Remains of trilobites and other species discovered in a volcanic ash bed of the end-Permian,Yangtze craton,South China

The remains of trilobites and other species were unexpectedly discovered in a volcanic ash layer beneath the Permian–Triassic Boundary (PTB). Based on a biostratigraphic investigation of the Zhongliangshan section in Chongqing, South China, the quantity of the species gradually decreased with subsequent volcanism. This finding provides an opportunity to further understand the disappearance of trilobites and the evolution of the mass extinction event. The temporal coincidence between the volcanic eruption event and the loss of trilobites and other species supports the idea of a cause-and-effect relationship. The species remains in the ash bed appeared before the disappearance of Clarkina yini and the climax of the negative carbon isotope excursion, which implies that the onset of the mass extinction occurred at the end-Permian. The explosive volcanic events caused massive releases of CO₂, toxic gases and volcanic ash and resulted in loss of habitat for certain species in the Tethys domain. This phenomenon may have led to the abrupt death of trilobites and the catastrophic collapse of biodiversity.     

The significance of Cambro-Ordovician trilobites from the Kalladeina 1 drill hole,Warburton Basin,South Australia

The significance of trilobites described from six cores from the Kalladeina 1 drillhole from the Warburton Basin, northeastern South Australia is discussed. The oldest are from a dark grey shale of Core 16 from a depth of 3453.7–3455.8 m; they are of possible early Drumian (Cambrian Series 3, middle Cambrian) age. The youngest come from a dark grey shale of Core 4 at a depth of 2017.2–2022.04 m. This fauna contains the trilobites Asaphellus? sp., Yosimuraspis sp., Conophrys sp. and Protopliomerops? sp. thus indicating a Tremadoc (earliest Ordovician) age. These are the youngest known trilobites from the Warburton Basin and the youngest known in South Australia.     

Ashgill trilobite faunas from the Llŷn Peninsula,North Wales,U.K.

The Crugan Mudstone Formation is formalized and the Dwyfor Mudstone Formation proposed for restricted Ashgill outcrops respectively near Llanbedrog and Llanystwmdwy in the Llŷn Peninsula. These mudstones contain brachiopods of the Foliomena fauna and a total of 26 determined genera of trilobites, one, Pseudosphaeroxochus represented by a new species, P. seabornei sp. nov. Elements of the trilobite fauna are used to demonstrate a Rawtheyan age for the mudstones. The generic assemblages both of brachiopods and of trilobites present in the mudstones indicate relatively deep water environments of deposition, that of the Dwyfor Mudstones probably being the deeper. The trilobites belong to what is here described as the Nankinolithus-Opsimasaphus association.     

Beecher's Trilobite Bed

Beecher's Trilobite Bed in the Upper Ordovician of New York State has yielded some of the most completely preserved trilobites. Abundant specimens of Triarthrus, and fewer of Cryptolithus, were buried in a single catastrophic influx of sediment, and the limbs and other features are preserved in pyrite. A recent re-excavation of the bed provided an opportunity to investigate the controls on the pyritization of soft tissues, an extremely rare phenomenon. Larval and juvenile specimens are common in the Trilobite Bed, allowing the life histories of the trilobites to be pieced together. The nature of the transition from larva to adult was significant in determining the trilobites' chances of surviving the mass extinction at the end of the Ordovician.     

Geological significance of middle Cambrian trilobites from near Melba Flats,western Tasmania

In western Tasmania, near Melba Flats, a basalt within the Serpentine Hill Complex is overlain unconformably by Dundas Group sediments (Hodge Slate). About 32 m above the unconformity, a siltstone contains agnostoid and polymerid trilobites that indicate correlation with the middle Cambrian Ptychagnostus punctuosus Zone (Drumian Stage of unnamed Epoch 3 of the Cambrian). This provides an upper limit of about 505.2–504.5 Ma to the emplacement of ultramafic allochthons in western Tasmania. Other fossils present include dendroids, inarticulate brachiopods, hyolithids, helcionellids and sponges. The widespread agnostoid trilobite Ptychagnostus affinis (Brøgger 1878) and the holocephalinid trilobites Meneviella and Holocephalina are recorded for the first time in Tasmania.     

