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.     

Palynomorphs from the Cambrian Series 3, Parahio valley (Spiti), Northwest Himalaya

We present low-diversity acritarch assemblages, including Alliumella baltica, Bavlinella faveolata, Brocholaminaria nigrita, Brocholaminaria sp., Eliasum sp., Leiosphaeridia minutissima, Leiosphaeridia spp., Lophosphaeridium sp., Pterospermella solida, Satka colonialica, Siphonophycus sp., and Synsphaeridium sp., as well as filamentous algae and cryptospore-like microfossils recorded from 45 rock samples around the Oryctocephalus indicus Zone (Cambrian Series 3) in the Cambrian Kunzam La (Parahio) Formation at the Kaltarbo locality in the Parahio valley, Northwest Himalaya. One new species Synsphaeridium parahioense was established. The acritarch assemblages at the basal part of the Cambrian Series 3 in the Kunzam La (Parahio) Formation were dependent on the local marine environment. The new discovery of cryptospore-like microfossils from the Cambrian Kunzam La (Parahio) Formation supports the extensive distribution of possible cryptospores in the Cambrian stratigraphic sequences.     

陕西洛南下寒武统辛集组疑难管状化石桶螺(Cupitheca)

本文对采自陕西洛南上湾剖面下寒武统辛集组疑难管状化石Cupitheca进行了研究,系统厘定和描述了两个种,分别为Cupitheca holocyclata和Cupitheca costellata,并对Cupithecidae科进行了重新修订。本文所描述标本的壳体多为次生磷酸盐化保存,壳体表面纹饰保存较好;C.holocyclata管体表面为横向纹饰,C.costellata管体表面为纵向纹饰。C.holocyclata全球广布,如南澳大利亚、南极乔治王岛、格陵兰东北部、加拿大纽芬兰等地皆有报道,产出于下寒武统中上部地层,具有一定的洲际地层对比意义。本文为C.holocyclata在中国首次发现。C.costellata仅发现于华北,即本文所研究的洛南辛集组及其同期地层安徽淮南与霍邱的雨台山组。     

The messaoudensis-trifidum acritarch assemblage (Ordovician: late Tremadoc-early Arenig) of the Barriga Shale Formation, Sierra Morena (SW-Spain)

The "messaoudensis-trifidum acritarch assemblage" is currently considered to be characteristic of latest Tremadoc-earliest Arenig cold-water environments on the periphery of Gondwana, at high latitudes in the southern hemisphere. An integrated biostratigraphical study on both acritarchs and graptolites was until now only available for localities in northwest England. Reinvestigation of the "messaoudensis-trifidum acritarch assemblage" from the Barriga Formation (Sierra Morena, southwestern Spain), which contains some graptolite horizons that can be attributed to the latest Tremadoc (pre-phyllograptoides and pre-approximatus graptolite biozones), strengthens the biostratigraphical potential of the "messaoudensis-trifidum assemblage", and the importance of some of the acritarch taxa recovered from these levels. It is concluded that the acritarch genera Coryphidium Vavrdová, 1972, Peteinosphaeridium Staplin et al., 1965 emend. Playford et al., 1995, Striatotheca, Burmann, 1970, and the Veryhachium lairdii group (rectangular veryhachiids) appear in the latest Tremadoc, and should not be considered as indicators of the base of the Arenig, as previously suggested.     

The acritarchs of the South Chinese Azygograptus suecicus graptolite Biozone and their bearing on the definition of the Lower–Middle Ordovician boundary

In southern China, well preserved acritarch assemblages have been recovered from numerous sections in the Yangtze Platform and the Jiangshan–Changshan–Yushan (JCY) area crossing the interval where the Lower–Middle Ordovician boundary should be defined, i.e., roughly at the base of the Chinese Azygograptus suecicus graptolite biozone. The acritarch taxa Aureotesta clathrata and Arbusculidium filamentosum first appear below the suecicus zone, while the genus Ampullula and the species Barakella felix have their First Appearance Datum (FAD) in the suecicus zone. These latter taxa, that are also present in Baltica and peri-Gondwana, respectively, thus probably indicate the base of the Middle Ordovician. To cite this article: J. Li et al., C. R. Palevol 1 (2002) 75–81.     

