Biodiversity patterns of Early–Middle Ordovician marine microphytoplankton in South China

Based on new materials from six sections and all available literature data, new diversity curves are presented for the phytoplankton (acritarchs) from South China, covering the Early–Middle Ordovician interval, when the Great Ordovician Biodiversification Event took place. The total diversity curve and the origination data imply that a major radiation of the phytoplankton occurred during the analysed interval. A peak of the total acritarch diversity curve appears in the A. suecicus graptolite biozone. The diversity changes vary in the different parts of the investigated area, most probably depending on the position of the analysed sections on the carbonate shelf or the slope, reflecting diversity differences due to the position on an inshore–offshore transect.The Early–Middle Ordovician diversity pattern of the phytoplankton is compared with those of several marine invertebrate groups. Compared with the diversity curve peak of the acritarchs, the conodonts and brachiopods reached their highest diversities before the acritarchs, while the highest diversity of the chitinozoans appears slightly later. The graptolites show two peaks during the Early–Middle Ordovician, while the trilobites diversity curve shows a peak only in the Sandbian. The different fossil groups, such as chitinozoans, conodonts, graptolites, brachiopods and trilobites show therefore different evolutionary patterns to that of the acritarchs, that are not yet fully understood, and correlations are so far difficult.The acritarch diversity changes can partly be compared to the local sea-level changes from four sections in South China. At a larger scale, the acritarch radiation coincides with a general transgression. At a regional or local scale, correlations are not straightforward, pointing out that more detailed data, based on both acritarch studies and more precise sea-level investigations, are necessary.     

Is Moyeria a euglenoid?

Moyeria , with a known age span of Middle Ordovician through Silurian, and normally placed in the acritarchs, may have euglenoid affinities. If this conclusion is correct it represents the oldest freshwater protozoan, because Moyeria is abundant in fluvial beds in Pennsylvania and New York, as well as in nearshore marine beds at many localities elsewhere. Paramecium is the only other typically freshwater protozoan with a pre-Cenozoic fossil record (a single, unillustrated Cretaceous report): euglenoids have rare Cenozoic records. As a freshwater taxon, or one capable of living in freshwaters, Moyeria , regardless of taxonomic assignment, provides further evidence about the early complexity of the freshwater ecosystem. □ Euglenoids, freshwater, Ordovician, Silurian, protozoans, acritarchs.     

Ordovician chitinozoans and acritarchs from southern and southeastern Turkey

Revision of the lithostratigraphy of Ordovician deposits in southern and southeastern Turkey led to a re-evaluation of the age assignments of formations identified in the subsurface and at outcrop. Previous datings were based on macrofauna (mainly trilobites and graptolites). The present paper focuses exclusively on organic-walled microfossils (chitinozoans and acritarchs), which provide numerous chronostratigraphical improvements, especially in successions barren or poor in macrofossils. Close to 200 samples were collected in the Taurus chain (i.e. from Kemer, Seydisehir, Ovacik, Kozan, to Sariz regions in southern Turkey) and in the Border Folds (Mardin and Hakkari regions), usually regarded as part of the Arabian Plate in palaeogeographical reconstructions. Many samples are productive and yield chitinozoans and/or acritarchs of extremely variable preservation, depending on their geographical and geological location. In the Taurus chain, the material is “coalified” and frequently fragmented whereas, in the Border Folds, maturation of the organic matter is much lower and preservation of the microfossils is good to excellent. Several Ordovician chitinozoan biozones (northern Gondwana zonation) as well as diagnostic acritarch assemblages are identified in southern and southeastern Turkey. These Ordovician formations are assigned here to the new global stages of the Ordovician chronostratigraphical scale. The Seydisehir (upper part), Sobova, and Kilgen Lake (lower part) formations are referred to the Darriwilian. The Kilgen Lake (upper part), Sort Tepe, and Bedinan formations are attributed to the Sandbian and to the Katian, and the Halevikdere Formation (glacio-marine part) is assigned to the Hirnantian. Reworking of Early Ordovician acritarchs is documented in pre-glacial and in glacial Late Ordovician deposits. They indicate that active erosive processes occurred during the Middle and Late Ordovician sedimentation. The organic-walled microfossils recorded in the Ordovician of south and southeastern Turkey belong to the northern Gondwana realm. Interestingly however, some Baltoscandian influences are noted in the Border Folds during Early Late Ordovician.     