Stratigraphy and facies development of the marine Late Devonian near the Boulongour Reservoir,northwest Xinjiang,China

Late Devonian to Early Carboniferous stratigraphic units within the ‘Zhulumute’ Formation, Hongguleleng Formation (stratotype), ‘Hebukehe’ Formation and the Heishantou Formation near the Boulongour Reservoir in northwestern Xinjiang are fossil-rich. The Hongguleleng and ‘Hebukehe’ formations are biostratigraphically well constrained by microfossils from the latest Frasnian linguiformis to mid-Famennian trachytera conodont biozones. The Hongguleleng Formation (96.8 m) is characterized by bioclastic argillaceous limestones and marls (the dominant facies) intercalated with green spiculitic calcareous shales. It yields abundant and highly diverse faunas of bryozoans, brachiopods and crinoids with subordinate solitary rugose corals, ostracods, trilobites, conodonts and other fish teeth. The succeeding ‘Hebukehe’ Formation (95.7 m) consists of siltstones, mudstones, arenites and intervals of bioclastic limestone (e.g. ‘Blastoid Hill’) and cherts with radiolarians. A diverse ichnofauna, phacopid trilobites, echinoderms (crinoids and blastoids) together with brachiopods, ostracods, bryozoans and rare cephalopods have been collected from this interval. Analysis of geochemical data, microfacies and especially the distribution of marine organisms, which are not described in detail here, but used for facies analysis, indicate a deepening of the depositional environment at the Boulongour Reservoir section. Results presented here concern mainly the sedimentological and stratigraphical context of the investigated section. Additionally, one Late Devonian palaeo-oceanic and biotic event, the Upper Kellwasser Event is recognized near the section base.     

昆明二村下奥陶统红石崖组顶部Cruziana rugosa遗迹化石群落的沉积环境和生态特征

Cruziana及其相关的遗迹化石是由底栖三叶虫在沉积物表面停歇、爬行所留下的遗迹,一般认为产于潮下带至浅海陆棚环境,具有古水深指示意义。然而,本文通过对昆明民富二村下奥陶统红石崖组顶部含Cruziana rugosa遗迹化石群落层位的沉积环境分析却发现,Cruziana rugosa遗迹化石群落的沉积环境为潮上带—潮间带,说明三叶虫可以在短暂暴露环境中活动。三叶虫的这种适应性活动可能为节肢动物从海洋走向陆地做好了进化准备。     

Unpairing metamorphic belts: P–T paths and a tectonic model for the Ryoke Belt, southwest Japan

