing sea level rise. Our case study resulted in the distribution model providing a general example of local factors masking those global and exemplifying a significance of differentiation between global and local causes of the GOBE. • Dendroid graptolites declined during the GOBE in the Prague Basin. • Decline of dendroids was ecologically controlled. • The main factor of the decline was disappearing proper substrate. • Bohemian Ordovician dendroids provide examples of local factors masking those global. • Palaeoecologic potential of dendroid graptolites is long-term underestimated. [ABSTRACT FROM AUTHOR]

Although caryocaridids experienced a significant biodiversity boost during the Ordovician and spread over broad trans-equatorial areas, they have been rarely reported from Siberia, Tarim and North China. Herein, we describe a new species of caryocaridids, Soomicaris ordosensis sp. nov., from the Upper Ordovician of western Inner Mongolia, China, which represents the first report of caryocaridids from the North China craton and extends the palaeogeographical distribution of caryocaridid phyllocarids to the northwestern margin of the Palaeo-Tethys. Detailed morphological and morphometric studies show that these specimens can be ascribed to different ontogenetic stages. After close inspection, we infer that Soomicaris ordosensis sp. nov. should have experienced a long pre-adult time, with several moulting stages, during which time the posterior margin of carapace gradually extends backward to form the posterodorsal spine, and the posterior margin of carapace gradually transitions from convex, linear, and finally to a rightward-sloping sigmoid curve. The taphonomic features of caryocaridids from the Upper Ordovician in the western Inner Mongolia suggest in-situ and parautochthonous preservation. The carapaces have experienced weak phosphatic mineralization during diagenesis. The Ordovician caryocaridids were most likely stenothermic which preferred to inhabit the cool- to cold-water environments from the upper mesopelagic to the lower epipelagic zones, and showed the polar emergence phenomenon. • The new specie Soomicaris ordosensis represents the first reported caryocaridid from the North China craton. • The ontogenetic series of phyllocarids was the first fully demonstrated. • Soomicaris ordosensis sp. nov. probably experienced a long pre-adult time, with several moulting stages during growth. • Ordovician caryocaridids represent a kind of cool- to cold-adapted stenothermic group (U. mesopelagic–L. epipelagic zones). [ABSTRACT FROM AUTHOR]

The Ordovician Radiation shaped Paleozoic marine ecosystems and led to an increase in the biodiversity of marine organisms. The onset of the Plankton Revolution in the late Cambrian is an important step within the Ordovician Radiation. During this revolution, various organisms invaded pelagic realms, which affected the marine trophic web through the introduction of phytoplankton, zooplankton, as well as plankton- and suspension-feeding animals. Trilobites, a main Paleozoic group of marine metazoans, played a major role during the Ordovician Radiation. Previous studies showed that some trilobites evolved planktic larvae during the Furongian and Early Ordovician. Herein, we quantify the appearance of trilobite planktic larvae by providing highly resolved Cambrian and Ordovician data on 144 trilobite species with well-known developmental sequences. We show that Cambrian trilobites were dominated by species and families with exclusively benthic early post-embryonic stages. On the contrary, Ordovician seas comprised a remarkable number of trilobites with one or more planktic stages. However, species and families with exclusively benthic early stages still constituted about half of the Ordovician trilobite diversity. The first trilobites with planktic larvae might have been present in the Miaolingian, but their earliest fossil record is from the mid-Furongian. The appearance of planktic forms was accentuated later in the Ordovician with many trilobite lineages adapting a multi-staged planktic mode of life – more than one developmental stage colonizing the water column during ontogeny. Between the Miaolingian and the Middle Ordovician, both species- and family-level data show a progressive increase in the number of taxa incorporating planktic larvae in their development, highlighting a gradual transition in the structure of marine ecosystems during the early Paleozoic. [Display omitted] • Early trilobites were characterized by exclusively benthic developmental stages and these dominated Cambrian ecosystems. • During the Miaolingian-Middle Ordovician interval several trilobite lineages independently evolved planktic larvae. • Trilobites with planktic larvae were a fundamental part of the Ordovician marine trophic web. [ABSTRACT FROM AUTHOR]