Progress in Irish lower palaeozoic palynology

Knowledge of Lower Palaeozoic palynomorphs in Ireland has improved greatly during the last decade, and acritarchs of most epochs from Vendian to Wenlock, together with Ordovician and Silurian chitinozoa and Silurian miospores, are now known. The present state of knowledge is summarised in terms of a number of distinctive palynological assemblages.     

The effects of the final stages of the Late Ordovician glaciation on marine palynomorphs (chitinozoans, acritarchs, leiospheres) in well Nl-2 (NE Algerian Sahara)

Palynomorph assemblages, especially chitinozoans and acritarchs, from the Upper Ordovician of well Nl-2 (north-east of the Algerian Sahara) are studied in order to precisely date the ultimate effects of the Late Ordovician glaciation and to document the impact of this major climatic stress on the diversity of the palynoplankton. References are made to stable isotope excursions and to global eustatic sea level variations in order to improve the local age determination. The Hassi el Hadjar Formation, i.e. glacio-marine diamictites, is interpreted as a transgressive event resulting from the melting of the northern Gondwana ice cap. It yields poorly preserved and moderately diverse chitinozoans of late Hirnantian age. Acritarchs are more abundant in the lower part of these "microconglomeratic clays", but display a low diversity and are badly preserved throughout the whole formation. Reworked individuals are recorded in both groups. The marine sediments of the M'Kratta Formation of latest Hirnantian age contain better preserved, more abundant and more diverse palynomorph assemblages, especially in the Upper Member. The composition of this palynoplankton indicates a fairly good faunal and phytoplankton recovery after the early Hirnantian climatic stress.The extinction of the Ordovician forms, and the appearance of Silurian type taxa occur only in the uppermost Hirnantian, i.e. following with a slight delay the glacial event. The overlying black shales of Wenlock age (lower part of the Oued Mehaiguène Formation) are indicative of marine anoxic environments. They yield a virtually exclusive, but enormous number of Tasmanacea. The very peculiar composition of this palynoplankton seems to be independent of the Late Ordovician glaciation and is most likely related to the factors that, later, generated and maintained anoxic conditions in this area.A new species of chitinozoan, Spinachitina oulebsiri sp. nov. from the latest Hirnantian M'Kratta Formation, is described and illustrated.     

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.     

Population dynamics of galeate acritarchs at the Cambrian–Ordovician transition in the Algerian Sahara

Galeate acritarchs are a major component of Late Cambrian and Early Ordovician palynological assemblages. The populations of galeate acritarchs from the Cambrian–Ordovician transition section in borehole Nl-2 of the Algerian Sahara show a gradual increase in process length and in complexity of the process structures through the succession. While the number of striate elements at the process base, the number of distal ramifications, and the number of membranes between the processes increases progressively, the central body diameter shows only minor variation within the succession. The galeate acritarchs are here interpreted as probably being resting cysts of a microorganism similar to modern dinoflagellates. Published laboratory culture experiments on living dinoflagellates document a considerable morphological variation with respect to the process morphology of the resting cysts that can be produced by a single biologically defined species. Based on these experiments, it is possible to interpret the morphological changes in the galeate acritarchs as being possibly related to changing ecological conditions. By analogy to the cyst distribution of modern dinoflagellates, changing salinity may have played an important role. Depending on environmental parameters, and perhaps on the maturity attained before rupture of the outer membrane during cyst formation, the galeate acritarchs may show a wide variety of process morphologies that have to date been used to describe four genera (morphogenera) and 84 species (morphospecies).     