Southwest Japan is divided into Outer and Inner Zones by the Median Tectonic Line (MTL), a major transcurrent fault. The Outer Zone is composed of the Sambagawa (high-pressure intermediate or high P/T type metamorphism), Chichibu and Shimanto Belts. In the Inner Zone, the Ryoke Belt (andalusite– sillimanite or low P/T type metamorphism) was developed mainly within a Jurassic accretionary complex. This spatial relationship between high P/T type and low P/T type metamorphic belts led Miyashiro to the idea that metamorphic belts were developed as ‘paired’ systems. Textural relationships and petrogenetically significant mineral assemblages in pelites from the Ryoke Belt imply peak P–T conditions of ≈5 kbar and up to 850 °C in migmatitic garnet–cordierite rocks from the highest-grade metamorphic zone. It is likely that the thermal anomaly responsible for metamorphism of the Ryoke Belt was related to a segment of the Farallon–Izanagi Ridge as it subducted under the eastern margin of the Asian continent during the Cretaceous. The sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dP/dT slope, inferred to have been generated by a nested set of hairpin-like ‘clockwise’P–T paths. These P–T paths are characterized by limited prograde thickening, minor decompression at peak-T , and near-isobaric cooling, features that may be typical of P–T paths in low P/T type metamorphic belts caused by ridge subduction. A ridge subduction model for the Ryoke Belt implies that juxtaposition of the high-P/T metamorphic rocks of the Sambagawa Belt against it was a result of terrane amalgamation. Belt-parallel ductile stretching, recorded as syn-metamorphic, predominantly constrictional strain in both Ryoke and Sambagawa Belt rocks, and substantial sinistral displacement on the MTL are consistent with left-lateral oblique convergence. Diachroneity in fast cooling of the Ryoke Belt is implied by extant thermochronological data, and is inferred to relate to progressive SW to NE docking of the Sambagawa Belt. Thus, an alternative interpretation of ‘paired’ metamorphic belts in Japan is that they represent laterally contemporaneous terranes, rather than outboard and inboard components of a trench/arc ‘paired’ system. Amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of ‘paired’ metamorphic belts in the geological record.     

Trilobite biostratigraphy in the Middle and Upper Ordovician of western Leningrad Region

Biostatigraphical significance of trilobites of the Middle and Upper Ordovician in the eastern Baltic area is discussed, based on a new material from numerous outcrops from Leningrad Region. The study of the Middle and Upper Ordovician trilobites in the Mednikovo, Viivikonna, Gryazno, Khrevitsa and Elizavetino formations allows establishing two trilobite interval-zones. The zone of Chasmops odini in the upper part of the Uhaku and Kukruse stages, and the zone of Chasmops marginatus in the lower part of the Haljala Stage (the Idavere Substage). The upper part of the Haljala Stage (J~ohvi Substage) is characterized by the occurrence of Rollmops wenjukowi. Distribution and biostratigraphical significance of some trilobite taxa (Asaphus (Neoasaphus) lepidus, Illaenus intermedius, Toxochasmops maximus) is discussed.     

New Advances in the Study of the Devonian-Carboniferous Boundary Stratotype in Muhua,Changshun,Guizhou——An Introduction to the Daposhang Devonian-Carboniferous Boundary Section

The Daposhang section at Muhua, Changshun. Guizhou. is an excellent and attractive DevonianCarboniferous boundary section. The transitional beds between the Devonian and Carboniferous of the sectionare continuous and well exposed. belonging to the deep-water basin facies Abundant fossil groups have beendiscovered from this section: conodonts. ammonoids. trilobites. ostracods. vertebrate microfossils and so on.So far as known. it has the most continuous and complete conodont zonation for the Devonian-Carboniferousboundary beds in the world. It is especially worth pointing out that both typical Siponodella pracsulcata andthe transitional forms between S. praesulcata and S. sulcata have been found from the upper pracsulcata Zoneof the Daposhang section. Evidently. we can not only prove the actual existence of the evolutionary lineagefrom S. praesulcata to S. sulcata, but also exactly define the level of the Devonian-Carboniferous boundary. Inthis paper. the development of the Devonian-Carboniferous boundary beds at the Daposhang section is dealtwith and the section is compared with the Muhua section and the Nanbiancun section which are the candidatesfor the Devonian-Carboniferous boundary stratotype. In the authous opinion the Daposhang section is obvi-ously better than the Muhua and the Nanbiancun sections. hence it can be recommended as one of the candi-dates for the international Devonian-Carboniferous boundary stratotype.     

