Presence de Graptolites de l'Arenig moyen a la basede la Formation de Postolonnec (Massif Armoricain); Consequences stratigraphiques et paleogeographiques

The lowest part of the Postolonnec Formation (western Brittany) can be assigned to the middle Arenig Didymograptus nitidus Zone on graptolite evidence. Preliminary information on the Chitinozoa agrees with this age assignment, which defines more precisely the age of the underlying Grès armoricain Formation and also demands reconsideration of the problems associated with early Ordovician palaeogeography and sedimentology in the Massif armoricain.     

Ordovician Bathyal Trace Fossils From Metasiliciclastics in Central Norway and Their Sedimentological and Paleogeographical Implications

Numerous trace fossils are recognized in metasiliciclastic rocks of the Lower-Middle Ordovician Lower Hovin Group, Upper Ordovician Ekne Group and on roofing slates from an unknown locality (probably Ordovician) from the Trondheim region, central Norway. The trace fossil assemblages are dominated by meandering pascichnial forms, i.e., Helminthoidichnites in the Lower Hovin Group and Helminthoidichnites and Nereites in the slates. Protovirgularia, Dictyodora, ?Planolites and ?Palaeophycus are less common. Alcyonidiopsis, ?Trichophycus, Chondrites, cf. Chondrites, Gordia, ?Phycodes, ?Helminthopsis, cf. Naviculichnium, Treptichnus, Saerichnites, Megagrapton and ?Paleodictyon (Squamodictyon) are rare. The trace fossils were collected or observed at 17 different localities and represent assemblages belonging to the Nereites ichnofacies (deep-sea flysch deposits with thinly bedded turbidites). They reflect a distal Nereites subfacies (fan-fringe or basin-plain deposits) transitional to a Nereites/Paleodictyon subfacies (proximal fan facies). The trace fossil assemblages indicate an opportunistic style of colonization related to fluctuations in food supply introduced by turbidity currents.     

Morphological variation in some lower palaeozoic dalmanellid brachiopods

Three predictions arising from the niche-variation model concerning morphological variation are tested. These are: increasing diversity accompanied by reduced species variability, variation directly proportional to species abundance and eurytopic species more variable than stenotopic forms. Using Ordovician and Silurian dalmanellid brachiopods, measurements were taken at different intervals through ontogeny of 5 brachial characters on 11 species ofBancroftina, Dalmanella andOnniella (all Ordovician andIsorthis (Silurian). Morphological variability decreases through ontogeny but is not related to species abundance as predicted. Indeed, evidence suggests the opposite. Less abundant species are generally more variable, this is again the opposite of the predictions. There is no apparent relationship between morphological variability, eurytopy and stenotopy. These results considered with other data suggest that other Lower Palaeozoic animals, particularly suspension feeders, have similar niche requirements and these may be fairly broad.     

Phylogenetic and biogeographic analysis of sphaerexochine trilobites

Background

Sphaerexochinae is a speciose and widely distributed group of cheirurid trilobites. Their temporal range extends from the earliest Ordovician through the Silurian, and they survived the end Ordovician mass extinction event (the second largest mass extinction in Earth history). Prior to this study, the individual evolutionary relationships within the group had yet to be determined utilizing rigorous phylogenetic methods. Understanding these evolutionary relationships is important for producing a stable classification of the group, and will be useful in elucidating the effects the end Ordovician mass extinction had on the evolutionary and biogeographic history of the group.

Methodology/Principal Findings

Cladistic parsimony analysis of cheirurid trilobites assigned to the subfamily Sphaerexochinae was conducted to evaluate phylogenetic patterns and produce a hypothesis of relationship for the group. This study utilized the program TNT, and the analysis included thirty-one taxa and thirty-nine characters. The results of this analysis were then used in a Lieberman-modified Brooks Parsimony Analysis to analyze biogeographic patterns during the Ordovician-Silurian.

Conclusions/Significance

The genus Sphaerexochus was found to be monophyletic, consisting of two smaller clades (one composed entirely of Ordovician species and another composed of Silurian and Ordovician species). By contrast, the genus Kawina was found to be paraphyletic. It is a basal grade that also contains taxa formerly assigned to Cydonocephalus. Phylogenetic patterns suggest Sphaerexochinae is a relatively distinctive trilobite clade because it appears to have been largely unaffected by the end Ordovician mass extinction. Finally, the biogeographic analysis yields two major conclusions about Sphaerexochus biogeography: Bohemia and Avalonia were close enough during the Silurian to exchange taxa; and during the Ordovician there was dispersal between Eastern Laurentia and the Yangtze block (South China) and between Eastern Laurentia and Avalonia.     