Cryptospore Assemblages from the Ordovician/silurian Boundary in the Puna Region, North-west Argentina

Palynomorph assemblages have been recovered from deposits believed to straddle the Ordovician/Silurian boundary, from the upper member of the Salar del Rincón Formation, in the Puna region of north-west Argentina. The palynomorph assemblages are dominated by terrestrial cryptospores, but also contain marine elements (acritarchs, prasinophycean algae and chitinozoans). The cryptospore assemblages are similar in composition to those described from coeval deposits worldwide, suggesting that the producers were cosmopolitan and tolerated a wide range of climatic conditions. They are correlated with the Imperfectotriletes spp. Interval Biozone (sub-biozone α) of the Imperfectotriletes spp.- Laevolancis divellomedia Assemblage Biozone, indicating a Hirnantian (latest Ordovician) age. Acritarchs include late Ordovician species such as Eupoikilosusa striata and Villosacapsula cf. setosapellicula , that coexist with the Llandovery species Dactylofusa estillis. Based on lithological and palynological evidence, an early Llandovery, or a late Hirnantian (post-glacial) age is proposed for the basal part of the upper member of the Salar del Rincón Formation.     

The Ordovician Biodiversification: revolution in the oceanic trophic chain

The Early Palaeozoic phytoplankton (acritarch) radiation paralleled a long-term increase in sea level between the Early Cambrian and the Late Ordovician. In the Late Cambrian, after the SPICE δ¹³C_carb excursion, acritarchs underwent a major change in morphological disparity and their taxonomical diversity increased to reach highest values during the Middle Ordovician (Darriwilian). This highest phytoplankton diversity of the Palaeozoic was possibly the result of palaeogeography (greatest continental dispersal) and major orogenic and volcanic activity, which provided maximum ecospace and large amounts of nutrients. With its warm climate and high atmospheric CO₂ levels, the Ordovician was similar to the Cretaceous: a period when phytoplankton diversity was at its maximum during the Mesozoic. With increased phytoplankton availability in the Late Cambrian and Ordovician a radiation of zooplanktonic organisms took place at the same time as a major diversification of suspension feeders. In addition, planktotrophy originated in invertebrate larvae during the Late Cambrian–Early Ordovician. These important changes in the trophic chain can be considered as a major palaeoecological revolution (part of the rise of the Palaeozoic Evolutionary Fauna of Sepkoski). There is now sufficient evidence that this trophic chain revolution was related to the diversification of the phytoplankton, of which the organic-walled fraction is partly preserved.     

Palynostratigraphic,palynofacies, organic geochemical and palaeoenvironmental analysis of the Silurian Tanezzuft Formation in the Ghadames Basin of north-west Libya

Two palynofacies associations are documented from the Silurian Tanezzuft Formation in the Ghadames Basin. These are characteristic of the basal ‘Hot Shale’ and the overlying deposits, referred to here as the Cold Shale. The former reflects deposition in distal suboxic anoxic conditions and is dominated by highly oil-prone amorphous organic matter (AOM) typical of deposition in generally anoxic, restricted marine basins. Only a few acritarchs, prasinophyte algae and chitinozoans occur in association: virtually no spores or cryptospores were recorded. Thick-walled prasinophytes are most numerous in this part of the Tanezzuft Formation in both the Ghadames and Murzuq basins, suggesting enhanced surface water productivity. Deposition took place after the melting of the Late Ordovician ice sheets, which led to a major marine transgression. The palynofacies recorded from the overlying ‘Cold Shale’ deposits indicate deposition in distal shelf and basin conditions that were also relatively anoxic. They contain more palynomorphs, especially acritarchs, and generally less AOM. The phytoplankton assemblages are dominated by simple and thin-walled prasinophyte algae (leiospheres), suggesting dysoxic–anoxic conditions. Overall the middle and the upper parts of the Tanezzuft Formation are regarded as being deposited in distal dysoxic–anoxic shelf, distal dysoxic–oxic shelf and distal suboxic–anoxic basin respectively. Because chitinozoans are very rare, age determinations of the samples investigated are based mainly on acritarchs. The Hot Shale is dated as early-mid Rhuddanian (early Llandovery) whereas the rest of the formation is considered to late Rhuddanian–Telychian in age. As documented previously from other samples of the Tanezzuft Formation in both the Ghadames and Murzuq basins, the Hot Shale has a very high TOC content and excellent source potential for liquid hydrocarbons, whereas rest of the formation is less rich in organic matter with larger terrestrial and oxidized components and hence reduced potential for sourcing hydrocarbons (both oil and gas).     