Ordovician graptolite fossils represent one of the first abundant, well-documented records of marine zooplankton. An accurate assessment of their stratigraphic ranges, biogeography, and taxon richness is crucial to studies on the Great Ordovician Biodiversity Event. Unfortunately, our understanding of graptolite richness from high paleo-latitudes is poor and recent work has demonstrated that many high paleo-latitude taxa are often misidentified endemic species. Hence, high paleo-latitude localities likely contain a hidden graptolite richness. This work uses CONOP9 to construct a range chart and correlation model from the graptolite occurrences at thirty-eight Ordovician stratigraphic sections from the Central Andean Basin of South America, and parts of Laurentia and Baltoscandia. Thirteen graptolite biozones spanning the early Tremadocian (Tr1)–middle Dapingian interval (Dp2) are identified. Some taxa in the Central Andean Basin composite exhibit unusual ranges and occur in biostratigraphically unexpected positions in the composite range chart. In order to examine the possible causes of these unusual ranges, sections from Baltica and Laurentia were added to the analyses. These additional data facilitated results that were more consistent with the generally accepted positions of certain taxon. The first raw taxon richness curve of graptolites from the Central Andean Basin obtained with CONOP9 shows a diversification phase in the early Floian that continues to a peak in the late early Floian. A second peak occurs at the beginning of the middle Floian. A decreasing trend is observed from the top of the Didymograptellus bifidus Biozone to the " Isograptus victoriae " Biozone. These results generally agree with the graptolite richness curves published for the same interval in Baltica, Australasia, and the Yangtze region of SW China, but differs from the patterns observed for the Jiagnan and Yiyang regions of SE China. The rarified regional taxon richness curve, obtained with CONOP64, is also in agreement with global curves previously proposed by different authors. • A first graptolite composite range chart for the Central Andean Basin is obtained. • Additional data enabled results more consistent for unlikely taxon range positions. • A first raw graptolite richness curve for the Central Andean Basin is presented. • Regional graptolite richness patterns are consistent with global arrangements. [ABSTRACT FROM AUTHOR]

Taphonomic sorting can be assessed directly in fossil assemblages by comparing expected and observed proportions of elements of multielement skeletons. Trilobites are model organisms for this approach because each individual possesses one cranidium (head) and one pygidium (tail). Departures from an expected 1:1 cranidia:pygidia (C/P) ratio reflect taphonomic processes such as size- or shape-sorting. We analyzed a dataset of > 16,000 secondarily silicified cranidia and pygidia from subtidal, storm-influenced facies of a highstand systems tract in the House Limestone (Lower Ordovician) in Utah. Species fall into four distinct isotaphonomic groups, which we define as sets of morphologically similar species likely to have similar responses to taphonomic processes. All isotaphonomic groups have median C/P ratios that depart significantly from expected proportions; micropygous groups show strong enrichment of cranidia in all samples, whereas isopygous groups include some pygidia-rich samples. Despite this, rank orders of abundances and C/P ratios are not correlated for any isotaphonomic group, indicating that sorting bias is not controlling abundance patterns. Cluster analysis of genus abundance data defined two biofacies, each of which included unique dominant taxa, and which characterized early and late highstand strata. The same groupings of samples were readily recognizable using ordination (non-metric multidimensional scaling). Rank orders of C/P and positions of samples along ordination axes are not correlated, so that sorting bias does not influence biofacies groupings. Rank order of species richness of samples, both before and after rarefaction also shows no correlation with C/P. The results indicate that paleoecological analysis is possible despite clear evidence of taphonomic sorting. In this case, sorting has shuffled sclerite ratios without having a significant impact on taxon abundances and species richness. However, taphonomic bias may be problematic in more proximal marine environments where frequent winnowing produces extensive sorting and differential breakage of skeletal material. [ABSTRACT FROM AUTHOR]

