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.     

Age of the Silurian Wuxiahe Formation in Langao,Northwest China: New conodont data

A recent study of conodonts from the Wuxiahe Formation (lower to middle part) in the Ziyang-Langao region suggested its age of middle Telychian (Llandovery) to lower Sheinwoodian (Wenlock), contradicted by subsequent graptolite studies indicating an age of late Telychian for the same interval. New samples from the Qiaoxi section for conodonts to re-access the age of the Wuxiahe Formation collected in this study show that the lower to middle part of the formation belongs to the Pterospathodus amorphognathoides amorphognathoides Biozone, suggesting the age of late Telychian; thus, the Llandovery–Wenlock boundary in the section is most probably higher than previously estimated, but its precise position is not determined since the identification of the Wenlock graptolite Cyrtograptus cf. lundgreni in the section is to be further confirmed. Based on the conodont faunas recognized in the Qiaoxi and Tianwancun sections, the base of the Wuxiahe Formation in the Ziyang-Langao region is diachronous, i.e., not higher than the upper Telychian Pterospathodus amorphognathoides amorphognathoides Biozone at Qiaoxi, but not lower than the lower Sheinwoodian Kockelella ranuliformis Biozone at the Tianwancun setion.     

Chitinozoan biostratigraphy of the basal Wenlock Series (Silurian) Global Stratotype Section and Point

Diverse and abundant assemblages of chitinozoans allow the recognition of the upper part of the Margachitina margaritana Biozone in the uppermost Llandovery and lowermost Wenlock series of the Hughley Brook section. The Cingulochitina bouniensis and Salopochitina bella biozones are identified in the lower part of the Buildwas Formation, Wenlock Series. The chitinozoan data indicate that the base of the Wenlock Series most probably correlates with a level in the upper centrifugus or lower murchisoni graptolite biozones. Chitinozoans also indicate that the base of the riccartonensis graptolite Biozone may occur within the Buildwas Formation and not the overlying Coalbrookdale Formation.     

Llandovery sporomorphs and graptolites from the Manbo Formation,the Mojiang County,Yunnan, China

Llandovery sporomorphs and graptolites have been recovered from the Manbo Formation, Mojiang area, western Yunnan, China, which belonged to the Indo-China Palaeoplate in the Palaeozoic. The graptolite fauna, including 8 genera and 12 species, is considered Mid Telychian, Llandovery in geological age, confirming the existence of Llandovery rocks in this region. The Silurian stratigraphical sequence of this area is reconsidered as in the ascending order: Manbo Formation (Llandovery–early Wenlock), Shuiqing Formation (late Wenlock–Ludlow) and unnamed formation (Pridoli?). The sporomorph assemblage from the Manbo Formation includes seven species in five genera. The dominant members of this assemblage are Tetrahedraletes medinensis and Laevolancis chibrikovae. Based on the low diversity and low abundance, the geological age of this assemblage is considered to be approximately the Telychian, Llandovery. The parent plants of sporomorphs probably inhabited the landmass near Mojiang area, these sporomorph-producing plants probably included bryophyte-like land plants and primitive vascular land plants. The sporomorph evidence shows that the South China and Indo-China palaeoplates may have been in close proximity (maybe with some continental bridges linking them) to each other at least in the Llandovery. They were also closely related with Gondwanaland in the Llandovery.     

志留纪笔石Oktavitesexcentricus(Bjerreskov,1975)的古生物学特征及地层意义*

陕西紫阳地区在古地理上位于扬子台地西北缘,区域内志留纪地层发育,其中志留系兰多维列统(Llandovery)特列奇阶(Telychian)笔石相地层出露较为完整,笔石带较为连续。Oktavites excentricus(Bjerreskov,1975)是特列奇阶Oktavites spiralis笔石带重要的伴生分子,在世界范围内广泛分布。文中通过对紫阳地区一系列特列奇阶剖面研究后认为,Oktavites excentricus较短的化石延限和广泛的古地理分布使其具备地层对比潜力,这将为Oktavites spiralis笔石带的进一步细分提供依据。同时,Oktavites excentricus胞管为典型的奥氏笔石式,其笔石体的盘旋方式又与稍晚的Cyrtograptus lapworthi(Tullberg,1883)(笔石带化石,弓笔石类的早期代表)十分接近,因此,对该种的演化过程的研究也为讨论弓笔石的起源过程提供了更多参考。     

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.     

