Restudy of some Ordovician–Silurian boundary graptolites from Anticosti Island, Canada, and their biostratigraphic significance

The Ellis Bay Formation on Anticosti Island has long been recognized for the biostratigraphic importance of its latest Ordovician conodont, palynomorph, and shelly fossil assemblages. However, a sparse record of graptolites has made it difficult to correlate these assemblages with the graptolite biozonation. Restudy of some previously described and examination of newly collected normalograptid specimens from the Ellis Bay and lower Becscie formations, however, shows that biostratigraphically important taxa are present and these species provide important constraints on the age of the strata. Normalograptus parvulus and N. minor have been identified in the upper half of the Ellis Bay Formation, suggesting that these strata are late Hirnantian in age ( N. persculptus Biozone). Normalograptus imperfectus and Normalograptus sp. aff. N. acceptus occur in the basal beds of the Becscie Formation, indicating that these strata are earliest Silurian. These graptolite data support the hypothesis that the positive carbon isotope excursion seen in the uppermost strata of the Ellis Bay Formation is isochronous with that seen in the Hirnantian strata at Dob's Linn, Scotland, and does not span much of the lower to mid-Hirnantian, as is the case in Arctic Canada and Nevada, USA.     Anticosti Island , Hirnantian, graptolites, Ordovician, Silurian.     

Discovery of Anticostia uniformis from the Xiazhen Formation at Zhuzhai,South China and its stratigraphic implication

The Xiazhen Formation is an Upper Ordovician lithostratigraphic unit in the Jiangshan-Changshan-Yushan (JCY) area, which contains series of Late Ordovician reef successions. The reef successions of the Xiazhen Formation at Zhuzhai are critical for evaluation of the Late Ordovician marine diversity and palaeoecology. However, their age has long been uncertain and generally is regarded as of upper Katian, based on the occurrences of shelly fossils and correlation with the stratigraphic equivalent Changwu Formation. The newly discovered graptolite species Anticostia uniformis, in the Xiazhen Formation, together with the combined evidence of brachiopods and sedimentology, indicates an age range for the graptolite locality from the Dicellograptus complanatus Biozone to the Diceratograptus mirus Subzone of late Katian, but the graptolites do not rule out the possibility that it is Hirnantian.     

湖南益阳早、中奥陶世的叶笔石属(Phyllograptus)

报道湖南益阳早、中奥陶世的叶笔石1属5种,包括Phyllograptus anna Hall,1865,Phyllograptus anna longus Ruedemann,1947,Phyllograptus anna ultimus Ruedemann,1904,Phyllograptus claviger Monsen,1937,Phyllograptus claviger acuminatus(Chen and Xia),1979。研究表明,该属主要产出于弗洛晚期至大坪中期,在早、中奥陶世笔石多样性演变事件第一阶段的后期(Corymbograptus deflexus带),叶笔石所代表的上攀伸展类型就已萌芽,且在第二阶段(Pseudophyllograptus angustifolius elongatus带-Isograptus victoriae divergens带)于均分笔石动物群中占据优势地位,对该动物群内部类群的演替起着承前启后的作用,同时也验证了笔石动物群自身的演替对于其多样性演变具有重要影响。     

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.     

Age constraints of the Hungshihyen Formation (Early to Middle Ordovician) on the western margin of the Yangtze Platform,South China: New insights from chitinozoans

The Hungshihyen Formation represents the Lower to Middle Ordovician near-shore siliciclastic deposits on the western margin of the Yangtze Platform, South China. However, its age is still debated, largely because of insufficient studies. Here we documented the Bursachitina maotaiensis–B. qianbeiensis chitinozoan assemblage from the lower part of the Hungshihyen Formation at Songliang, Qiaojia County, eastern Yunnan Province. This assemblage is remarkably similar to an assemblage that roughly correlates to the middle part of the Corymobograptus deflexus graptolite Biozone in northern Guizhou Province, indicating that the Hungshihyen Formation at Songliang is probably of late Floian age and may extend into the early Dapingian. Additionally, a revision on the chitinozoans previously documented from the strata assigned to the same formation in the Wuding area was made according to the latest classification. The revision allows the recognition of the Euconochitina symmetrica–Eremochitina brevis assemblage, and therefore suggests a late Tremadocian age for this interval based on regional and global chitinozoan biostratigraphic correlations. Considering also its lithologic succession, we argue that the chitinozoan-bearing strata in the Wuding area previously assigned to the Hungshihyen Formation correspond most likely to the uppermost part of the underlying Tangchi Formation or the lowermost part of the Hungshihyen Formation. The base of the Hungshihyen Formation appears therefore to be diachronous in eastern Yunnan Province.     