One of the earliest endosymbiotic associations with stromatoporoids occurs in the Late Ordovician Xiazhen Formation of southeastern China. Bajgolia , an auloporid tabulate coral characterized by dichotomous branching due to longitudinal fission, is represented by free-living as well as endobiontic forms in various lithofacies representing a wide range of environments. Only two of 11 stromatoporoid genera ( Clathrodictyon and Ecclimadictyon ) hosted Bajgolia , mainly in reef and related facies. Bajgolia –stromatoporoid associations occur occasionally in the lower part of the formation, but eventually become persistent in the upper part. Such associations were initiated by larval settlement of the coral on the growth surface of the stromatoporoid. Growth of Bajgolia usually kept pace with its host, but the coral's ability to change growth direction and grow faster prevented its envelopment and termination by the stromatoporoid, allowing the establishment and recurrence of an ongoing endosymbiotic relationship between the two organisms. Endobiontic Bajgolia was able to survive with its corallites protruding from the host; in some cases, the growth form of the stromatoporoid changed in response to the coral. The relationships between Bajgolia and stromatoporoids were probably commensal, but there is also evidence for mutualism and/or parasitism. Bajgolia –stromatoporoid associations represent an important stage in the development of complex ecological relationships and community structure, prior to the common and widespread syringoporid (“caunopore tubes”)–stromatoporoid associations in the Siluro-Devonian. [ABSTRACT FROM AUTHOR]

Exceptionally preserved fossils yield crucial information about the evolution of Life on Earth. The Fezouata Biota from the Lower Ordovician of Morocco is a Konservat-Lagerstätte of major importance, and it is today considered as an ‘Ordovician Burgess Shale.’ This biota was discovered only some 15 years ago, but geological studies of the area date back to the beginning of the 20th century. Pioneering geological investigations lead to the discovery of Ordovician strata in the Anti-Atlas (1929) and ultimately resulted in their formal subdivision into four main stratigraphic units (1942). In the Agdz area, the presence of fossiliferous Tremadocian (Lower Ordovician) strata was suspected as early as 1939, but only definitively confirmed in 1955. In the 1960s–1990s, Jacques Destombes provided the first detailed biostratigraphic framework for the Lower Ordovician of the Anti-Atlas, and collected thousands of fossils that were subsequently described in a series of monographs. In the early 2000s, exceptionally preserved fossils were discovered in the Fezouata Shale (Tremadocian–late Floian) in the central Anti-Atlas by Mohamed ‘Ou Saïd’ Ben Moula. This biota, now known as the Fezouata Biota, is of utmost importance, for it demonstrates the extent in the fossil record of non-biomineralising animals typical of the ‘Cambrian Explosion’ into the Ordovician, during the ‘Great Ordovician Biodiversification.’ Although most components are still in need of formal descriptions, a fairly good picture of the composition and organisation of this biota, and how it contributes to our understanding of the early evolution of metazoan communities, can now be depicted. Moreover, recent studies have substantially clarified the biostratigraphical and palaeoenvironmental context of the Fezouata Shale, and are now being followed up by detailed investigations of the taphonomy, geochemistry and micropalaeontology of this unique Konservat-Lagerstätte . These efforts will soon greatly benefit from the recovery of fresh, unweathered samples from drill cores. [ABSTRACT FROM AUTHOR]

Echinoderms are one of the major components of benthic faunas in the Lower Ordovician sequence near Zagora, central Anti-Atlas, Morocco. The Fezouata Shale (Tremadocian–late Floian) has yielded numerous, exquisitely preserved echinoderm assemblages, ranging through several stratigraphic levels and palaeoenvironmental conditions. These associations offer a unique opportunity to document both evolutionary and palaeoecological aspects of echinoderm diversification in high-latitude, siliciclastic-dominated western Gondwana sediments, where rapid in situ burials facilitated excellent faunal census conditions. Lower shoreface deposits of the Fezouata Shale provide the most complete record of successive echinoderm faunas. In late Tremadocian times, these relatively shallow shelf deposits show the progressive replacement of low-diversity, opportunistic, Cambrian-like, dwarfed communities dominated by cornute stylophorans, in unhospitable, dysoxic environmental conditions by higher diversity benthic assemblages, more typical of the Great Ordovician Biodiversification Event and dominated by blastozoans, on well-oxygenated sea-floors. The turnover of the Ordovician radiation was apparently slightly delayed in more proximal settings. Eocrinoid meadows persisted in shallower environmental conditions up to the middle Floian. In the late Floian, they were replaced by diploporite-dominated communities, typical of later Ordovician high-latitude peri-Gondwanan faunas. From a palaeobiogeographic point of view, low-diversity assemblages display relatively strong affinities with cosmopolitan late Cambrian echinoderm faunas, whereas high-diversity communities are dominated by peri-Gondwanan taxa. [ABSTRACT FROM AUTHOR]