Geophysical properties and seismotectonics of the Tohoku forearc region

We investigated the detailed three-dimensional (3-D) isotropic and anisotropic structures of the crust and upper mantle under the NE Japan forearc region using a large number of P and S wave arrival-time data from onshore and offshore earthquakes. The suboceanic earthquakes used in this study are well relocated using the sP depth phases. We also determined the 3-D distribution of Poisson’s ratio, crack density and saturation rate using the 3-D P and S wave velocity model obtained by this study. The relatively complex anisotropic structures in the megathrust zone may reflect the complex geological structures, lithological variations and fluids in the accretional prism under the forearc region. The tomographic images reflect strong lateral heterogeneities in the megathrust zone under the Tohoku forearc. Areas with low velocity, high Poisson’s ratio, high crack density and high saturation rate may be due to entrapment of fluid-filled, unsolidated sediments on the plate interface close to the Japan Trench. Most of the large megathrust earthquakes since 1900 (M ⩾ 6.0) and the large 2011 Tohoku-oki earthquakes (M 6.0–9.0) are located in areas with high velocity, high Poisson’s ratio, low crack density and high saturation rate, which may represent strongly-coupled asperities in the megathrust zone resulting from the subducted oceanic ridges and/or seamounts. In contrast, the areas with high Poisson’s ratio may indicate that the fluids have infiltrated into the strongly coupled patches. We think that the great Tohoku-oki earthquakes were caused by not only the stress concentration but also the in situ structural heterogeneities in the megathrust zone.     

Quantitative taxonomy and biostratigraphy of Middle Cambrian trilobites from Montana and Wyoming

This study was made to revise the taxonomy and biostratigraphy of Middle Cambrian trilobites from Montana and Wyoming. Both numerical and conventional analyses were made of the taxonomic and biostratigraphic data. Numerical taxonomy was performed on a sample of 210 OTU's (Operational Taxonomic Units) using 66 characters which were determined on cranidia of Middle Cambrian ptychopariid trilobites. These characters consisted of linear measurements which were transformed to indices proportional to glabellar length, quantitative-qualitative shape or angle measurements, and multistate attributes. The similarity coefficients calculated were average taxonomic distances and Pearson's product-moment correlation coefficient. The matrices of similarity coefficients were clustered by single-linkage and unweighted pair-group algorithms. The coordinates of OTU's in a three-dimensional A-space defined by the first three principal axes also were calculated. A phenogram of the distance matrix clustered by UPGMA (which yielded the highest cophenetic correlation coefficient) and a three-dimensional pin-and-ball model were used to interpret the trilobite taxonomy. Insufficiently clear clustering in both representations of taxonomic structure necessitated also a conventional taxonomic study guided by the numerical phenetics. Numerical biostratigraphy was performed with 79 collections as OTU's and 43 trilobite genera as characters. Jaccard coefficients were used as similarity coefficients, and the results were clustered by UPGMA. The resultant phenogram was readily interpretable, and seven faunal assemblage zones (Albertella, Glossopleura, Ehmaniella, Bolaspis-Glyphaspis, Ehmania, Parehmania, and Bolaspidella)are defined for the Middle Cambrian in the study area. Two of the zones (Ehmaniellaand Parehmania)are new and replace part of the previously defined “Bathyuriscus-Elrathina”zone. Bolaspis-Glyphaspis, Ehmania,and Parehmaniazones may have only regional validity.     

Bryozoan-rich stromatolites (bryostromatolites) from the Silurian of Gotland and their relation to climate-related perturbations of the global carbon cycle