Dissepiments in the graptolite Monograptus turriculatus

An example of the graptolite Monograptus turriculatus (Barrande, 1850) from the crispus Biozone (Silurian, Llandovery) of the Girvan district, Scotland, shows the first recorded instance of dissepiment formation in graptoloid graptolites. The dissepiments are formed by the fusion of thecal spines with the dorsal wall of the adjacent part of the succeeding whorl of this spiral graptolite. The formation of these structures appears easiest to envisage using models of the graptolite colony that do not involve extrathecal tissue. □ Graptolithina , Monograptus, morphology, functional morphology, Silurian, Llandovery.     

The nautiloid cephalopod Order Endocerida in the Silurian

The rare Silurian representatives of the Order Endocerida are reviewed, described afresh, and illustrated. The Welsh Llandovery speciesTretoceras bisiphonatum is assigned to the order. Canadian Llandovery to Wenlock forms,Cameroceras hudsonicum and species ofHumeoceras, complete the record as presently known.     

Palaeontologic and biogeochemical characterization of the Cyrtograptus lundgreni event in the black shales of eastern Mid-Sardinia, Italy

A succession of biotic and geochemical changes that occurred during the Cyrtograptus lundgreni Event (Late Wenlock) have been recorded from the 'pelagic' black-shales in the Goni section, eastern mid-Sardinia, Italy. The studied interval encompasses the Cyrtograptus rigidus to Pristiograptus dubius-Gothograptus nassa zones. The fossil association includes graptolites, chitinozoans and microplankton i.e. probable linings of agglutinated foraminifera and radiolaria capsular membranes. Analysis of the chitinozoan distribution revealed a succession of several chitinozoan associations with low species diversity and dominated by opportunistic species. Three chitinozoan faunal turnovers and three extinction events have been recorded. Two of them coincide with graptolite extinctions whereas one probably is of local significance. Disappearance of the chitinozoan and microplankton associations occurred during four consecutive graptolite zones. Geochemical data (trace elements analysis) showed significantly higher (up to c. 100%) values for Co and Cd in the sedimentary organic matter (SOM) than in the whole rock samples. Possible relationships between peaks of metal enrichment, the major faunal changes among chitinozoans, extinction events among chitinozoans and graptolites and, to a certain extent, oceanic events may be inferred. The first extinction datum is older that those occurring in Gotland, Sweden and Thüringen, Germany and is so far considered to be of local significance. The second extinction datum of Sardinia can be matched with Datum 1 of Gotland and Thüringen. A close correlation between the third extinction datum of Sardinia and Datum 2 of Thüringen and Gotland reinforces the importance of these events at global scale.     

Microplankton changes through the early Silurian Ireviken extinction event on Gotland, Sweden

This paper documents and analyses the extinction and origination patterns of acritarchs and prasinophyte algae at the Llandovery/Wenlock boundary transition in the Lower Silurian on the island of Gotland, Sweden. Closely spaced samples were collected from two parallel sections: Lusklint 1 and Lickershamn 2, spanning the upper part of the Lower Visby Beds and almost all of the Upper Visby Beds (i.e. the uppermost Llandovery and lowermost Wenlock).

At least eight extinctions affecting the conodont record have been reported at these levels. This turnover (named the Ireviken Event) has been interpreted as an example of the change from a P to an S climate state, reflecting large changes in the ocean/atmosphere system.

The palynomorph data show a significant turnover in the phytoplankton, with most of the extinctions at the end of the event (85.3% in the top 4 m of the Lusklint 1 section). The originations are more numerous than the extinctions and they are distributed through the whole of the Ireviken Event. There is an uneven distribution across the event with more originations in the Lower Visby Beds forming a convex pattern.

Comparison of these data to other palynological studies suggests that there was a slightly higher number of migrations out of Gotland than in. The P and S model does not successfully explain all the changes recorded, but is the model most inclusive of all the climatic variables available at this time.     