The Tarim Basin, located in NW-China, represents one of China's major palaeoplates. Its palaeogeographic position in the Ordovician, however, is debated. In this study we present biostratigraphically controlled stable carbon isotope data from some 600 samples spanning almost the entire Ordovician in order to provide a tool that can be used for future chemostratigraphic correlations. The data correlate well with results from South China, and several major global δ 13 C excursions such as the MDICE (Dw2/3), SAICE (Sa2), GICE (Ka1), and Whitewater excursion (Ka4) are documented for the first time from the Tarim Basin. Major sedimentary gaps are detected in the lower Floian between the Penglaiba and Yinshan formations, in the upper Darriwilian to lower Sandbian between the Dawangou and Saergan (or Kanling) formations, and in the latest Sandbian between the Kanling and Qilang formations as well as between the Tumuxiuke and Lianglitag formations. Three scenarios for the depositional history of the late Darriwilian to early Sandbian Saergan Formation, which represents an important hydrocarbon source rock in the Tarim Basin, are discussed with respect to their bio- and chemostratigraphic implications. The mid-upper Ordovician rocks recording high δ 13 C values have been deposited during transgressive or highstand intervals, whereas the regressive intervals are mostly missing. This results in a stacking of several positive excursions, and thus chemostratigraphic correlation without biostratigraphic control might produce misleading results, especially for shallow-water carbonates. [ABSTRACT FROM AUTHOR]

Middle–Late Ordovician sequences from the Appalachian Basin and Arbuckle Mountain regions of North America were analyzed for carbonate-associated sulfate (δ 34 S CAS ) and pyrite (δ 34 S pyr ) paired with carbonate (δ 13 C carb ) and organic matter (δ 13 C org ) chemostratigraphy. Two major negative drops in δ 34 S CAS (12‰ excursions) are recognized: the older decline in δ 34 S CAS occurs within the Histiodella holodentata–Phragmodus polonicus Conodont Zones and the younger drop is within the Cahabagnathus sweeti–Amorphognathus tvaerensis ( Baltoniodus gerdae subzone) Zones. These overall these negative shifts in δ 34 S CAS have an antithetical relationship with positive shifts in δ 34 S pyr (~+ 10‰) and δ 13 C carb (~+ 2‰) recorded in the same successions. The older negative δ 34 S CAS shift is coincident with the widely documented mid-Darriwilian δ 13 C excursion (MDICE), and the younger negative δ 34 S CAS shift is coincident with another positive δ 13 C carb shift in the early Sandbian. Geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative δ 34 S CAS trends. Additionally, a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends. While increased crustal weathering is broadly consistent with a sea-level lowstand, and the seawater 87 Sr/ 86 Sr isotope record of change in continental weathering in the late Darriwilian Stage of the Ordovician, geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in δ 34 S CAS . These antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates. Pulses of oceanic ventilation would have expanded habitable environments for marine organisms, and thus is broadly consistent with major increases in biodiversification during this period of the Ordovician. [ABSTRACT FROM AUTHOR]

Patterns of change in the isotope composition of sulfur-bearing minerals play a key role in reconstructing the marine sulfur cycle. Determining marine sulfate concentration, in particular, is critical to understanding the linkages between oxygenation of the Earth's atmosphere and oceans, and the history of life. Much of our current understanding of marine oxygenation relies on a traditional single reservoir model, wherein the isotopic composition of marine sulfate is controlled primarily by burial of sedimentary pyrite. Utility of this model is limited, however, during times of persistent marine euxinia, which marks a fundamental decoupling between oxic and anoxic marine sulfur reservoirs. At these times, short-term fluxes that act between the two reservoirs (e.g., sulfate reduction and sulfide oxidation processes) often dominate over the long term fluxes (e.g., weathering and pyrite burial) that control the single reservoir ocean model. Ordovician strata from Argentina, western Newfoundland, and South China illustrate how the marine sulfur cycle is affected by the presence of a persistent euxinic reservoir. Regional euxinic reservoirs remain generally stable from the Floian through the Dapingian, despite evidence for periodic short-term oxygenation events. Dramatic reorganization of the marine sulfur cycle in the early Darriwilian, however, reflects dynamic disequilibrium between oxic and anoxic (euxinic) marine reservoirs, driven by ocean ventilation. Ventilation resulted in a rapid change in the isotopic composition of marine sulfate in surface oceans, and ultimately led to the near depletion of hydrogen sulfide within the anoxic reservoir, as marked by formation of superheavy pyrite. Ventilation occurred coincident with a decline in sea surface temperatures and may represent the onset of climatic change that ultimately led to late Ordovician glaciation. [ABSTRACT FROM AUTHOR]

