Late Wenlock (Middle Silurian) global Bioevent: Possible chemical cause for mass graptolite mortalities

Graptolites nearly became extinct in the latest Wenlock in all preserved stratigraphic sequences of this age. Graptolite mortalities occurred along the western coast of Laurentia and at sites that surrounded the Proto‐Tethys. Graptolite mass mortalities took place among deep‐water, open ocean dwelling organisms. After the mass mortalities, only the Pristiograptus dubius group and retiolids surface or near‐surface dwellers, survived. For a period of time, little speciation or diversification occurred. The base of the Ludlow is marked by diversification, with appearances of S. colonus, M. nilssoni and other groups which occur in near surface waters. None of the extensive plate movements postulated for the Silurian readily explain the mass extinctions that occurred. During the Silurian, global temperatures were warmer than present and atmospheric oxygen concentrations were lower, creating extensive oceanic anoxia. Below the oxygenated surface layers of the ocean, was an anoxic, non‐sulfidic zone (i.e. nitrate‐reducing) above a sulfidic zone. Graptolites lived over a range of depth from the oxygenated zone to either near or in the nitrate‐reducing zones. As the oxygen concentration declined through the Silurian, the depth of the oxic zone would have become shoaler with expanding anoxia. Late Wenlock graptolites that were unable to migrate to shallower depths, living in borderline oxygen conditions, could have been killed, resulting in the mortalities of the late Wenlock. Only those graptolites that were surface dwellers survived, adapted and reradiated.     

Astogenetic aspects of the dendroid graptolite Airograptus (Late Cambrian-Early Ordovician) and their phylogenetic implications

Based on a Backscatter Electron (BSE) examination of partially 3-dimensional graptolite specimens of Airograptus from the Tremadocian (Early Ordovician) Yehli Formation at Dayangcha, Baishan of Jilin Province, NE China, the thecal structure and astogenetic aspects of this genus (except for the missing proximal part) are described and illustrated in detail. In the present collection, no furcating but only peltate thecal apertural processes are confirmed. A comparison between the presumably benthic dendroid Airograptus and Rhabdinopora, the earliest planktonic graptolite, indicates that there are remarkable similarities in astogenetic aspects between them, although there are also differences in the development of dissepiments and thecal apertural processes. A possible ancestor-descendent relationship between the two genera is herein proposed.     

Ecostratigraphy, zonation and global correlation of earliest Ordovician planktic graptolites

The succession of earliest Ordovician planktic graptolites (graptoloids) in 11 key sequences around the world is reviewed and used to assess their distribution in depth facies, in paleolatitude and in time. An ecostratigraphic model shows the pattern of distribution of species across the shore-to-ocean profile in time. Fluctuations in this pattern are related to eustatic change; the Acerocare, Black Mountain, Ceratopyge and possibly, the Peltocare Regressive events are recognized. Surface-water temperature is found to have had little significant effect on distribution of Early Tremadoc species, and once the effects of facies is allowed for, their biostratigraphic utility can be assessed. A global suite of four chronozones is proposed for the Early Tremadoc and a further five zones comprise a provisional suite for the Late Tremadoc. The nine zones enable precise correlation of Tremadoc graptolitic sequences. The zones replace the previously used graptolite assemblages, are correlated with key conodont horizons and are, in upward sequence, the Zones of Rhabdinopora praeparabola, R. flabelliformis parabola, Anisograptus matanensis, R. f. anglica, Adelograptus, Paradelograptus antiquus, Kiaerograptus, Araneograptus murrayi-Araneograptus pulchellus , and Hunnegraptus copiosus.     

探讨笔石反射率对奥陶系、志留系烃源岩成熟度的指示作用

通过对塔里木盆地、鄂尔多斯、江苏等地区奥陶系,志留系笔石反射率的研究,以该地区有机地球化学等综合分析指标为中介,分析笔石反射率与等效镜质体反射率之比关系,建立笔石反射率对奥陶系,志留系烃源岩成熟度的评价标准。认为笔石最大反射率,随机反射率对奥陶率,志留系烃源岩成熟度有良好的指示作用。发现笔石有机质在热演化过程中,由于笔石纺锤层等生物结构的存在,使温度成为影响笔石热演化程度的主要因素。笔石的热演化有     

Stratigraphy of the Lungmachi black shales from the Huaying drill core in southwestern China and its palaeogeographic implications