Devonian features in Ludlovian graptolites — no guide to ancestry

A diverse group of Ludlovian Monograptus species is characterized by the retarded growth of thecal hoods, either proximally [ M. colonus (Barrande), M. chimaera (Barrande) and M. haupti Kühne] or throughout the rhabdosome [M. micropoma (Jaekel)]. In all these species the hoods are secondary structures, i.e. later growth additions to the thecae. By contrast, in the Devonian Monograptus species the thecal hoods are primary structures, i.e. the outward extension of the dorsal thecal wall. and are formed by undelayed growth that typically is completed before growth of the succeeding theca begins. It is among Ludlovian ( M. uncinatus Tullberg) and Pridolian ( M. simih Přibyl) forms with homologous hoods that the roots of the majority of the Devonian Monograptus species might be found. The Ludlovian M. micropoma and M. haupti should not be affiliated to the group of Monograptus vomerinus (Nicholson), which is highly characteristic of, and restricted to, the late Llandovery and Wenlock graptolite faunas.     

Pattern of some Silurian bioevents

The best‐known Silurian bioevent occurred at the end of the Wenlock: the lundgreni event, together with the nassa‐ludensis crisis, was established among planktic graptolites. The East Baltic data show several peaks of high diversity (especially triangulatus, turriculatus, scanicus zones) and three levels of low diversity of graptolites (antennularius, radians, ludensis zones). These are more or less coincident with sea‐level changes. Energetic innovation of the shallow shelf corals started in the early Silurian. The late Wenlock regression seems not to affect them seriously but the late Silurian decline coincides with the aridization of the climate and a regression of the shelf seas. The Agnatha had strong radiations in the Wenlock (ludensis Zone) and Ludlow (leintwardinensis Zone), the fishes in the Pridoli. Many Silurian vertebrates were long‐ranging and extinction rate was relatively low; only at the late leintwardinensis level and in the latest Ludlow did considerable extinctions occur.     

A Wenlock ancestor of the micropoma phylogenetic lineage

A new graptolite species, Monoclimacis praemicropoma , from the Upper Wenlock of Poland is described. It is an ancestor of the Lower Ludlow M. micropoma (Jaeckel). The phytogeny of the praemicropoma-micropoma lineage is discussed. M. micropoma nannopoma (Jaeger) has been assigned to a separate evolutionary lineage. Graptolites, Wenlock, phytogeny .     

Evaluation of the Salinic I tectonic, Cancañiri glacial and Ireviken biotic events: Biochemostratigraphy of the Lower Siluriansuccession in the Niagara Gorge area, Canada and U.S.A.

Well-preserved brachiopods from the Niagara Gorge area, Anticosti Island, Britain, Gotland and Estonia were utilised to delineate a complex isotopic evolution for Llandovery-Wenlock seawater. The Sr-isotope record reflects the Salinic I tectophase of the Late Llandovery in the continuous increase in ⁸⁷Sr/⁸⁶Sr values from 0.708070 to 0.708346. The Salinic II tectophase is marked by relative constancy of Sr isotope values until the Late Wenlock when it rises from 0.708345 to 0.708430. The second tectonic phase was therefore likely only of a regional nature. The carbon isotopes during the Llandovery fall within a band of about − 1‰ to + 3‰, a range comparable to modern low-latitude brachiopods. A large positive δ¹³C excursion of about 3‰, identifies the Ireviken event/excursion, characterizes the Early Wenlock. The biotic crisis and the isotope excursion itself may be ultimately related to the onset and duration of the Cancañiri glaciation, although a direct causative scenario is as yet unknown. The oxygen isotopic trends of well-preserved brachiopods clearly reflect a warm climate interval during the latest Llandovery associated with the Silurian sea level highstand. Subsequently, in the Early Wenlock, the sea level fell with the onset of the Cancañiri glaciation in the southern hemisphere. This is reflected in a significant positive δ¹⁸O excursion, particularly in brachiopods from the Niagara Gorge area. Brachiopods from lower latitudes were awash in warm tropical currents and therefore exhibit somewhat more negative δ¹⁸O values, indicating a lesser cooling gradient.     