Upper Ordovician microphytoplankton of the Bill's Creek Shale and Stonington Formation,Upper Peninsula of Michigan,U.S.A.: Biostratigraphy and paleogeographic significance

An abundant, diverse, and well-preserved organic-walled microphytoplankton assemblage is described from the Upper Ordovician Bill's Creek Shale and the lower Stonington Formation (Bay de Noc Member) in the Upper Peninsula of Michigan, U.S.A. Based on graptolite and conodont evidence, the Bill's Creek Shale and Stonington Formation are Richmondian (=Ashgill) in age. The assemblage is dominated by acritarchs, which comprise 29 species (including the enigmatic palynomorph Gloeocapsomorpha prisca) assigned to 20 genera. The prasinophyte phycomata are represented by undifferentiated species of Leiosphaeridia and Tasmanites. In addition, chitinozoans are abundant, and scolecodonts and graptolite fragments are common. Paleontologic-palynologic and sedimentologic evidence indicates that the Bill's Creek Shale was deposited in a low-energy, shallow, nearshore marine environment. The overlying Bay de Noc Member of the Stonington Formation also accumulated in a low-energy, normal marine environment, but in a more offshore, somewhat deeper water setting. Both formations experienced minor transgressive and regressive episodes as indicated by fluctuations in the composition of the palynoflora. The combined Bill's Creek/Stonington acritarch assemblage closely resembles those described from the Richmondian-aged Maquoketa Shale (Missouri and Kansas), Sylvan Shale (Oklahoma), and Vauréal Formation (Anticosti Island, Québec, Canada). The overall composition of the acritarch assemblage from these four formations reflects a distinctive, recognizably Laurentian character. Nonetheless, many of the Bill's Creek/Stonington acritarchs have been reported from Upper Ordovician localities elsewhere, providing additional evidence for Late Ordovician cosmopolitanism of the marine microphytoplankton community. Additionally, the restricted stratigraphic range of many of the taxa further enhances their biostratigraphic application, both regionally and globally, and reaffirms the Richmondian (=Ashgill) age of the Bill's Creek Shale and Stonington Formation.     

Horizon of the oldest known bryozoans (Ordovician)

Among the six species of fossil bryozoans described by Xia et al. [Xia, F.S., Zhang, S.G., Wang, Z.Z., 2007. The oldest bryozoans: new evidence from the late Tremadocian (Early Ordovician) of East Yangtze Gorges. Journal of Paleontology 81 (6), 1308–1326] from the Fenghsiang Formation of the Chenjiahe section of Yichang and the Guanzhuangping section, Liujiachang Town of Songzi, the Yangtze Gorges, Nekhorosheviella nodulifera occurs in the pristinus Conodont Subzone (lower Subzone of deltifer Conodont Zone) and the five other species are associated with the conodont Paltodus deltifer deltifer and the graptolites Acanthograptus sinensis and A. erectoramus. Based on the conodonts and graptolites, this bryofauna stratigraphically corresponds to the A. sinensis Graptolite Zone or P. deltifer Conodont Zone. The bryofauna is of Tremadocian Age (Early Ordovician) and represents the oldest bryozoans so far as known.     

Late Devonian Acritarchs from the Gneudna formation in the Western Carnarvon basin,Western Australia

The Gneudna Formation is a Late Devonian(Frasnian) sequence of marine calcareous sediments that occurs in the Carnarvon Basin, Western Australia. The present palynological study is based upon subsurface silty strata from a borehole (Pelican Hill or Bibbawarra Bore) that was drilled early this century near the western coastal limit of the Carnarvon Basin.The subject strata have previously been attributed to the Gneudna Formation on lithostratigraphic grounds. They contain a rich and varied assemblage of marine microphytoplankton (acritarchs), associated with trilete miospores of which Geminospora lemurataBalme, 1962 is the dominant form. Forty-seven species of acritarchs are recognizable in the palynoflora, which corresponds very closely with that described recently (Playford & Dring, 1981) from the Gneudna Formation in the vicinity of its type section on the opposite (eastern) side of the Carnarvon Basin. The apparently parochial complexion of the Gneudna acritarch suite is probably illusory, insofar as early Late Devonian acritarchs have not been studied extensively or intensively from either the northern or southern hemispheres.The following new species of acritarchs areformally instituted herein: Elektoriskos villosa, Lophosphaeridium pelicanensis, and Pterospermella tenellula.     