Ordovician phyllocarids (Arthropoda; Crustacea) from Argentina

New phyllocarids are described from the Ordovician (early Arenig to lower Caradoc) of the Precordillera and Eastern Cordillera of Argentina, namelyCaryocaris delicatus n. sp.,Caryocaris sp. 1, ?Caryocaris sp. 2,Pumilocaris granulosus n. gen., n. sp., ?Pumilocaris sp. and an undetermined form. They all occur in graptolite-bearing shales. A critical review of the South American phyllocarids formerly described in the literature is made andCaryocaris delicatus n. sp. (Llanvirn) is compared with congeneric species known from the Ordovician (Tremadoc-Caradoc) of North America, Europe, China and Australia. The presence ofCaryocaris in South America is therefore attested, which confirms the view that the genus had a worldwide distribution in the lower Ordovician in relation with a possible pelagic lifestyle. A new definition ofCaryocaris is proposed and the former subdivision of the genus [C. (Caryocaris) andC. (Rhinopterocarys)] abandoned. A set of original characters (e.g. lack of rostral plate, short leaflike furcal rami, short telson, extremely thin carapace) strongly indicates thatCaryocaris lies outside the Ceratiocarididae and may form the basis of a new taxonomic unit.     

Preliminary palynological investigation of Saudi Arabian Upper Ordovician glacial sediments

An Ordovician stratigraphically admixed palynomorph assemblage that contains palynomorphs eroded from Middle through Upper Ordovician strata characterizes the Hawban Member (restricted) of the Sarah Formation in central Saudi Arabia. This distinctive assemblage, combined with detailed sedimentology, helps identify the presence of Hirnantian Gondwanan glacial sediments on the Arabian Plate. Similar Ordovician admixed assemblages have been recognized from Upper Ordovician glacial sediments elsewhere along the Gondwanan margin. Within Saudi Arabia the composition of reworked assemblages depends upon the stratigraphic succession exposed to glacial erosion. Sylvanidium? hawbanense, which is one of the acritarchs found in glacial sediments, is newly described from Arabian Upper Ordovician strata.     

Marine ostracod provinciality in the late ordovician of palaeocontinental laurentia and its environmental and geographical expression

Background

We examine the environmental, climatic and geographical controls on tropical ostracod distribution in the marine Ordovician of North America.

Methodology/Principal Findings

Analysis of the inter-regional distribution patterns of Ordovician Laurentian ostracods, focussing particularly on the diverse Late Ordovician Sandbian (ca 461 to 456 Ma) faunas, demonstrates strong endemicity at the species-level. Local endemism is very pronounced, ranging from 25% (e.g. Foxe basin) to 75% (e.g. Michigan basin) in each basin, a pattern that is also reflected in other benthic faunas such as brachiopods. Multivariate (ordination) analyses of the ostracod faunas allow demarcation of a Midcontinent Province and a southern Marginal Province in Laurentia. While these are most clearly differentiated at the stratigraphical level of the bicornis graptolite biozone, analyses of the entire dataset suggest that these provinces remain distinct throughout the Sandbian interval. Differences in species composition between the provinces appear to have been controlled by changes in physical parameters (e.g. temperature and salinity) related to water depth and latitude and a possible regional geographic barrier, and these differences persist into the Katian and possibly the Hirnantian. Local environmental parameters, perhaps operating at the microhabitat scale, may have been significant in driving local speciation events from ancestor species in each region.

Conclusions/Significance

Our work establishes a refined methodology for assessing marine benthic arthropod micro-benthos provinciality for the Early Palaeozoic.     

Drosophila Cuticular Hydrocarbons Revisited: Mating Status Alters Cuticular Profiles

Most living organisms use pheromones for inter-individual communication. In Drosophila melanogaster flies, several pheromones perceived either by contact/at a short distance (cuticular hydrocarbons, CHs), or at a longer distance (cis-vaccenyl acetate, cVA), affect courtship and mating behaviours. However, it has not previously been possible to precisely identify all potential pheromonal compounds and simultaneously monitor their variation on a time scale. To overcome this limitation, we combined Solid Phase Micro-Extraction with gas-chromatography coupled with mass-spectrometry. This allowed us (i) to identify 59 cuticular compounds, including 17 new CHs; (ii) to precisely quantify the amount of each compound that could be detected by another fly, and (iii) to measure the variation of these substances as a function of aging and mating. Sex-specific variation appeared with age, while mating affected cuticular compounds in both sexes with three possible patterns: variation was (i) reciprocal in the two sexes, suggesting a passive mechanical transfer during mating, (ii) parallel in both sexes, such as for cVA which strikingly appeared during mating, or (iii) unilateral, presumably as a result of sexual interaction. We provide a complete reassessment of all Drosophila CHs and suggest that the chemical conversation between male and female flies is far more complex than is generally accepted. We conclude that focusing on individual compounds will not provide a satisfactory understanding of the evolution and function of chemical communication in Drosophila.     