The Guttenberg Isotope Carbon Excursion (GICE), a positive carbon isotope excursion that occurs near the base of the Katian Stage, is thought to be a global event possibly related to Late Ordovician cooling. Documenting how much regional and global variability exists in carbon isotopic trends prior to and during the GICE is a critical aspect in understanding the implications of the excursion for interpreting changes in the global carbon cycle, paleoclimate, and chemostratigraphic correlation during the Ordovician. To investigate carbon isotopic trends along the southern margin of the Laurentian carbonate platform during the Late Ordovician, we measured bulk carbonate δ 13 C carb and δ 18 O carb as well as organic carbon δ 13 C org values from four locations in Alabama. These sections are excellent study sites because they are well exposed, contain the regionally well-correlated Deicke and Millbrig K-bentonites, and are in a region where δ 13 C trends have not been studied. Carbonate δ 13 C carb results from one section (i.e. Fort Payne) in northeastern Alabama record a 1.5‰ positive excursion above the Millbrig K-bentonite. We interpret the 1.5‰ positive excursion as a primary feature and identify it as the GICE. The characteristic GICE excursion is absent in the other study sections and we attribute this absence to the presence of unconformities (either non-depositional or erosional) in these sections. We find neither evidence for sea level control on δ 13 C trends nor conclusive evidence for δ 13 C DIC gradients along the southeast margin of Laurentia's epeiric sea. Combined with the proximity to the Iapetus Ocean, these results suggest that carbon isotopic trends in Alabama are a reliable proxy for open ocean conditions. Therefore, despite the absence of GICE excursion in all but one of our study sections, increasing δ 13 C values in Alabama prior to the GICE provides evidence for a steady 13 C enrichment of the global surficial carbon reservoir consistent with a drawdown of atmospheric CO 2 during the early Late Ordovician [ABSTRACT FROM AUTHOR]

Paired measurements of bulk carbonate (δ 13 C carb ), organic matter (δ 13 C org ), and their difference (Δ 13 C) can be used to estimate changes in isotopic fractionation through time as a function of O 2 /CO 2 in the atmosphere. However, because local scale processes can also affect Δ 13 C, it is essential to compare sections from widely separated water masses. Here we present new δ 13 C org data from Ordovician carbonate rocks from the Great Basin, Oklahoma, and Appalachian Basin and compare with published δ 13 C carb records from these sections and paired δ 13 C values from other carbonate successions around North America. These new data complement previous studies that focused on Upper Ordovician δ 13 C carb excursions and now provide a composite Ordovician δ 13 C org record. New Lower Ordovician (Tremadocian Stage) δ 13 C org data range from ca. − 26 to − 28‰, decreasing throughout the Lower–Middle Ordovician (Floian–Dapingian Stages) to ca. − 29 to − 31‰. δ 13 C org values remain at their lowest throughout the Sandbian and are similar to other published Upper Ordovician (Sandbian–Katian) δ 13 C org data from North America. Δ 13 C 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 δ 13 C excursions known as the Guttenberg (GICE) and Hirnantian (HICE) events. The overall Δ 13 C 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 p CO 2 decreased during this time, suggesting that the effect of p CO 2 on Δ 13 C may have been overwhelmed by other controls, such as an in increase in p O 2 or a higher O 2 /CO 2 ratio during this biodiversification event. [ABSTRACT FROM AUTHOR]