湖北宜昌黄花场早奥陶世疑源类的新类型

该文报道了湖北宜昌黄花场下奥陶统大湾组下段的疑源类（海洋微体浮游植物化石）新类型,描述和建立３新属：Ｐａｐｉｌｌｉｆｅｒｕｍｇｅｎ．ｎｏｖ．,１ｇｅｎ．ｎｏｖ．和Ｙｉｃｈａｎｇｉａｇｅｎ．ｎｏｖ．;２新种：Ｐａｐｉｌｌｉｆｅｒｕｍｉｒｒｅｇｕｌａｒｅｇｅｎ。ｅｔｓｐ．ｎｏｖ．,Ｙｉｃｈａｎｇｉａｂｉｐｏｒｏｓａｇｅｎ;ｅｔｓｐ。ｎｏｖ。及重新修改和组合的３属、种：Ｐａｐｉｌｌｉｆｅｒｕｍｒｈａｂｄｏｃｌａｄｉｕｍ（Ｌｕ）ｃｏｍｂ．ｅｔｅｍｅｎｄ．ｎｏｖ．,Ｐ。ｓｔｒｉａｔｕｍ（Ｌｕ）ｃｏｍｂ．ｅｔｅｍｅｎｄ。ｎｏｖ。和Ｓｔｅｌｏｍｏｒｐｈａｅｒｃｈｕｎｅｎｓｉｓ（Ｆａｎｇ）ｃｏｍｂ;ｅｔｅｍｅｎｄ．ｎｏｖ．这些疑源类新类型目前仅见于我国西南的部分地区的早奥陶世地层。它们为探讨早奥陶世海洋微体浮游植物的区域分布及扬子地台的古地理位置,提供了重要的证据．     

湖北宜昌奥陶系庙坡组疑源类

湖北宜昌地区庙坡组是达瑞威尔阶/桑比阶界线附近的地层,该段地层产出分异度较高,丰度适中的疑源类组合。该组合包括16属,28种,其中7个未定命名种,可与国内、外同期疑源类组合进行对比。庙坡组疑源类组合既产出晚奥陶世特征分子,也产出阿伦尼格期(弗洛阶上部—达瑞威尔阶底部)的特征分子,显示出一定的过渡特色。组合以Baltisphaeridium(17%—52%),Leiosphaeridia(6%—78%)占优势;反映了其沉积环境为离岸较远的外陆棚环境。     

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.     

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.     

Paleoecology,biostratigraphy and biogeography of late Silurian to early Devonian acritarchs and prasinophycean phycomata in well A161, Western Libya,North Africa

Diverse and well preserved acritarch and prasinophycean phycomata assemblages were recovered from the late Silurian to Lower Devonian strata of well A161 in western Libya, and four distinct acritarch biozones are recognized, based on the stratigraphic distribution of 156 species. The palynoflora is independently dated by means of chitinozoans, and allows discussion of the evolution of acritarchs and prasinophyte phycomata across the Ludlow–Přı́dolı́ boundary in relation to probable major climatic change, as well as in the early and middle Přı́dolı́, and the lower Lochkovian. Correlations are proposed with the British Isles, Baltica, and Algerian Sahara. Sedimentation occurred in shallow high-energy conditions throughout, but with periodic rise of sea level. The changes in marine to terrestrial palynomorph ratios through the section document the relationship between marine palynomorph assemblages and sea surface conditions in these marginal marine environments. The major drift of Gondwana towards low latitudes during the Ludlow–Přı́dolı́ transition seems to have been the driving force behind homogenization of assemblages on the two sides of the Rheic ocean, and explains the similarities between phytoplanktonic assemblages of the north Gondwanan margin and the South of Baltica. The data suggest that the Rheic ocean was almost closed by the late Silurian, and had become restricted to a moderately deep sea. In the Lochkovian the microflora are strongly facies-dependent and delineate more restricted provinces such as the Ibarmaghian domain in the sense of Plusquellec (1987) including the Maghreb and Ibero–Armorican areas.Nine new species are described: Arkonia nova, Arkonia paulumstriata, ?Cymatiosphaera florida, Cymatiosphaera nimia, Dactylofusa hispidusa, Disparifusa quasibernesgae, Evittia areolata, Multiplicisphaeridium verticisum, and ?Villosacapsula steemansii. In addition, three new combinations are suggested: Visbysphaera bonita (Cramer) comb. nov., Visbysphaera jardinei (Cramer) comb. nov. and Visbysphaera albanega (Cramer et al.) comb. nov.     