Graptolithen aus ordovizischen Geschieben und die frühe Stammesgeschichte der Graptolithen

Primitive Ordovician Graptolites are compared with the recent PterobranchRhabdopleura. It is concluded that these tree-like primitive Ordovician Graptolites had creepingRhabdopleura-like ancestors. Probably all Graptolites developed fromRhabdopleura-like ancestors. — The soft parts of Graptolites had two arms likeRhabdopleura. Observations on the outer surface of Graptolites supportBeklemishev’s hypothesis, that Graptolite individuals were able to leave their tubes: moving outside on their tubes the individuals deposited material by their cephalic shields, forming the so called Cortex. A similar secondary deposition on the housing is known in recent Pterobranchs.     

Stratigraphical distribution of the Ordovician graptolite Azygograptus Nicholson & Lapworth in the Central Andean Basin (northwestern Argentina and southern Bolivia)

We analyzed new occurrences of Azygograptus lapworthi from the Cordillera Oriental, Argentina. The bearer sandstones levels, corresponding to the Acoite Formation, are overlying the deposits, in which the Didymograptellus bifidus Biozone (Lower Ordovician, upper Floian, Fl3) was previously recognized, and are overlain by younger pelitic levels yielding Xiphograptus lofuensis (Middle Ordovician, early Dapingian, Dp2). Previous records from the Central Andean Basin are also reviewed in detail and accurately correlated, allowing us to conclude that the Azygograptus lapworthi Biozone corresponds to the Middle Ordovician (lower Dapingian, Dp1). This biostratigraphic framework documents that the transition between the Lower and Middle Ordovician deposits occurs in the uppermost levels of the Acoite Formation in the Argentine Cordillera Oriental. It is additionally integrated with up to date conodont records to establish a high-resolution regional correlation, with equivalent deposits from the Puna of northwestern Argentina and Cordillera Oriental of Bolivia, and to discuss new insights for global correlation.     

Fossil microphytoplankton dynamics across the Ordovician–Silurian boundary

A critical review of all available data on acritarch biostratigraphy and diversity dynamics across the Late Ordovician through the early Silurian, permits a better appreciation of the potential of acritarchs for the recognition of the systemic boundary. This analysis also reveals the response of marine microphytoplankton populations to the Late Ordovician palaeoenvironmental crisis (Hirnantian glaciation). Previous zonal schemes are improved, and an updated acritarch biostratigraphic chart is proposed, plotted against the most recent chronostratigraphic subdivisions. Sections from Anticosti Island (Québec, Canada), Algeria, Morocco, and Estonia preserve the best palynological record for the investigated interval. The present analysis shows that no true mass-extinction event occurred in latest Ordovician times in connection with the well known glacial event. “Pre-glacial” Ashgill acritarch suites are dominated by species of Baltisphaeridium, Multiplicisphaeridium, Ordovicidium, Orthosphaeridium, and netromorph acritarchs. An important proportion of these taxa (excluding Ordovicidium and Orthosphaeridium) survive the onset of glacial conditions in Hirnantian (latest Ordovician) times and continues through the early Silurian. The development of morphological polymorphism appears as a response (a survival strategy?) to the establishment of glacial conditions.In glacial-related sediments of Hirnantian age in North Africa, acritarch assemblages display a burst of relative abundance and intra-specific morphological variability (polymorphism) of long-ranging taxa such as Veryhachium, Multiplicisphaeridium, Dactylofusa, Poikilofusa, and Evittia. The extinction of several species characteristic of Upper to uppermost Ordovician strata occurs near the boundary, in “post-glacial” Ashgill (uppermost Hirnantian). This extinction event is counterbalanced by the almost contemporaneous (within the limits of stratigraphic resolution) appearance of several new taxa showing already a clear “Silurian affinity”, e.g., Tylotopalla, Cymbosphaeridium, and Visbysphaera. This origination event is observable in, and correlatable between the North African, the Bohemian and the Anticosti sections, making it global in extent.The completion of the palynological turnover and the establishment of a diverse Silurian acritarch suite occurs well above the base of the Silurian, during Aeronian times. The strong survival capability of the oceanic plankton through periods of palaeoenvironmental crisis in latest Ordovician times (but also throughout the Phanerozoic) could have played an important role in the post-extinction rebounds of metazoan clades, by assuring the continuity of marine trophic resources to consumers and avoiding irreversible disruptions of the trophic chains.     

