Aeronian (Llandovery, Lower Silurian) palynomorphs and graptolites from the Lipeón Formation, Eastern Cordillera, north-west Argentina

The lower levels of the Lipeón Formation, in the Eastern Cordillera, north-west Argentina, yield a marine-dominated palynomorph assemblage, together with graptolites of mid to late mid Llandovery age (Demirastrites convolutus and probably Stimulograptus sedgwickii zones). The palynomorph assemblage is dominated by acritarchs, but also contains algae and terrestrial cryptospores. Crassiangulina variacornuta, considered a potentially good global biostratigraphical marker for the Upper Llandovery is recovered for the first time from the Silurian of Argentina. The occurrence of this species in strata not younger than late Aeronian, and independently dated by graptolites, indicates an early first appearance for Crassiangulina variacornuta, in the Lipeón Formation, below the Aeronian/Telychian boundary. The lower part of the unit corresponds to a quiet marine environment; thus supporting that Crassiangulina variacornuta is a facies-sensitive acritarch.     

Reef reconstruction after extinction events of the latest ordovician in the Yangtze platform,South China

Summary Early Silurian reef reconstruction on the Yangtze Platform, in the northern part of the South China Block, is preceded by a combination of regional and global processes. During most of Ashgill time (Late Ordovician), the area was dominated by Wufeng Formation deep water graptolitic black shales. Reefs largely disappeard in the middle of the Ashgill Stage, from the northwestern margin of Cathaysian Land (southeastern South China Block), in advance of the Late Ordovician glaciation and mass extinction, due to regional sea-level changes and regional uplift, unrelated to the mass extinction itselt. Late Ordovician microbial mudmound occurrence is also found in the western margin of the Yangtze Platform, its age corresponding to theDicellograptus complexus graptolite biozone of pre-extinction time. On the Yangtze Platform, a thin, non-reef-bearing carbonate, the Kuanyinchiao Formation (=Nancheng Formation in some sites), thickness generally no more than 1m, occurs near several landmasses as a result of Hirnantian regression. Reappearance of the earliest Silurian carbonates consisting of rare skeletal lenses in the upper part of Lungmachi Formation, are correlated to theacensus graptolite biozone, early Rhuddanian of Shiqian, northeastern Guizhou, near Qianzhong Land. Carbonate sediments gradually developed into beds rich in brachiopods and crinoids in the lower part of Xiangshuyuan Formation, middle Rhuddanian. In the middle part of Xiangshuyan Formation, biostromes, containing abundant and high diversity benthic faunas such as corals, crinoids and brachiopods, show beginnings of reconstruction of reef facies. Substantial reef recovery occurred in the upper part of Xiangshuyuan Formation, lower Aeronian, as small patch reefs and biostromes. During the late Aeronian, carbonate sediments, especially reefs and reef-related facies, expanded on the upper Yangtze Platform, and radiation of reefs occurred in Ningqiang Formation, upper Telychian. The long period of reef recovery, taking several million years, remains difficult to explain, because redistribution of any refugia faunas would be expected to take place soon after the extinction. Reefs and reef-related facies subsequently declined after Telychian time due to regional uplift of the major portion of the Yangtze Platform. Carbonate facies are therefore uncommon in South China during the rest of Silurian time.     

A proposed new global stratotype for Aeronian Stage of the Silurian System: Hlásná Třebaň section,Czech Republic

The current Global Stratotype Section and Point (GSSP ) for the Aeronian Stage (Llandovery Series, Silurian System), on Trefawr track in the Llandovery area of Wales, is an inadequate marker for precise, global, correlation. The International Subcommission on Silurian Stratigraphy has, therefore, undertaken the selection of a new GSSP for this level. The lowest occurrence of the graptolite Demirastrites triangulatus , 1.38 m above the base of the black‐shale succession of the ?elkovice Formation at the Hlásná T?ebaň section in Central Bohemia, is proposed to mark the base of the Aeronian Stage. The section, which fulfils all formal requirements for stratotype of a chronostratigraphical unit, should be considered as a candidate for the new GSSP . An abundant, well‐preserved, diverse graptolite fauna occurs through the section along with common chitinozoans, which indicate that the Spinachitina maennili Biozone spans the boundary interval. The section comprises the lower–middle Aeronian (D. triangulatus–Lituigraptus convolutus graptolite biozones) along with underlying Rhuddanian (Akidograptus ascensus–Coronograptus cyphus biozones) and Hirnantian strata. Several graptolite genera of primary importance in global correlation (Demirastrites , Petalolithus , Rastrites and Campograptus ) first appear in the lower part of the triangulatus Biozone. The structurally simple section is somewhat condensed, but there is a uniform succession of black shale without any evidence of disconformity in the broad boundary interval. The C_org isotope record exhibits a minor positive excursion just above the base of the triangulatus Biozone, whereas TOC and N isotope and elemental geochemical records provide evidence for uninterrupted sedimentation under stable, anoxic conditions.     