Revised conodont stratigraphy of the Cellon section (Silurian,Carnic Alps)

The Cellon section, located in the Carnic Alps, is a reference section for the Silurian of the world. The conodont association of the section is revised according to the most recent taxonomy and the biostratigraphy updated in the basis of the recently published zonation schemes. Seventy taxa (species and sub‐species) belonging to 23 genera have been identified, allowing the discrimination of 15 biozones from the upper Llandovery to the end of the P?ídolí . However, some of the uppermost Llandovery and Wenlock biozones, corresponding to black shale intervals, have not been documented.     

Symbiotic interactions in the Silurian of Baltica

Thirteen symbiotic associations occur in the Silurian of Baltica. Symbiosis was especially prominent among colonial animals, most commonly with stromatoporoids. These sponges hosted the most diverse fauna of endobiotic symbionts (including rugosans, Syringopora, ‘polychaetes’, cornulitids and lingulids). This pattern can be explained by the abundance of stromatoporoids in the Silurian of Baltica and their large skeletal volume, making them attractive hosts for smaller invertebrates. There is an evolutionary trend of an increasing number of different pairs of symbiotic taxa from the Llandovery to the Ludlow, with a remarkable increase in the Ludlow. This is likely related to an increase in the number of mutualistic taxa that could have had evolutionary advantages over organisms less amenable to symbiosis. The number of different pairs of symbiotic taxa also increased in the Wenlock, which may be linked to delayed recovery from the end‐Ordovician mass extinction.     

Chinese origin and radiation of the Palaeozoic crinoid family Petalocrinidae

The Ordovician to Devonian family Petalocrinidae includes 28 species belonging to 5 genera. This family is unique because the arm plates are fused into a large fan or cylinder. Paleobiogeographic occurrences of this family include Laurentia, Baltica, Avalonia, South China, Sibumasu, and Perunica blocks. The family has the oldest petalocrinids from China. An early radiation of this clade resulted in three genera during the Llandovery of China. Petalocrinus became cosmopolitan during the Llandovery and Wenlock, and the youngest genus is present in the Lower Devonian of the Czech Republic. Taxonomic determination for the Petalocrinidae is based on the fused arm plates instead of cup plates. The diverse morphology of these arm plates suggests a variety of aerosol suspension-feeding modes may have been used by petalocrinids.     

Le Silurien et le Dévonien basal dans le Nord de la province de Seville (Espagne)

In the studied area, on the southern slope(«Ossa-Morena Zone) of the Sierra Morena (Spain) the Silurian and earliest Devonian (Lochkovian = approximately Gedinnian) consist of about 130 m of black, argillaceous, largely euxinic graptolitic slates, which, by the intermezzo of the dark «Limestones with Orthoceras and Scyphocrinus (late Ludlow and Pridolian), are divided into two inequal units, namely the Lower and Upper Graptolitic Slates. The occurence of 21 Graptolite zones and horizons was established, starting off with the basal Llandovery (A. acuminatus zone) and ending up with the Upper Lochkovian (M. hercynicas zone). This condensed tripartite graptolitic sequence, both lithologically and faunally, compares closely with the typical development of the Palaeotethys, as known from Thuringia, Carnic Alps, Sardinia and Northwest Africa. Certain graptolite species and details of the graptolitic succession (for instance across the Wenlock-Ludlow junction) that were hitherto known only or essentially from Thuringia, are recognized also in southern Spain.     

A high-precision global biostratigraphy of myodocope ostracods for the Silurian upper Wenlock Series and Ludlow Series

The wide, trans-oceanic geographical distribution of myodocope ostracods during the Silurian (especially during the Ludlow and Pridoli epochs), and their widespread preservation in rocks of that age, permits the establishment of a transcontinental biostratigraphy of comparable resolution to coeval graptolite/chitinozoan/conodont biozones. Seven myodocope biozones, extending from the Homerian Stage, upper Wenlock Series Cyrtograptus lundgreni graptolite biozone to the middle part of the Ludfordian Stage of the Ludlow Series, enable a time-resolution for each biozone of circa 1 million years. These biozones can provide high-resolution correlation across Europe into Arctic Russia and Central Asia. There is also the potential for a myodocope biostratigraphy applicable from the uppermost Silurian (Pridoli) to the Carboniferous.