Analysis of 161 nodular and bedded cherts shows that shallow-water biosiliceous facies, which were common in the Lower Ordovician, had mostly retreated from peritidal–lagoonal facies to deeper settings by the Middle Ordovician. For decades, this retreat was thought to have spanned the Cambrian and Ordovician Periods. Our data suggest that the basinward shift may have taken approximately 2–14 m.y. in the late Lower to early Middle Ordovician. A likely cause for the departure of these often siliceous-sponge cherts from shallow waters was depletion of shallow-water dissolved silica (DSi). Increased use of DSi by radiolarians may have commenced in the Middle Ordovician when radiolarians increased just as a pulse of radiolarite deposition began. Siliceous sponges may have been forced to relocate to deeper waters because radiolarians depleted shallow-water DSi. Apparent low relative sea level in the Middle Ordovician may have hampered the formation and/or preservation of shallow-water cherts by reducing available shelf area to host such deposits. However, the resurgence of shelf chert in the Upper Ordovician while peritidal cherts remained virtually absent suggests that biology was critical to driving the basinward shift of cherty facies. Formation and preservation of abundant latest Darriwilian through Sandbian cherts may have been favored in deep-water settings during a possible warming interval marked by sea-level rise and extensive anoxia and euxinia that coincided with an ongoing decline in 87 Sr/ 86 Sr ratios and increased sea-floor volcanic activity that probably enriched the DSi content of deep-ocean waters. Ocean circulation associated with the onset of cooling in the Katian may have optimized chert accumulation by stimulating surface water biosiliceous productivity before DSi-rich chert-preserving anoxic basins were ventilated. As Late Ordovician glaciation intensified, deep-ocean ventilation by thermohaline sinking of oxygen-rich waters probably decreased preservation potential of biosiliceous deposits while increasing oceanic recycling of silica. [ABSTRACT FROM AUTHOR]

Life on Earth radiated dramatically during the Ordovician Period (485–444 Ma) at all taxonomic levels during an evolutionary episode known as the ‘Great Ordovician Biodiversification Event’. Abundant scientific literature has documented the pattern of biodiversification for numerous organism groups. However, fully understanding this event also requires detailed knowledge about the spatial distribution of the organisms. The full potential of the palaeobiodiversity patterns remains under-explored due to the lack of knowledge about past ocean circulation. Surface circulation significantly conditions the dispersal of organisms, in creating preferential ocean migration pathways or, in contrast, by isolating specific regions and thus establishing major physical barriers. These uncertainties persist because neither direct observational data, nor proxy data independent from the fossil record, are available to reconstruct the ocean surface circulation. Here we present new maps of Ordovician ocean surface circulation based on simulations using the general circulation model FOAM. This is a coupled ocean-atmosphere general circulation model and represents a methodological improvement over previously published work. In addition, we use the most up-to-date palaeogeographical reconstructions to increase compatibility with recent palaeontological databases. In providing clear, synthetic maps of ocean surface circulation in addition to raw model outputs, we aim to facilitate data-model comparison, to assist in interpretation of palaeontological datasets, and to promote renewed discussion about Ordovician biogeography. Finally, we investigate the sensitivity of the circulation pattern to the atmospheric CO 2 content, which remains poorly constrained in the Ordovician, potentially ranging from more than 15 times the preindustrial atmospheric CO 2 level (PAL) to ~ 5 PAL. Maps are systematically constructed for a high (16 PAL), medium (8 PAL) and low (4 PAL) CO 2 value. We show that the circulation pattern is much more sensitive to CO 2 than previously suggested. [ABSTRACT FROM AUTHOR]

The Upper Ordovician and Silurian dark to black shales and claystones in the northern Holy Cross Mountains (Poland) are transgressive facies developed in the early Sandbian, early Katian and early Sheinwoodian times, respectively. They were deposited under dysoxic to anoxic bottom waters interrupted, however, by short-term periods of oxygen increase. They contain lenticular and oval (and subordinately circular) structures composed of angular and well sorted micron-size quartz grains bound by cryptocrystalline silica cement. These structures are interpreted as remnants of agglutinated benthic foraminifera which have been members of opportunistic community inhibiting the subsurface sediment. They were able to survive of temporal anaerobic conditions at the bottom of sedimentary basin. Their high frequency in the Upper Ordovician black/dark claystones was facilitated by absence of grazing pressure from predators due to shift of the oxygen gradient above the sediment/water interface. However, in the case of the Sheinwoodian black shales, a high activity of predators and destructive impact of storm currents appear to be an important agents responsible for much lower frequency of foraminifera specimens. [ABSTRACT FROM AUTHOR]