A Lower Silurian (Aeronian) radiolarian assemblage from black cherts of the Armorican Massif (France)

The Chalonnes-sur-Loire outcrop is the most complete Llandovery section in the Ligerian domain (Armorican Massif, NW France); it displays a ca. 10 m-thick sequence of organic-rich black bedded cherts alternating with black graptolitic shales, which were deposited above glaciomarine diamictites of the Hirnantian glaciation. It is likely the black cherts accumulated under eutrophic waters, on the outer shelf part (distal offshore) of a Gondwanan margin, situated at intermediate to high latitudes of the Southern hemisphere. Hydrofluoric acid processing allowed the extraction of radiolarians; seven species are identified in the best preserved sample discovered so far. All seven are common species in tropical assemblages known from Alaska and Nevada, which are characteristic of the Orbiculopylorum assemblage, of Aeronian to early Telychian age. This age is in good agreement with independent age control from graptolites in the Chalonnes section, suggestive of an Aeronian age for the radiolarian-yielding level.     

塔里木盆地塔中台地上奥陶统海百合茎化石及其古生态*

聂拉木是西藏南部志留纪含笔石地层(石器坡组)的主要产地。近期在该地亚来村新发现一批笔石标本, 包括Normalograptus sp.、Glyptograptus sp.、Campograptus lobiferus、C. cf. lobiferus、C. cf. obtusus、C.? circularis、Lituigraptus cf. convolutus、Rastrites cf. perfectus、Stimulograptus sedgwickii、Streptograptus sp.和Torquigraptus decipiens。该组合面貌确定其属于志留系兰多维列统埃隆阶上部Lituigraptus convolutus带至Stimulograptus sedgwickii带。这一新材料的发现, 使西藏南部的志留纪地层得以作全球对比。     

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.     

黔东北石阡香树园剖面“龙马溪组”灰岩中的生物多样性

黔东北石阡香树园剖面志留系底部的"龙马溪组"沉积于上扬子区黔中古陆近岸浅海带,与其它远岸相的龙马溪组下部的黑色笔石页岩差异很大,厚度仅3.3m,为黄色、黄绿色含粉砂质页岩夹6个灰岩薄层或透镜体。从灰岩中识别出叠层石、海绵、床板珊瑚、四射珊瑚、三叶虫、介形类、腕足类和棘皮类等化石碎片类型,指示了奥陶纪末赫南特冰期结束后,适宜于各类底栖生物生长的生态条件在近岸区最早出现。     

Conodont biostratigraphy and carbon isotope chemostratigraphy of the Middle to Upper Ordovician on the western Yangtze Platform,South China

Middle and Late Ordovician strata in the nearshore area of the western Yangtze Platform are investigated. In total, 241 δ¹³C_carb samples and 108 conodont samples from the Songliang and Tuanjie sections in the northeastern Yunnan Province were collected for chemostratigraphic and biostratigraphic studies. Altogether, 21 species belonging to 16 conodont genera were recovered from the two sections, among which Ansella jemtlandica, Drepanoistodus basiovalis, Hamarodus brevirameus, and Microzarkodina hagetiana are important age indicators within the Dapingian to early Katian interval. Four carbon isotope shifts are recognized in the studied sections: a negative shift in the early Darriwilian, the middle Darriwilian carbon isotope excursion (MDICE), the early Katian Guttenberg carbon isotope excursion (GICE), and a further positive carbon isotope shift later in the early Katian which is a new record of this event in South China. Integrated carbon chemostratigraphy and conodont biostratigraphy indicate early Darriwilian and early Katian ages for the base and top, respectively, of the Huadan Formation at the Songliang section, and a Darriwilian to early Katian age for the Shihtzupu Formation at the Tuanjie section.     