Influence of the Late Ordovician glaciation on basin configuration of the Yangtze Platform in China

The Late Ordovician paleogeographic changes in the Yangtze Platform indicate a widespread regression and consequent subaerial exposure probably caused by the 'locking up' of substantial amounts of marine waters during the Late Ordovician glaciation. The subsequent rapid paleogeographic changes on the Yangtze Platform during the earliest Silurian suggest a rapid melting of the ice. The relatively cool waters in the partially closed Yangtze basin during the latest Ordovician were replaced by open and warm waters during the earliest Silurian. Sedimentation rates and composition of faunal assemblages underwent marked changes during latest Ordovician and Early Silurian. Glaciation, Ordovician, Silurian, Paleogeography, sedimentary rate, faunal change.     

Early Paleozoic (Late Cambrian-Early Ordovician) acritarchs from the metasedimentary Baden-Baden-Gaggenau zone (Schwarzwald, SW Germany)

The metasediments of the low-grade metamorphosed Baden-Baden-Gaggenau zone of the northern Schwarzwald (southwestern Germany) have been analyzed palynologically. From 133 samples representing different metasedimentary units, only three samples of the upper part of the Traischbach Serie provide extremely poorly-preserved palynomorphs. The assemblage consists of the galeate acritarch genera Caldariola, Cymatiogaleaand Stelliferidium, as well as specimens of diacromorph and polygonomorph acritarchs. Although determinations are difficult at the generic level and essentially impossible at the specific level, the assemblage can be attributed to an interval between the Late Cambrian and Early Ordovician. This is the first biostratigraphical age assignment for the metasediments of the Baden-Baden-Gaggenau zone, which can possibly be correlated with the Villé Unit of the northern Vosges Mountains (eastern France).     

Upper Ordovician and Lower Silurian chitinozoans from central Nevada and Arctic Canada

Late Ordovician and Early Silurian chitinozoans from the uppermost Vinini Formation, and the Hanson Creek Formation in central Nevada and the lower Cape Phillips Formation, Cape Manning section, in Arctic Canada have been re-investigated and a new chitinozoan biozonation is proposed. The Upper Ordovician of central Nevada can easily be correlated to that of Arctic Canada through the common occurrence of the Ordochitina nevadensis biozone in both regions. No such correlation, however, is possible with the Late Ordovician of Anticosti Island in eastern Canada because of the absence of the index Upper Ordovician chitinozoan zonal species of central Nevada such as Belonechitina tenuispinata sp. nov, Ordochitina nevadensis sp. nov. and Nevadachitina vininica gen. nov., sp. nov. in the former area.One new genus, Nevadachitina, and nine new species, Eisenackitina ripae, Belonechitina martinica, Nevadachitina vininica, Nevadachitina praevininica, Ordochitina nevadensis, Belonechitina tenuispinata, Belonechitina parvispinata, Tanuchitina laurentiana, Angochitina hansonica are described and illustrated in this paper and four species are left in open nomenclature.     