Volkhov graptolites from the lower-middle Ordovician boundary beds of the St. Petersburg region,Russia

The lower and lowermost Middle Ordovician strata of the Volkhov Stage exposed in the vicinity of St. Petersburg, Russia, yield a graptolite fauna at several stratigraphic levels, being more often confined to the mud-mound clays than to the normally stratified, Condensed carbonates. Overall taxonomic diversity of the Volkhov graptolites is relatively low and associations include no more than seven species in ail localities. TheTetragraptus, Didymograptus andXiphograptus species found are pandemic and have quite extensive stratigraphie ranges within the mid-Arenig Series. Based on the graptolite zonation, the Volkhov Stage can be correlated to thePseudophyllograptus angustifolius elongatus andDidymograptus (Expansograptus) hirundo graptolite biozones of Baltoscandia and to the upperDidymograptus simulans toDidymograptus (Expansograptus) hirundo biozones of the British graptolite succession. TheIsograptus victoriae lunatus lower zonal boundary probably falls within the graptolite-bearing interval of the lower Volkhov Stage. As the result of indirect biostratigraphic and interpretative lithological correlations, the first appearance ofUndulograptus austrodentatus at the base of the Darriwilian Stage is deemed to be close to the boundary of theMegistaspis simon andM. limbata trilobite biozones and the lowerBaltionodus norrlandicus conodont Biozone of the upper Volkhov Stage. Seven dichograptid species belonging to four genera are described, one of them,Azygograptus volkhovensis n. sp., new.      

Proposal for adoption of the base of the Undulograptus austrodentatus Biozone as a global Ordovician stage and series boundary level

The base of the Undulograptus austrodentatus Biozone appears to be a synchronous event that is widely recognizable within graptolitic facies around the world. It occurs within an interval in which graptolite species ranges are now well known and in which there is a rapid turnover in the composition of graptolite faunas. This turnover reflects the rapid evolutionary radiation of the Diplograptacea simultaneously with the appearance of several distinctive pseudisograptid and glossograptid species. These events provide the basis for the recognition of two thin but widely applicable subzones; a lower Arienigraptus zhejiangensis Subzone and an upper U. sinicus Subzone. The occurrence of the lower boundary of the U. austrodentatus Biozone within a succession of first appearances also permits accurate and reliable identification of the boundary as well as assessment of stratigraphic completeness across the boundary interval in correlated sections. Diverse graptolite faunas of late Yapeenian and early Darriwilian age occur in association with the Histiodella altifrons Biozone of the North American midcontinent conodont zonation and the Paroistodus originalis and Microzarkodina parva biozones of the North Atlantic conodont zonation. They also occur in association with the shelly-fossil zonations developed for several different continents. These features of the base of the U. austrodentatus Biozone make it a suitable level for use as the boundary level for a global stage. Its stratigraphic position within the Ordovician System relative to other likely global stages as well as its coincidence with one of the major events in graptolite evolutionary history suggest that this level also may be a suitable level for the base of a global Middle Ordovician Series.Ordovician System, Ordovician stages, graptolite zonation, chronostratigraphy, international correlation. Charles E. Mitchell and Jörg Maletz, Department of Geology, State University of New York at Buffalo, Buffalo, New York 14260-1550, USA; 13th July, 1994; revised 22nd May, 1995.     