Late Ordovician–Early Silurian facies development and environmental changes in the Subpolar Urals

The continuous Upper Ashgill–Sheinwoodian carbonate succession in the most eastern Kozhym River area in the Subpolar Urals comprises the Yaptikshor (Rawtheyan), Kamennaya baba (Hirnantian), Ruchej and Manyuku (Llandovery–?Sheinwoodian) formations. The facies of the deep subtidal Yaptikshor Fm. mark an abrupt sea‐level rise following emergence of the Bad’ya reef (Rawtheyan). Carbonate breccias at the base of the Kamennaya baba Fm. correlate with the beginning of the Hirnantian glaciation and change upwards towards the Ordovician–Silurian boundary with the development of light‐grey massive boundstone/packstone shoal deposits. An abrupt change in facies to the Rhuddanian–Aeronian Ruchej Fm. continental slope environment marked the start of a long‐term sea‐level rise. The uppermost Aeronian–?Sheinwoodian is represented by submarine canyon carbonate conglobreccias of the Manyuku Fm. unconformably underlying the Balban’yu reef. The rapid facies changes at the base of the Hirnantian and at the Ordovician–Silurian boundary were of global eustatic origin. In contrast, the abrupt changes in the Rawtheyan and the formation of the Manyuku Fm. conglobreccias were of local or regional origin associated with tectonics. They were followed by the start of a regional transgression (Yaptikshor Fm.) and a global transgression marked by the initiation of the Balban’yu Reef in the Sheinwoodian.     

重庆石柱漆辽地区中、上奥陶统牙形刺序列—揭示达瑞威尔阶下部至桑比阶之间的沉积间断

本文详细描述了重庆石柱漆辽剖面奥陶系达瑞威尔阶(Darriwilian)牯牛潭组至凯迪阶(Katian)宝塔组的牙形刺序列及其对比。牙形刺序列由上而下可分为:宝塔组Hamarodus brevirameus生物带和Amorphognathus superbus生物带;大田坝组的Baltoniodus alobatus,Baltoniodus variabilis和Yangtzeplacognathus jianyeensis生物带;牯牛潭组的Yangtzeplacognathus crassus生物带和Lenodus variabilis生物带。其中Hamarodus brevirameus生物带至Baltoniodus alobatus生物带上部为凯迪阶,B.alobatus生物带下部至Yangtzeplacognathus jianyeensis生物带上部为桑比阶(Sandbian),而Yangtzeplacognathus jianyeensis生物带下部,Yangtzeplacognathus crassus生物带和Lenodus variabilis生物带归入达瑞威尔阶。根据文中所识别的牙形刺序列,该剖面显然缺失了达瑞威尔阶中上部的牙形刺生物带如Pygodus serra生物带(即Yangtzeplacognathus protoramosus和Y.foliaceus生物带),Eoplacognathus suecicus和Dzikodus tablepointensis生物带。因此,在重庆石柱漆辽剖面奥陶系牯牛潭组和大田坝组之间的地层是不连续的,存在一明显的沉积间断,缺失了相当于牯牛潭组标准剖面中上部的地层(达瑞威尔阶中上部)。而这一沉积间断所发生的时间正好与达瑞威尔中晚期全球海平面下降相吻合。     

Trilobites from the Silurian “Lower Red Beds” of Wuhan,South China: stratigraphic and paleogeographic implications

The age of the Silurian “Lower Red Beds” (LRBs), which occur widely in South China, has long been controversial because of the scarcity of index fossils. Here, we report a fauna with abundant trilobites from the LRBs in Wuhan, Middle Yangtze region, consisting of Aulacopleura (Paraaulacopleura) pijiazhaiensis, ‘Encrinuroides’ yichangensis, Gaotania hubeiensis, Hyrokybe gaotanensis, Japonoscutellum yichangense, Latiproetus latilimbatus, and Ptilillaenus lojopingensis. These trilobites are common in the Leijiatun, upper Xiangshuyuan, Shihniulan, and Lojoping formations in the Upper Yangtze region, and belong to the middle to upper Aeronian Ptilillaenus or ‘Encrinuroides’–Ptilillaenus–Latiproetus assemblages. The discovery of this trilobite assemblage suggests that the LRBs are probably partly of middle–late Aeronian age in the Middle Yangtze region. Thus, the lower boundary of the LRBs is likely diachronous in the Yangtze region, ranging from the middle–late Aeronian to the early Telychian. The discovery of the Ptilillaenus assemblage in the Wuhan LRBs expands the paleogeographic distribution of this assemblage and provides evidence of deeper depositional environment of the LRBs in the study area versus shallow waters in other areas of the Yangtze region.     