A nearshore–offshore trend in acritarch distribution from the Early–Middle Ordovician of the Yangtze Platform, South China

The stratigraphical interval of the late Early Ordovician Didymograptus deflexus and the early Middle Ordovician Azygograptus suecicus graptolite Biozones was investigated from seven sections from the upper Yangtze Platform, southern China. These are located on different parts of the platform, between the nearshore environments of the Kunming area, Yunnan Province, and the offshore carbonate shelf of the Yichang area, Hubei Province. The assemblages recovered from the different parts of the platform vary both in terms of diversity and composition. The nearshore environments show low diversity assemblages with about 10 acritarch species, whereas the offshore shelf environments reflect higher diversities with about 40 species. The composition of the assemblages also changes from simple morphologies (micrhystrids, leiosphaerids, fusiform acritarchs) in nearshore environments to specimens with longer and more complexly branched processes on the shelf. The polygonomorph acritarchs are common over all the upper Yantze Platform, while the acanthomorph genera Baltisphaeridium and Peteinosphaeridium are the most abundant taxa on the offshore carbonate shelf area. Phylogenetic or sea-level changes are probably not responsible for the compositional and diversity changes that occurred during the investigated interval. This study confirms previous interpretations that poorly diversified Palaeozoic acritarch assemblages occur in neritic environments and more complex, highly diversified assemblages are found on the shelf.     

Phanerozoic changes in the high-rank suprageneric diversity structure of brachiopods: Linear and non-linear effects

Phanerozoic evolution of brachiopods produced many linear (established by a comparison of successive geologic time units) and non-linear (established by a comparison of non-successive geologic time units) effects, which can be examined quantitatively by using the similarity coefficients (Czekanowski's Quantified Coefficient and Gower Index) and correlation tools. The high-rank suprageneric diversity structure accounts for a number of superfamilies in each of 26 orders for every epoch of geological time. The intensity of turnovers in this structure was generally low during the entire Phanerozoic. It was slightly stronger during the Early Paleozoic, but close to zero during the Cenozoic, when the high-rank suprageneric diversity structure of brachiopods stabilized finally. Significant turnovers took place at the Middle Cambrian–Early Ordovician, the Late Ordovician–Early Silurian, the Late Silurian–Early Devonian, the Middle Devonian–Mississippian, and the Permian–Triassic transitions. Influences of mass extinctions, both major like those End Ordovician or Permian/Triassic and minor like Early Jurassic or Jurassic/Cretaceous, on the high-rank suprageneric diversity structure of brachiopods is registered. The strongest was the consequences of the Permian/Triassic catastrophe, which perhaps even reset the brachiopod evolution. No evident direct relationships are established between intensity of turnovers and eustatic fluctuations. However, the changes in the diversity structure recorded with the Gower Index provide evidence that eustatic lowstands were more favorable for intensification in these changes.     

UPPER ORDOVICIAN BRYOZOANS OF THE PIN FORMATION (SPITI VALLEY, NORTHERN INDIA)

Abstract: Twenty‐nine species of bryozoans from the Upper Ordovician–Lower Silurian Pin Formation (Spiti, India) have been identified. Eight of these are new: Trematopora minima, Ulrichostylus bhargavai, Ptilodictya exiliformis, Phaenopora ordinarius, Oanduellina himalayaica, Pesnastylus? vesiculosum, Ralfina? originalis and Pinocladia triangulata. The fossil record and facies analyses of the area investigated indicate shallow‐water conditions within the subtropical–tropical realm. The distribution pattern of fossils among the Ordovician/Silurian succession on the Northern Gondwana shelf and the influence of the Late Ordovician cooling phases on marine organisms are distinctive owing to a dramatic reduction in diversity globally. As far as the bryozoan taxa of Spiti are concerned, only one (Helopora fragilis) of the 29 species was recorded above the Ordovician/Silurian boundary. Observed bryozoan communities are very similar to faunas of Laurentia, the Baltic, Siberia and southern China of early–late Ordovician age.     

Acritarchs from the Dadas Formationin Southeast Turkey

Dadas Formation in Southeast Turkey yields abundant well preserved acritarchs, chitinozoans, tasmanites and scolecodonts. Encountered acritarchs suggest Upper Llandoverian-Ludlovian age for this formation. The acritarch association of the Dadas Formation resembles to the Middle Silurian assemblages of Tunisia and Libya. Thus Southeast Turkey falls in the Transitional-Iberian realm of Neoveryhachium carminae facies.     