An Early Ordovician Trilobite assemblage from the Lashkarak Formation, Damghan area, northern Iran

A low diversity Lower Ordovician trilobite assemblage is described from the middle part of the Lashkarak Formation at Simeh-Kuh, northwest of Damghan, northern Iran. Co-occurrence with conodonts of the lowermost Paroistodusproteus Biozone suggests an uppermost Tremadocian age. This assemblage includes four taxa, including one new genus and two new species, Damghanampyxginteri nov. gen. nov. sp. and Asaphellusfecundus nov. sp. The third diagnosable species from the formation is Taihungshaniamiqueli (Bergeron, 1894) which has previously been reported from southern France and the Turkish Taurides, suggesting north Gondwanan faunal affinities.     

A chitinozoan morphological lineage and its importance in Lower Silurian stratigraphy

Margachitina margaritana is a distinctive, chain-forming chitinozoan taxon of the uppermost Telychian Stage (Llandovery Series) to lower Homerian Stage (Wenlock Series). M. margaritana is shown to be the end member of a morphological lineage that developed from Calpichitina densa via the intermediate Margachitina banwyensis sp. nov. A local M. banwyensis Biozone, which correlates with the upper lapworthi graptolite Biozone, is proposed for the Banwy River section. The morphological lineage described provides a tool for the enhanced subdivision of the uppermost Llandovery Series, which may prove global in extent. This research suggests also that the base of the Wenlock Series may correlate with a level below the centrifugus graptolite Biozone and it also brings into question the systematic status of Calpichitina and Margachitina, although both genera are retained herein.     

贵州思南奥陶纪末至志留纪初大息场组的笔石

贵州思南奥陶纪末至志留纪初大息场组的笔石研究结果表明,大息场组共产出笔石6属7种:Styracograptus chiai(Mu,1949),Normalograptus mirnyensis(Obut and Sobolevskaya,1967),Normalograptus ajjeri(Legrand,1977),Metaclimacograptus hughesi(Nicholson,1869),Avitograptus avitus(Davies,1929),Atavograptus atavus(Jones,1909)和Korenograptus laciniosus(Churkin and Carter,1970)。自下而上可识别出相当Dicellograptus complexus带、Akidograptus ascensus带和Cystograptus vesiculosus-Coronograptus cyphus带的3个含笔石地层带。虽然大息场组的笔石动物群分异度较低,但全都是上扬子区奥陶-志留系之交的常见属种,除S.chiai(Mu)之外,其它都是全球广布的物种。     

The Lerhamn drill core and its bearing for the graptolite biostratigraphy of the Ordovician Tøyen Shale in Scania,southern Sweden

Maletz, J. & Ahlberg, P. 2011: The Lerhamn drill core and its bearing for the graptolite biostratigraphy of the Ordovician Tøyen Shale in Scania, southern Sweden. Lethaia, Vol. 44, pp. 350–368. A drill core through the Lower Ordovician Tøyen Shale Formation at Lerhamn, NW Scania, southern Sweden, provides important new information for the precise biostratigraphic resolution of the Floian to lower Darriwilian time interval in southern Scandinavia. The Hunnegraptus copiosus, Tetragraptus phyllograptoides, Cymatograptus protobalticus (new), Baltograptus vacillans (new), Baltograptus sp. cf. Baltograptus deflexus (new), Baltograptus minutus (new), Isograptus victoriae, Undulograptus austrodentatus (Arienigraptus zhejiangensis and Undulograptus sinicus Subzones) and the (?)Corymbograptus retroflexus (new) biozones are differentiated in the core and their correlation in Scania is discussed. The Lerhamn drill core provides the most detailed graptolite record of the Floian Stage in Scandinavia. The interval is dominated by a number of species of the genus Baltograptus, endemic to the Atlantic Faunal realm and highly useful for regional biostratigraphic correlation. The biostratigraphic framework is based on endemic and pandemic faunal elements. The mixture of both elements in the drill core allows a more precise inter‐continental correlation of Lower to Middle Ordovician graptolite faunas and may – in the future – provide information as to the climatic history of regions dominated by Baltograptus faunas in the Floian. The (?)C. retroflexus Biozone is based on species originally described from Bohemia, but the record of the zone in the Lerhamn drill core indicates a wider distribution of its fauna. □Biostratigraphy, graptolites, Ordovician, Scania, Sweden.     