Palaeoecology and palaeogeography of Late Ordovician (Katian–Hirnantian) cephalopods of the Boda Limestone,Siljan district,Sweden

The carbonates associated with the Late Ordovician (Katian–Hirnantian) Boda mounds of the Siljan district, Sweden, contain a rich cephalopod fauna. Cephalopods are rare in the micritic stromatactis facies of the Katian Boda Core Member, but are concentrated together with other molluscs and trilobites in synsedimentary fillings of caves and crevices of the lithified mound limestone. More than 60 cephalopod species are known from these fillings. The assemblage is dominated in richness and abundance by small orthoconic proteoceratids of the genus Isorthoceras and by curved barrandeocerids, and is considerably less endemic than the benthic fauna of the Boda mounds. Similarities are remarkable at the species level with assemblages from elsewhere in Baltica, with Avalonia and south‐western Kazakhstan. A genus‐level comparison reveals affinities with Laurentia. However, the genus‐level similarities are interpreted as reflecting mainly similarities of the depositional environment and of physicochemical conditions. The presence of calcareous algae in the Boda Limestone and maximum hydrostatic septal strength of cephalopods indicate a depositional depth of <100 m, well within the euphotic zone. The dominance and diversity of cephalopods with vertical migrant lifestyle and the absence of actinocerids indicate a cool, nutrient‐rich deeper neritic environment. The post‐Katian sediments contain a drastically reduced diversity, dominated by orthocerids. The maximum hydrostatic septal strength of the Hirnantian cephalopods and sedimentological features indicate a shallowing during the Hirnantian, but a continuation of high food availability for cephalopods in the water column.     

A late Rhuddanian (early Llandovery,Silurian) trilobite association from South China and its implications

A recovery trilobite association after the end-Ordovician mass extinction, the ‘Encrinuroides’ Association, is documented from the lower Niuchang Formation (upper Rhuddanian, lower Llandovery, Silurian) of Gaojiang, Meitan, northern Guizhou, South China paleoplate, including 10 genera of eight families. Three subassociations of this association could be differentiated, i.e., ascendingly the ‘Encrinuroides’-Eoleonaspis Subassociation, the ‘Encrinuroides’-Aulacopleura (Paraaulacopleura) Subassociation, and the ‘Encrinuroides’-Meitanillaenus Subassociation. The succession of these subassociations in the section indicates a shallowing upward trend of the environment as a result of regional tectonic activities (i.e., the Qianzhong Uplift) against the background of the Rhuddanian global transgression. A relatively complete trilobite succession in South China across the Ordovician–Silurian transition has been summarized and its implications for the trilobite macroevolution of this particular time interval are discussed in detail.     

Late Ordovician brachiopods from eastern North Greenland: equatorial offshore migration of the Red River fauna