Core samples from the Huaying well, Sichuan Province, have been analyzed for both graptolite biostratigraphy and carbon isotope chemostratigraphy. The result shows a stratigraphic sequence of Lituigraptus convolutus, Stimulograptus sedgwickii and Spirograptus guerichi biozones, and records a strong positive shift of δ¹³C_org at the base of the Stimulograptus sedgwickii Biozone, which coincides with those from Cornwallis Island (Arctic Canada), Arisaig (Nova Scotia, Canada), and the Barrandian area (Bohemia). Two outcrops were also studied in the Huaying region. The Yanwanggou section shows a continuous, but complicated, stratigraphical sequence, from the upper Katian to the lower–middle Rhuddanian, then a sedimentary hiatus of ∼3.26 Ma, and finally a continuous sequence from the upper Aeronian to the lower Telychian, whereas the Liziya section and the Huaying well exhibit significant hiatus, which lasted from Hirnantian to middle Rhuddanian or early Aeronian times. Therefore, the eastern boundary of the Central Sichuan Oldland during the Ordovician and Silurian transition can be precisely constrained to a narrow area between the Huaying well-Liziya section and the Yanwanggou section, which indicates that the Central Sichuan Oldland was much larger than previously considered. The present data also indicate the existing of a so far unidentified stage of the Kwangsian Orogeny, which resulted in the uplift of some portions of the Yangtze Platform region and the consequent delay of the transgression in those places. The multi-stage hiatus, spanning at least from the upper Katian to middle Aeronian in the study area, resulted from a combined effect of the Hirnantian glacio-eustatic sea-level fall and the regional tectonic activity of the Kwangsian Orogeny.     

Les assemblages de graptolites du llandovery dans le synclinorium du Ménes-Bélair (Massif Armoricain)

Ampelites in the lower member of the Formation 0839 de la Lande-Murée (Silurian of the Ménez-Bélair Syncline) have yielded rich and well-preserved graptolite faunas. This material, and that in older collections, indicates the presence of all or part of the Turriculatus, Crispus and Griestoniensis Zones. Earlier Llandovery graptolite zones are not represented in the Ménez-Bélair Syncline and breaks encompassing the Lower Llandovery and the earliest part of the Upper Llandovery is envisaged.     

Flotation devices in planktic graptolites

Four unique graptolite specimens representing Dicellograptus gurleyi, Pseudoclimacograptus angularus, Glossograptus ciliatus , and Climacograpnrs tubuliferus are described. They are rhabdosomes in which compressed circular objects interpreted as floats are attached to the nemata. Several additional objects isolated from carbonates and found on shale surfaces are described and interpreted as floats that functioned in juvenile rhabdosomes but became detached later in astogeny. They could be mistakenly identified as palynomorphs. These specimens serve as additional supporting evidence for previous work in which a float attached to a juvenile rhabdosome was described and a passive buoyant mode of life for planktic graptolites was suggested.     

Tremadocian (Ordovician) biostratigraphy and graptolites at Dayangcha (Baishan,Jilin, NE China)

Based on a systematic study of Tremadocian graptolites from the Early Ordovician Yehli Formation at Dayangcha, Baishan of Jilin, NE China, a revised graptolite zonation is proposed for the Tremadocian rocks in the area. In ascending order, the graptolite zones include theRhabdinopora flabelliformis parabola Zone, theAnisograptus matanensis Zone, thePsigraptus jacksoni Zone and theAorograptus victoriae Zone. The first three zones are discussed with their definitions and durations clarified, whereas the last zone is introduced for the first time as a replacement of the formerAdelograptus-Clonograptus Zone. The entire Yehli Formation is considered to be older than the late Tremadocian Hunneberg Substage of Scandinavia, the latter being represented by the carbonate-dominated basal part of the overlying Liangchiashan Formation where no graptolites have so far been discovered. The Tremadocian succession in the Dayangcha area is regarded as being representative of the North China (Sino-Korea) Palaeoplate, and a correlation of the Tremadocian graptolite sequence with those of other continents is suggested. Nine species belonging toDendrograptus, Aspidograptus, Callograptus andAirograptus of the family Dendroidae, and eight species ofPsigraptus, Adelograptus, Aorograptus andKiaerograptus of the family Anisograptidae are described and some are revised in the present paper. Among the important revisions are the reconstructed proximal development ofPsigraptus jacksoni based on partially three-dimensional specimens from the area, and the complex thecal structures ofAspidograptus.      

扬子区中奥陶统大湾阶底界精细生物地层分带与对比

系统研究湖北宜昌黄花场大湾组下段和重庆城口翁溪沟湄潭组下部笔石、牙形石和几丁虫生物分带及其对比关系。精细的生物地层学研究结果证明,在我国大湾阶底界界线层型剖面上的生物群组合存在冷水型分子与暖水型分子共生的特点,以牙形石Baltoniodus triangularis首现所确定的中奥陶统底界较nipodus laevis的首现低0．2m,位于笔石Didymograptus protobifidus／Corymbograptus deflexus带或几丁虫Conochitina langei／C．brevis带中部。中奥陶统底界上述牙形石、笔石和几丁虫的对比关系不受沉积相限制,在重庆城口翁溪沟含笔石碎屑岩相的湄潭组下部同样存在。宜昌黄花场剖面和该剖面上Baltoniodus triangularis的最初出现应是理想的全球中奥陶统底界线层型剖面和点。     

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.