A pioneer δ 13 C org study through the upper Sandbian and Katian (Upper Ordovician) succession in the Röstånga 1 drill core in the classical geological outcrop area at Röstånga in southernmost Sweden produced a wealth of new carbon isotope data which are useful for local and regional correlations. Among the Upper Ordovician positive δ 13 C excursions, the Guttenberg (GICE), Waynesville (Saunja), Whitewater (Moe), Paroveja, and Hirnantian (HICE) isotopic carbon excursions are recognized but the Kope (Rakvere) δ 13 C excursion is missing, suggesting a stratigraphic gap. All these isotopic excursions are tied closely to biostratigraphy, especially graptolite biostratigraphy, and in the case of the Waynesville (Saunja) and Whitewater (Moe) excursions, for the first time anywhere in the world. The Röstånga GICE δ 13 C org curve from the upper Sularp Shale shows a striking similarity to that of the Katian GSSP in Oklahoma, suggesting the potential of trans-Atlantic correlation. Based on a projection from the Katian GSSP, the previously poorly constrained position of the base of the Katian in southern Sweden appears to be in the uppermost Sularp Shale in strata of the upper Diplograptus foliaceus Zone. Previous interpretations of the relations between K-bentonite successions in southern Scandinavia are somewhat revised and the Kinnekulle K-bentonite is recognized for the first time in Scania. Based on new radiometric dates, this very prominent and widespread ash bed appears to be slightly older than the Deicke and Millbrig K-bentonites in eastern North America. [ABSTRACT FROM AUTHOR]

Stromatoporoids hosted a diverse fauna of symbiotic endobionts during the Silurian and Devonian. Assemblages of symbiotic endobionts from the Ordovician, Silurian and Devonian differ significantly. The only symbiotic association in the Ordovician is cornulitid-stromatoporoid. The diversity of Silurian and Devonian symbiotic associations is similar, but the taxonomic compositions of the Silurian (i.e. rugosans, syringoporids, cornulitids, lingulids, Chaetosalpinx and Helicosalpinx ) and Devonian (i.e. syringoporids, Torquaysalpinx , Chaetosalpinx , Streptindytes , and rugosans) associations are different. Symbiotic corals dominated both Silurian and Devonian associations, rugosans dominated the Silurian associations, and syringoporids dominated the Devonian associations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A diverse, typical Hirnantia brachiopod fauna from terminal Ordovician strata in Latvia extends the distribution of the Kosov Province across much of the Baltic Palaeoplate into the deeper-water facies of the East Baltic. The new data emphasise the co-occurrence of the core elements of the fauna, Eostropheodonta , Dalmanella , Cliftonia , Hindella , Plectothyrella and Hirnantia , in a siliclastic shelfal setting. The fauna has close similarities with faunas elsewhere on Baltica, including Jämtland, the Oslo Region and Västergötland and plot within the Kosov Province in analyses of the global distribution of Hirnantian assemblages. Carbonate facies, first, in Estonia (lower Hirnantian) and, second, in Oslo together with Östergötland (upper Hirnantian) support quite different faunas related to the margins of Laurentia and the Edgewood Province of the midcontinent, respectively. Stable isotope curves, when used with caution, have helped correlate the sections across the eastern Baltic into Sweden. [ABSTRACT FROM AUTHOR]

Body size is an important ecological parameter that can be understood in both evolutionary and environmental terms. We present a database of changes in overall body size in brachiopods and trilobites through the Ordovician and earliest Silurian of the Oslo Region, Norway. In contrast with global studies, the limited geographical extent of our data allows correlation with environmental parameters such as climate and water depth, and simplifies taxonomical standardization and stratigraphical correlation. In our data set there is a clear indication of increasing size both for brachiopods and trilobites during most of the Ordovician, although this could result from an unbiased random walk. The size increase is followed by a reduction in size during the latest Ordovician. Trilobites show a strong increase followed by a decrease in body size during the Middle Ordovician, while brachiopod body size peaks later, in the Late Ordovician. These changes are partly correlated with the changes in species diversity in the Oslo Region. We discuss the results in terms of two models for change in body size — Cope's rule and the Lilliput effect. Cope's rule is the theory of an increasing body size through geological time, while the Lilliput effect describes a decrease in body size in the aftermath of a mass extinction or a severe environmental disturbance. We find no clear correlations between body size and sea level. [ABSTRACT FROM AUTHOR]