Bryozoan–stromatolite associations (bryostromatolites) formed conspicuous reef structures throughout the Sheinwoodian (Wenlock) to Ludfordian (Ludlow) stratigraphy on Gotland but have not been described so far. They are mainly composed of encrusting bryozoans forming a complex intergrowth with porostromate and spongiostromate microbes and are different from the abundant stromatoporoid–coral–algal reefs with respect to their composition. In the bryostromatolite different growth stages can be identified. The observed succession can be taken as evidence for cyclic environmental changes during reef formation. Stenohaline reef-dwelling organisms, such as echinoderms, sponges, corals and trilobites, indicate fully marine salinities. Ten localities exposing bryostromatolites were discovered. Individual bryostromatolites are small with few decimetres up to one metre in size, and occur solely in shallow marine areas. Common features of these reefs on Gotland are cauliflower-like growth, a high bryozoan diversity, a high abundance of phosphatic fossils and components such as bryozoan pearls and inarticulate phosphatic brachiopods, enhanced bioerosion, Palaeomicrocodium crusts, vadose silt and gypsum pseudomorphs. The high abundance of Palaeomicrocodium, as well as the alternation with other crust-forming contributors, suggest that it could have been formed directly at the palaeo-sea surface, probably in times of minor but high-frequency sea-level fluctuations. Vadose silt and pseudomorphs after gypsum in reef cavities indicate subaerial exposure shortly after reef growth. The high amount of phosphatic components indicates a high nutrient input, probably by dust. All bryostromatolites were formed in times of strongly elevated δ¹³C values. The unusual combination of sedimentological and palaeoecological features, as well as their occurrence exclusively during strong positive δ¹³C excursions, are evidence that the bryostromatolite development responded to climatic/oceanographic changes, which may have played an important role in reef control.     

Incipient subduction and deduction along the eastern margin of the Japan Sea

The eastern margin of the Japan Sea has been under convergent tectonics since sometime after the Pliocene. The suture line is on the ocean-continent boundary between the Japan-Yamato Basins and the Tohoku (Northeast Japan) Arc. Incipient subduction, and obduction, of the oceanic and sub-oceanic crust are observed along the suture line with the occurrence of numerous N-S trending, fault-bounded ridges and troughs. Focal mechanism solutions of several compressional type earthquakes with magnitudes of M = 6.9 to 7.7 which have occurred along the zone are consistent with the observation of geological thrust faults. The convergent stresses in the Japan Sea may be due to India-Eurasia collision and its associated intra-plate or inter-microplate movements in East Asia. The eastward movement of the Amurian Plate causes Baikal extension along its western margin and the Japan Sea convergence along its eastern margin.     

Trilobite tracks and other trace fossils from the Upper Cambrian of North Wales

Trace fossils are recorded from eight localities within Upper Cambrian (Ffestiniog Stage) strata in North Wales. Trilobite furrows (Cruziana), resting impressions (Rusophycus), walking and striding tracks (Diplichnites), sideways grazing tracks (Dimorphichnus) and swimming grazing marks (Monomorphichnus, gen. nov.) are described, largely on the basis of over 500 collected specimens, and are figured together with examples of other trace fossil genera including Rouaultia, Phycodes and Monocraterion. The trilobite traces are used to investigate the varied methods of locomotion of the animais. They also indicate the morphology of some of the trilobite hard and soft parts from which it is inferred that most of these traces were probably made by a population consisting of a small number of closely allied species of trilobites belonging to the Olenid family.     

A new deep‐water Upper Ordovician (Caradocian) trilobite fauna from south‐west Wales

A deep‐water trilobite fauna has been discovered in the otherwise graptolitic Mydrim Formation near Clarbeston Road, Pembrokeshire, south‐west Wales. Associated graptolites indicate an early Caradocian age. The trilobites comprise three species with eyes reduced or absent, representing an atheloptic assemblage with benthic life habits, which appeared during a short period of relative oxygenation of the Welsh Basin. The trilobite Platycalymene dilatata (Tullberg) is redescribed from the type material from Sweden, and new material from Wales is assigned to this species. Two new species are described in addition. Rorringtonia multisegmentata sp. nov. possesses twelve thoracic segments, which has implications for the classification of the Rorringtoniidae within the Aulacopleuroidea. Trinucleus conollyi sp. nov. differs from T. fimbriatus in the proliferation of the radial sulci. Copyright © 2006 John Wiley & Sons, Ltd.     