Apparatus structure of the Ordovician conodontDecoriconus peselephantis (Lindström 1955)

The Lower Ordovician conodont species first described asScolopodusl peselephantis Lindström 1955, has a seximembrate apparatus of laterally compressed, variously curved and flexed coniform elements. These are very small and invariably striated or multicostate. The taxon is reassigned toDecoriconus Cooper 1975, a genus hitherto known by species from Upper Ordovician and Silurian strata. These younger species share several characteristic morphological features with the older species investigated in this work. When studied in detail, it was discovered thatD. peselephantis was the first species in a lineage which also included two successive new species. Of these,D. mercurius n. sp. occurs in the middle Arenig, whileD. pesequus n. sp. ranges from the late middle Arenig and at least through the Llanvirn, possibly even into the Caradoc. These new species, which were previously included intoS.? peselephantis, are described and their geographic distribution investigated. In addition, a probable homeomorphic form ofD. peselephantis, Toxotodus? gabriellae n. sp., is diagnosed.     

A possible abrograptid graptolite (Abrograptidae,Graptoloidea) from western Newfoundland

Jiangshanites (?)dubius n. sp. is the oldest abrograptid graptolite yet described. It was found in theUndulograptus austrodentatus Biozone of western Newfoundland. The specimens possess an extended rutellum and an isograptid orartus type proximal development with prosicular origin of th 1¹. No indication of a virgellar spine is seen. The proximal development structures of the specimens show the relation to Sigmagraptidae or Dichograptidae.     

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.     

川西汉源-荥经地区下奥陶统大乘寺组的笔石

对川西汉源-荥经地区CYD2井下奥陶统弗洛阶大乘寺组的笔石进行研究,结果表明Baltograptus deflexus笔石带在这一覆盖区发育良好。CYD2井的该带笔石动物群共含2属5种。根据下垂对笔石类的始端发育型式,修正前人对波罗的笔石属(Baltograptus)和巅峰笔石属(Corymbograptus)内一些种的归属,并对CYD2井的5种笔石进行描述。可能受制于钻心标本的限制,该地区的Baltograptus deflexus带笔石动物群分异度较低。川西汉源-荥经-峨眉山地区的大乘寺组与上覆二叠系梁山组呈假整合接触,表明该地区长期遭受沉积间断和剥蚀作用,B.deflexus带的确定,指示了这一长期剥蚀作用的最大深度。     

A proposed new global stratotype for Aeronian Stage of the Silurian System: Hlásná Třebaň section,Czech Republic

The current Global Stratotype Section and Point (GSSP ) for the Aeronian Stage (Llandovery Series, Silurian System), on Trefawr track in the Llandovery area of Wales, is an inadequate marker for precise, global, correlation. The International Subcommission on Silurian Stratigraphy has, therefore, undertaken the selection of a new GSSP for this level. The lowest occurrence of the graptolite Demirastrites triangulatus , 1.38 m above the base of the black‐shale succession of the ?elkovice Formation at the Hlásná T?ebaň section in Central Bohemia, is proposed to mark the base of the Aeronian Stage. The section, which fulfils all formal requirements for stratotype of a chronostratigraphical unit, should be considered as a candidate for the new GSSP . An abundant, well‐preserved, diverse graptolite fauna occurs through the section along with common chitinozoans, which indicate that the Spinachitina maennili Biozone spans the boundary interval. The section comprises the lower–middle Aeronian (D. triangulatus–Lituigraptus convolutus graptolite biozones) along with underlying Rhuddanian (Akidograptus ascensus–Coronograptus cyphus biozones) and Hirnantian strata. Several graptolite genera of primary importance in global correlation (Demirastrites , Petalolithus , Rastrites and Campograptus ) first appear in the lower part of the triangulatus Biozone. The structurally simple section is somewhat condensed, but there is a uniform succession of black shale without any evidence of disconformity in the broad boundary interval. The C_org isotope record exhibits a minor positive excursion just above the base of the triangulatus Biozone, whereas TOC and N isotope and elemental geochemical records provide evidence for uninterrupted sedimentation under stable, anoxic conditions.     