Eleven symbiotic associations between taxa occur in the Ordovician of Baltica, most involving colonial animals. Bryozoans formed the most numerous associations (including possible polychaete bioclaustrations, possible polychaete borings, conulariids, and crinoids) and had the highest number of symbiosis cases. In Baltica, the Late Ordovician symbiotic associations differ from those in the Middle Ordovician by a remarkable increase in the numbers of recorded cases. This is most likely explained by the Great Ordovician Biodiversification Event (GOBE). The higher faunal diversity may have created more opportunities for symbiotic relationships. These changes in the symbiotic interactions in the Ordovician of Baltica were probably not related to climate change, but had an evolutionary cause. [ABSTRACT FROM AUTHOR]

Several Ordovician tunnel valleys are exposed in the Moroccan Anti-Atlas Mountains, including the Alnif and the Foum Larjamme tunnel valleys, located 150 km away from each other. Sedimentological and deformational analyses of these two glacial troughs reveal that differing processes lead to their formations. The Alnif tunnel valley contains numerous deformation structures within sediments both below and above the main glacial erosion contact surface. Ball-structures and clastic dykes occur within preglacial sediments down to 35 m below glacial incisions while overlying glacial sediments contain fluted surfaces, clastic dykes, dewatering structures, folds and radial step normal faults. The characteristics of the Alnif tunnel valley can be explained by a porewater pressure-driven model of formation where the localized increase of basal shear stress and porewater pressure underneath subglacial deforming zones lead to the development of a dense hydrofracture network in the preglacial bed. These processes of hydraulic brecciation promoted subglacial remobilization of the preglacial material and contributed to the formation of the tunnel valley. The Foum Larjamme tunnel valley displays undisturbed preglacial sediments and few dewatering structures at the base of the glacial sedimentary infill which suggests relatively low porewater pressures within the tunnel valley during formation. This second type of tunnel valley where porewater pressure remained relatively low appears to have been formed by meltwater erosion. The undulating base of the Foum Larjamme tunnel valley implies progressive erosion by a stable subglacial braided network of Nye-channels, or alternatively by channels migrating laterally during episodic minor subglacial outbursts. These two tunnel valleys highlight the regional variability of processes involved in the formation of tunnel valleys. The distribution of palaeo-ice streams in North Africa illustrate that morphologies and processes involved in the formation of tunnel valleys vary between ice stream and inter-ice stream zones due to variations in meltwater availability, the topography and bed lithological properties. [ABSTRACT FROM AUTHOR]

The Late Ordovician world experienced a series of huge volcanic eruptions, recorded as the big Deicke, Millbrig and Kinnekulle bentonites, together with numerous thinner beds. The Kinnekulle event can be traced across northwestern Europe. U–Pb zircon ages are here presented of both the Kinnekulle K-bentonite and the uppermost recorded tephra layer in the Upper Ordovician of the Oslo Region, Norway. The tephras are located in the upper part of the Arnestad Formation (Sandbian) south of Oslo and gave ages of 454.52 ± 0.50 Ma (the Kinnekulle K-bentonite) and 453.91 ± 0.37 Ma (the upper Grimstorp K-bentonite). The dated tephras are separated by a 7 m thick shale succession with subordinate nodular limestone beds. Although the two U–Pb ages overlap within errors, statistical considerations indicate a most likely time interval of about 600 kyr between the tephras. High-resolution magnetic susceptibility logging in the same section shows cycles that likely represent changes in sediment supply in response to astronomical forcing. Spectral analysis shows the presence of long (400 kyr) and short (100 kyr) eccentricity bands, and obliquity components in the 30 kyr band. Precessional cycles are not detected. Based on this method, it is possible to estimate a time interval of 766 kyr between the two tephra events, and the radiometric dating therefore does not exclude a Milankovitch interpretation of the cycles. This opens new possibilities for understanding the evolution of one of the world's best preserved Ordovician marine systems. [ABSTRACT FROM AUTHOR]

The earliest known macroscopic endobiotic symbionts are described from Middle Ordovician bryozoans of Osmussaar Island, Estonia. The bryozoan symbionts represent a new ichnospecies and ichnogenus, Anoigmaichnus odinsholmensis ichnosp. nov., which occurs only in Mesotrypa bystrowi . It most likely represents a parasite. The shape of the trace is consistent with a worm-like animal, making the annelids likely candidates for the trace maker. The increase in predation pressure may have been among the direct causes leading to the appearance of various macroscopic endobiotic symbionts in the Ordovician. [ABSTRACT FROM AUTHOR]