A Tremadocian (Early Ordovician) palaeoscolecidan worm from graptolitic shales in Hunan Province,South China

A new locality exposing Tremadocian (Early Ordovician) graptolitic shales in Hunan Province, South China, has yielded an exceptionally well‐preserved annulated worm. This palaeoscolecidan is described as Waflascolex changdensis gen. et sp. nov. and reveals extremely fine detail of the cuticle organization and plating array. The new taxon is characterized by three critical characters: incomplete plate rows that occur only on the posterior end of the worm (except in the posterior‐most area) and do not extend over the entire circumference or along the entire trunk; a regular rhomboidal array of platelets around intercalations; and reduced cuticular organization at the posterior termination. The unique cuticular organization and platelet ornamentation in the new taxon offer insight into functional differentiation of plates in the scleritome. Palaeoscolecid distribution through the early Palaeozoic is reviewed, showing that the worms were widespread in the Cambrian and Ordovician, but became more restricted during the Silurian. Ordovician palaeoscolecidans are diverse in scleritome architecture, and strikingly different from Cambrian taxa, indicating that this group diversified as part of the Great Ordovician Biodiversification Event.     

新疆哈密地区晚泥盆世疑源类化石的发现及地质意义

从新疆哈密地区石城子北剖面7件样品获得分异度较高、有机质壁显著炭化保存的疑源类化石,根据其形态特征,共鉴定出14个形态属和15个形态种(其中8个未定种,3个比较种)。结合国内外古生代晚泥盆世已知疑源类组合进行比较,清楚表明当前获得的疑源类组合代表了晚泥盆世海洋微体浮游植物群面貌。我国涉及晚古生代疑源类生物地层的调查研究相对薄弱,而有关晚泥盆世疑源类化石的发现和报道更为匮乏,当前疑源类化石的发现,填补和丰富了新疆乃至国内晚泥盆世疑源类化石研究资料。该发现佐证了关于卡拉麦里洋在早石炭世闭合的认识;作为基础食物链的海洋微体浮游植物是重要成烃生物,晚泥盆世疑源类的保存预示研究区域具有石油、天然气勘察的前景。     

扬子区中、晚奥陶世和兰多维利世四射珊瑚起源、扩散及生物古地理关系

扬子区中奥陶世—志留纪兰多维利世四射珊瑚产出丰富 ,尤以兰多维利世最为繁盛。目前已报道的1 2 3属 (包括中奥陶世 4属 ,晚奥陶世 2 5属 ,兰多维利世 94属 )中 ,有 30属最早出现在扬子区 ,尔后扩散到欧洲、北美和其它邻区。例如 :Calostylis最早出现在川南的中奥陶世 (Llandeilo)地层中 ;Aphyllum和Cantrillia最早出现在浙西的晚奥陶世中Ashgill地层 ;扭心珊瑚类Briantelasma ,Pycnactis和Tunguselasma等 ,最早出现在黔东北晚Ruddanian地层 ;柱珊瑚类Ceriaster、Stauria、Amplexoides、Synamplexoides等 ,泡沫珊湖类Maikottia ,Rhizophyl lum等均最早出现在黔东北的中兰多维利统 (MiddleLlandovery)。根据这些资料 ,我们认为扬子区应该是奥陶纪和志留纪四射珊瑚起源中心之一。文中论述扬子区中奥陶世—兰多维利世四射珊瑚动物群特征及其生物古地理关系。该区中奥陶世珊瑚以穿孔珊瑚类的Calostylis和Yohophyllum为特征。下扬子区浙西晚奥陶世三衢山组 (中Ashgill)四射珊瑚群有某些澳大利亚分子Hillophyllum和Bowanophyllum ;而上扬子区晚Ashgill观音桥层的四射珊瑚动物群与北欧同期珊瑚群有高度的相似性。这表明扬子区当时与欧洲具有较密切的古生物地理关系。扬子区兰多维利世四射珊瑚群与西伯利亚。