A middle Cambrian shallow water trilobite fauna from the Comstock Formation,near Queenstown,western Tasmania

A limestone lens within the Comstock Formation, from low in the Tyndall Group near the Lyell Comstock Mine near Queenstown, western Tasmania, contains a small trilobite fauna. The trilobites include Ammagnostus cf. laiwuensis, Kootenia sp., Sudanomocarina? sp. and a possible member of the Monkaspidae. These suggest an age somewhere in the range from the upper Ptychagnostus atavus Zone to the Lejopyge laevigata Zone, with regional stratigraphic correlations suggesting the latter zone. This is only the second Tasmanian Cambrian fossil assemblage described from limestone; almost all previously described Tasmanian Cambrian faunas are found in siltstones and shales.     

1600–1500 Ma hotspot track in eastern Australia: implications for Mesoproterozoic continental reconstructions

Mesoproterozoic A‐type magmatic rocks in the Gawler Craton, Curnamona Province and eastern Mount Isa Inlier, form a palaeo‐curvilinear belt for reconstructed plate orientations. The oldest igneous rocks in the Gawler Craton are the Hiltaba Granite Suite: c. 1600–1575 Ma. The youngest in the Mount Isa Inlier are the Williams‐Naraku Batholiths: c. 1545–1500 Ma. The belt is interpreted as a segment of a hotspot track that evolved between c. 1600 and 1500 Ma. This hotspot track may define a quasilinear part of Australia’s motion between 1636 and 1500 Ma, and suggests that Australia drifted to high latitudes. An implication of this interpretation is that Australia and Laurentia may not have been fellow travellers leading to the formation of Rodinia. A hotspot model for A‐type magmatism in Australia differs from geodynamic models for this style of magmatism on other continents. This suggests that multiple geologic processes may be responsible for the genesis of Proterozoic A‐type magmas.     

西准噶尔克拉赛勒克山和布克河组晚泥盆世化石的发现及其地层归属

西准噶尔地区广泛分布的和布克河组已被证明是洪古勒楞组的晚出异名,本文在西准噶尔克拉赛勒克山原划为下石炭统"和布克河组"的碎屑岩地层中发现了丰富的动、植物化石,包括:三叶虫Phacops sp.,菊石Manticoceras sp.,腕足类Tylothyris sp.,Aulacella sp.,"Mucrospirifer"sp.,Cyrtospirifer sp.,植物Leptophloeum rhombicum,Syringodendron sp.,Knorria sp.,其时代显示为晚泥盆世。根据岩性组合特征将其厘定为上泥盆统铁列克提组,而非"和布克河组"的替代名称洪古勒楞组。通过区域地质调查工作,证明了西准噶尔玛依力山地层小区内没有洪古勒楞组的分布,其晚泥盆世地层仅由铁列克提组组成,洪古勒楞组只分布在沙尔布尔提山地层小区,两个地层小区的界线为孟布拉克北侧至谢米斯台山南坡的深大断裂。晚泥盆世两个地层小区沉积分异明显,直到早石炭世早期才进入统一的盆地演化阶段,地层小区的分区性也随之消失。     

Trace fossils from the Upper Ordovician (Caradoc) of the Berwyn Hills,Central Wales

A suite of Caradocian trace fossils from the Berwyn Hills, Central Wales, are examined. They include forms produced by trilobites (Cruziana, Rusophycus, Trichophycus, Monomorphichnus), pelecypods (Lockeia), ?gastropods (?Imponoglyphus) and traces of debatable affinity probably produced by oligochaete and polychaete annelids, amphipods and crustaceans (Skolithos, Planotites, Teichichnus, Diplocraterion, Arenicolites, ?Gyrochorte, ?Palaeophycus, Vermiforichnus). Representatives of the Skolithos and Cruziana communities coexist in this assemblage and no subdivision into discrete ichnofacies is therefore possible. The trace fossils are examined in terms of their relationship to the structure and composition of Upper Ordovician communities and it is concluded that substrate and ecological controls were more important limiting environmental parameters than depth.