The effects of the final stages of the Late Ordovician glaciation on marine palynomorphs (chitinozoans, acritarchs, leiospheres) in well Nl-2 (NE Algerian Sahara)

Palynomorph assemblages, especially chitinozoans and acritarchs, from the Upper Ordovician of well Nl-2 (north-east of the Algerian Sahara) are studied in order to precisely date the ultimate effects of the Late Ordovician glaciation and to document the impact of this major climatic stress on the diversity of the palynoplankton. References are made to stable isotope excursions and to global eustatic sea level variations in order to improve the local age determination. The Hassi el Hadjar Formation, i.e. glacio-marine diamictites, is interpreted as a transgressive event resulting from the melting of the northern Gondwana ice cap. It yields poorly preserved and moderately diverse chitinozoans of late Hirnantian age. Acritarchs are more abundant in the lower part of these "microconglomeratic clays", but display a low diversity and are badly preserved throughout the whole formation. Reworked individuals are recorded in both groups. The marine sediments of the M'Kratta Formation of latest Hirnantian age contain better preserved, more abundant and more diverse palynomorph assemblages, especially in the Upper Member. The composition of this palynoplankton indicates a fairly good faunal and phytoplankton recovery after the early Hirnantian climatic stress.The extinction of the Ordovician forms, and the appearance of Silurian type taxa occur only in the uppermost Hirnantian, i.e. following with a slight delay the glacial event. The overlying black shales of Wenlock age (lower part of the Oued Mehaiguène Formation) are indicative of marine anoxic environments. They yield a virtually exclusive, but enormous number of Tasmanacea. The very peculiar composition of this palynoplankton seems to be independent of the Late Ordovician glaciation and is most likely related to the factors that, later, generated and maintained anoxic conditions in this area.A new species of chitinozoan, Spinachitina oulebsiri sp. nov. from the latest Hirnantian M'Kratta Formation, is described and illustrated.     

Latest Ordovician graptolites from the Mandalay Region,Myanmar

The graptolite fauna of the Mandalay Region, Myanmar, is poorly known. In this paper, 13 species in the genera Avitograptus, Korenograptus, Metabolograptus, Neodiplograptus and Normalograptus are described from the uppermost Ordovician strata of the area. Two of these species, Korenograptus selectus X. Chen n. sp. and Neodiplograptus mandalayensis X. Chen n. sp., are new. On the basis of the graptolite faunas, a biostratigraphic scheme for the uppermost Ordovician (Hirnantian: Metabolograptus extraordinarius Biozone and Metabolograptus? persculptus Biozone?) strata in this area is erected.     

Dendroid graptolites from the Lower Ordovician (Tremadocian) of the Yichang area, Hubei, China

The Yichang area of Hubei Province, China provides important information on Lower to Middle Ordovician fossil faunas and is a center for research on Ordovician graptolites due to the sometimes excellent preservation of the copiously available material. New material collected during a research project on Lower to Middle Ordovician biota in the Yichang Region provided among others specimens a number of dendroid (benthic) graptolites of the genera Acanthograptus, Aspidograptus, Dendrograptus, and Koremagraptus showing unexpected and unknown structural features, useful for structural and evolutionary investigations.     

First Ordovician vertebrate from south America

The first Ordovician vertebrate from South Americais described from the Anzaldo Formation (Caradoc, Upper Ordovician) of Central Bolivia. It is referred to a new taxon, Sacabambaspis janvieri nov. gen. nov. sp., within the jawless vertebrate group Heterostraci, and displays some resemblance to both Astraspis and Arandaspis. It differs from all other known heterostracans by having very broad sensory-line grooves on the exoskeleton. Consideration of the palaeobiogeography of Ordovician vertebrates suggests that the early forms have been restricted to warm regions.     

Correlation of eustatic and biotic events in the Ordovician paleobasins of the Siberian and Russian platforms

Nine sedimentary sequences are recognized in the Ordovician of the Siberian Platform. These sequences correspond to sea level fluctuations of the 3rd order, from 1 to 6 My. Correlation with the sequences recognized in the Ordovician of the Russian Platform suggest their possible eustatic nature. Cold water nontropical carbonates are suggested in the Ordovician of the Tungus Syneclise, which may be explained by the upwelling of cold oceanic waters. The upwelling was caused by re-distribution of oceanic currents due to largescale tectonic events in the mid-Ordovician. The Ordovician evolution of the Siberian Platform was much more similar to that of the North American Platform than of the Russian Platform.     