The early Ludfordian leintwardinensis graptolite Event and the Gorstian–Ludfordian boundary in Bohemia (Silurian,Czech Republic)

The late Gorstian to early Ludfordian hemipelagic succession of the south‐eastern part of the Prague Synform preserves a rich fossil record dominated by 28 species of planktic graptoloids associated with pelagic myodocopid ostracods, pelagic and nektobenthic orthocerid cephalopods, epibyssate bivalves, nektonic phyllocarids, rare dendroid graptolites, brachiopods, crinoids, trilobites, sponges and macroalgae. Faunal dynamics have been studied with particular reference to graptolites. The early Ludfordian leintwardinensis graptolite extinction Event manifests itself as a stepwise turnover of a moderate diversity graptolite fauna rather than an abrupt destruction of a flourishing biota. The simultaneous extinction of the spinose saetograptids Saetograptus clavulus, Saetograptus leintwardinensis and the rare S. sp. B. at the top of the S. leintwardinensis Zone was preceded by a short‐term acme of S. clavulus. Cucullograptus cf. aversus and C. rostratus vanished from the fossil record in the lower part of the Bohemograptus tenuis Biozone. No mass proliferation of Bohemograptus has been observed in the postextinction interval. Limited indigenous speciation gave rise to Pseudomonoclimacis kosoviensis and Pseudomonoclimacis cf. dalejensis. Egregiograptus (the only novel element of cryptic origin) made its earliest occurrence in association with the latest cucullograptids. A postextinction graptolite assemblage of moderate diversity, composed for the most part of long‐ranging taxa, persisted through the middle and upper tenuis Biozone until new rediversification in the Neocucullograptus inexpectatus Biozone. Unlike the graptoloids, the planktic, epiplanktic, nektonic and nektobenthic shelly fauna did not suffer significant extinction in the early Ludfordian. The Gorstian–Ludfordian boundary is placed at the lowest occurrence of S. leintwardinensis in spite of the very modest graptolite record available from the Ludfordian GSSP where no graptolites occur below the basal bed of the Lower Leintwardine Formation. Elsewhere, the Gorstian–Ludfordian boundary has been placed at the base of the Saetograptus linearis Biozone which has been considered roughly correlative with the leintwardinensis Biozone. Indeed, our morphometric study places the worldwide biozonal index species S. linearis in synonymy with S. leintwardinensis and thus considerably enhances the biostratigraphical utility of the latter index species. Pseudomonoclimacis antiqua sp. nov. is described.     

Upper Ordovician bryozoans from the Xiazhen Formation of Yushan,northeastern Jiangxi,East China

The bryozoan fauna from the Xiazhen Formation (Katian, Upper Ordovician) of northeast Jiangxi Province, southeast China is reported here. Seventeen species of bryozoans belonging to fifteen genera and four orders are identified: Homotrypa yushanensis, Homotrypa sp., Prasopora yushanensis, Trematopora sp., Monotrypella sp., Rhombotrypa sp., Orbignyella sp., Constellaria jiangxiensis, Constellaria sp., Stictopora nicholsoni, Trigonodictya parvula, Ptilodictya ensiformis, Stictoporella sp., Pseudopachydictya sp., Nematopora sp., Arthrostylidae sp. indet., and Chasmatoporidae sp. indet. Four of these genera have been reported previously but nine genera (Trematopora, Monotrypella, Rhombotrypa, Orbignyella, Trigonodictya, Ptilodictya, Stictoporella, Pseudopachydictya, and Nematopora), one rhabdomesine and one fenestrate are found for the first time in the Late Ordovician strata of South China. Our palaeogeographical analysis suggests that the bryozoan association is typical for the Katian, which is mostly widespread in Laurentia, Siberia, Baltica and Mediterranean, and displays palaeobiogeographical relationships to the Laurentia–Siberia Province.     

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.