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.     

Organic-walled microfossils from Cambrian Stage IV in the Jiaobang section,eastern Guizhou,China

The lower Cambrian succession in the Jiaobang section, Jianhe County, eastern Guizhou, China, includes, in ascending order, the Bianmachong, Balang, and Tsinghsutung formations, with a total thickness of about 645 m. Twenty-six morphological genera (including one new genus) are identified from the Balang and the underlying Bianmachong formations, many of which are common and widely distributed. Six acritarch assemblages are discerned in the Balang Formation. They are, in ascending order, the Adara alea‒Skiagia ornata, the Acrum radiale‒Pterospermella velata, the Comasphaeridium molliculum‒Solisphaeridium baltoscandium, the Corrugasphaera perfecta n. sp.‒Pterospermella vinctusa n. sp., the Acrum novum‒Heliosphaeridium oligum, and the Acrum membranosum‒Adarve diafanum acritarch assemblages. An obvious change of organic-walled microfossil assemblages occurred in the interval between 84 m and 98 m from the bottom of the Balang Formation which roughly corresponds to the boundary between the Oryctacarella duyunensis trilobite Zone and the overlying Arthricocephalus chauveaui trilobite Zone. In addition, organic-walled microfossils are scarce in about 24 m thick from the bottom of the Balang Formation. One new genus and five new species including Plagasphaera balangensis n. gen. n. sp., Asteridium tubulus n. sp., Cymatiosphaera spina n. sp., Corrugasphaera perfecta n. sp., and Pterospermella vinctusa n. sp. are described.     

Early Cambrian organophosphatic brachiopods from the Xinji Formation,at Shuiyu section,Shanxi Province,North China

Abundant and diverse small shelly fossils have been reported from Cambrian Series 2 in North China, but the co-occurring brachiopods are still poorly known. Herein, we describe seven genera, five species and two undetermined species of organophosphatic brachiopods including one new genus and new species from the lower Cambrian Xinji Formation at Shuiyu section, located on the southern margin of North China Platform. The brachiopod assemblage comprises one mickwitziid (stem group brachiopoda), Paramickwitzia boreussinaensis n. gen. n. sp., a paterinide, Askepasma toddense Laurie, 1986, an acrotretoid, Eohadrotreta cf. zhenbaensis Li and Holmer, 2004, a botsfordiid, Schizopholis yorkensis (Holmer and Ushatinskaya in Gravestock et al., 2001) and three linguloids, Spinobolus sp., Eodicellomus cf. elkaniiformis Holmer and Ushatinskaya in Gravestock et al., 2001 and Eoobolus sp. This brachiopod assemblage suggests a late Age 3 to early Age 4 for the Xinji Formation and reveals a remarkably strong connection with coeval faunas from East Gondwana, particularly the Hawker Group in South Australia. The high degree of similarity (even at species level) further supports a close palaeogeographic position between the North China Platform and Australian East Gondwana during the early Cambrian as indicated by small shelly fossil data.     

陕西梁山地区宝塔组介形类

本文报道了陕西省南郑县梁山地区中奥陶统宝塔组介形类8属10种,其中包括3新属8新种。这些介形类化石的发现,为该时代地层的划分、对比增添了新的古生物依据。     

湖北宜昌地区下-中奥陶统界线疑源类生物地层学意义

根据牙形类Baltoniodus? triangularis的首现,中奥陶统和大坪阶底界已界定于湖北宜昌黄花场剖面大湾组底界之上10.57m.对黄花场剖面以及邻近的大坪剖面大湾组疑源类研究表明,某些疑源类属种具有生物地层学意义,其首现可用于识别奥陶系下/中统界线.