Ordovician biogeography and continental drift

The changeable biogeography of the Ordovician is reviewed, quantitatively analysed and used to assess possible positions and relative movements of the continental plates. Plate boundaries are defined as precisely as possible using geological and palaeontological data. The trilobites, corals, brachiopods, cephalopods, echinoderms, graptolites and ostracods are found to be useful in defining plate boundaries and relative plate movements. Oceanic barriers are considered to be the simplest explanation for the maintenance of faunal provincialisms. The faunal barriers between South America and Africa and between Australia and Gondwanaland may have been climatic and land barriers. The present Asian continent is divided into Kazakhstan, the Siberian Platform, South Asian, Northern China, India and the Jano-Kolymian block. These areas had different faunal histories and are considered to have had different drift histories. The North China and South Asian plates were separated by the Tsinling Ocean, the Northern European plate from the North American plate by Wilson's Proto-Atlantic and the Siberian Platform plate from the Northern European by the Uralian Ocean. Southern and Central Europe are shown to have been joined to Africa and separated from the Northern European plate by a Mid-European Ocean. If Australia is considered as part of an Ordovician Gondwanaland then the best explanation for the faunal histories of most plates is provided by an anticlockwise rotation of Gondwanaland about the South palaeo-pole.     

The type species of the ordovician trilobitesymphysurus: systematics,functional morphology and terrace ridges

The type of the widespread Ordovician trilobiteSymphysurus, S. palpebrosus Dalman, 1827, is redescribed from the type Swedish material; its distribution is documented.Symphysurus is one of very few trilobite genera to be found in both Ordovician Baltica and Gondwana, in more peripheral sites relative to the platform areas.S. palpebrosus has complex coaptative devices to ensure tight enrollment. Analysis of the functional morphology shows that it spent at least part of its life with thorax and pygidium buried in the sediment in the bumastoid stance, although it may have fed outside its burrow. Terrace ridges are well developed on certain parts of the cuticle; their positioning and geometry shows that only a few of them could have functioned in direct engagement with the sediment. Those associated with petaloid thoracic facets may have permitted respiration in the enrolled condition.     

The Ordovician acritarch genus Coryphidium

The acritarch genus Coryphidium Vavrdová, 1972 is one of the most frequently recorded acritarch taxa in the Ordovician. The original diagnoses, stratigraphical ranges and geographical distribution of all Coryphidium species are critically evaluated in a review of published literature supplemented by studies of material from the British Isles, Belgium, the Czech Republic, Germany, Spain, Morocco, Algeria, Tunisia, Libya and China, including sections from type areas. The taxonomic concept of the genus is here rationalized: the genus Coryphidium is emended and the informal category of coryphid acritarchs is introduced to include all morphotypes with the characteristic vesicle shape of the two genera Coryphidium and Vavrdovella Loeblich and Tappan, 1976. Nine of the previously described species can be attributed to the genus, and two other species possibly belong to it. The attribution to Coryphidium of the species C. sichuanense Wang and Chen, 1987 is rejected here. Intraspecific variability is very important and the attribution of Coryphidium specimens at the specific level is sometimes difficult. The genus is found in all palaeoenvironments from nearshore to offshore settings and apparently does not occupy specific palaeoecological niches. Coryphidium is very useful biostratigraphically and palaeobiogeographically. The review indicates that the genus first appears in the uppermost Tremadocian Araneograptus murrayi graptolite Biozone and is common through the upper Lower Ordovician and the Middle Ordovician, while Upper Ordovician occurrences might be the result of reworking. Palaeogeographically, Coryphidium is an indicator of the peri-Gondwanan acritarch “palaeoprovince” during the Early/Middle Ordovician.     

New Ordovician ostracodes from Himalaya and their palaeobiological and palaeogeographical implications

Silicified representatives of the ostracode orders Binodicopa, Palaeocopa and Podocopa have been recovered from the late Ordovician Pin Formation, Spiti, NW Himalaya area. Benthic binodicopes are distinctly more abundant than the palaeocope and podocope ostracodes. The binodicopes show biogeographical relations to Australia at the generic level, which supports the close palaeogeographic position of these continents. By contrast, the palaeocope and podocope taxa show close relations to Baltica and Bohemia even at the species level, which may suggest a pelagic or epipelagic mode of life since these regions are separated from each other by the broad and deep waters of Palaeotethys. New taxa are Vendona crassiplicata nov. sp., Glossomorphites himalayaensis nov. sp., Pilla pinensis nov. sp. and Pinpilla u-formis nov. gen. nov. sp.