Abstract: Late Ordovician rhynchonelliformean brachiopods, typical of the North American Red River fauna, are found sporadically in the Børglum River Formation of the Centrum Sø area, Kronprins Christian Land, eastern North Greenland. The geographical distribution of this characteristic brachiopod fauna is thus extended to the easternmost extremity of the Laurentian craton. The assemblage compares specifically with the Hiscobeccus brachiopod fauna, based on key taxa such as notably Hiscobeccus gigas (Wang, 1949), and indicates a late Katian age for this part of the succession. For the first time, this typically inland, shallow‐water fauna is found associated with genera like Bimuria, suggesting a transitional marginal facies with outer shelf benthos. The current study describes a Hiscobeccus fauna that lived at the seaward edge of its preferred habitat. Furthermore, an unpublished Hiscobeccus fauna, from the Børglum River Formation of Peary Land, central North Greenland, as well as several occurrences from the Kap Jackson and Cape Calhoun formations in various parts of Washington Land, western North Greenland, are described here. These show a distinct shift from older strata containing H. capax (Conrad, 1842) to younger strata exclusively yielding specimens of H. gigas. As H. gigas occurs in the upper part of the Cape Calhoun Formation in Washington Land, it indicates that the upper boundary of the Cape Calhoun Formation is considerably younger than previous estimates, reaching into the uppermost Katian (middle Cautleyan–Rawtheyan). The Cape Calhoun Formation correlates with the upper member of the Børglum River Formation and further demonstrates that the Hiscobeccus fauna was widespread in Laurentian marginal settings of North Greenland. Even though the Hiscobeccus fauna was pan‐continental during the late Katian (Richmondian), it possesses a strong provincial signal during the later Ordovician. The new occurrences indicate that this fauna extended to the north‐eastern margin of the Laurentian Craton. It lived in close association with cosmopolitan faunal elements that may have been the earliest sign of the succeeding invasion of migrants from Baltica that arrived later during the Hirnantian. The offshore migration of this atypical Hiscobeccus fauna likely demonstrates the path of warm‐water currents as the Centrum Sø locality was located at the equator during the Late Ordovician.     

华南奥陶纪末大灭绝后腕足动物是否存在"小型化效应"?

小型化效应（Lilliput effect）一般是指灭绝事件后生物个体相对于事件前明显变小的一种演化现象。为了更深入研究,文中将它分为狭义与广义两种类型：狭义型是指它的原始定义,专门针对种级分类单元的小型化,因研究条件苛刻而导致相关研究很少;广义型是指高于种级分类单元的小型化,已有一些实例。文章对华南奥陶纪末大灭绝前后“Katian晚期”、“Hirnantian”及“Rhuddanian初期”3个时期的腕足动物个体大小数据,依“属”、“超科”及“目”3级分类单元有选择地进行比较。分析结果表明,奥陶纪末大灭绝后残存期里较低级分类单元（属级）在个体大小变化方面显示较大的差异性,这与二叠纪末大灭绝后普遍的小型化有很大差别,这可能是因为奥陶纪末的灭绝强度远低于后者所致;同一较高级分类单元中不同属级分子个体变化趋势表现出较为一致的现象,反映出它们可能具有相似的灾难应对机制。     

MIDDLE LLANDOVERY (AERONIAN) GRAPTOLITES OF THE WESTERN MURZUQ BASIN AND AL QARQAF ARCH REGION, SOUTH-WEST LIBYA

Abstract:  The taxonomy and biostratigraphy of the Aeronian graptoloid graptolites of the Tanezzuft Formation (Murzuq Basin and Al Qarqaf Arch area) is presented and discussed with respect to their palaeoenvironmental setting and palaeobiogeographical links. The gregarius-libycus , ' leptotheca ', convolutus and sedgwickii assemblage biozones and tenuis Subzone are recognized and correlated with the generalized zonal scheme and with graptoloid successions in peri-Gondwanan Europe. The Rhuddanian/Aeronian boundary is tentatively placed at the base of the gregarius-libycus Biozone. Telychian faunas have not been identified. Fourteen of the 23 species recorded herein belong to the middle Aeronian convolutus Biozone assemblage. The overall low diversity may be the result of inhabiting unstable, occasionally turbulent and/or oxic environments. The lowest diversity, but with abundant graptoloid rhabdosomes, is observed in proximal silty and sandy deposits. The presence of ' Paraclimacograptus' libycus suggests biogeographical links to Aeronian graptolite faunas of Jordan and South America. The convolutus Zone assemblage is very similar to the coeval faunas of the Saudi Arabian Qusayba shales. The occurrence of several species endemic to northern and north-western Gondwana and peri-Gondwana provides further evidence for a distinct palaeoclimatic/palaeolatitudinal control on graptolite distribution.     

西藏南部聂拉木志留纪笔石新材料*

西沙群岛琛航岛琛科二井(CK-2)有878.3 m厚的新生代珊瑚礁灰岩(Cenozoic reef limestone)。新生代珊瑚礁灰岩自上而下包括了第四系的全新统和更新统以及新近系的上新统和中新统。其中, 中新统厚514.3 m, 产19属石珊瑚(Scleractinia corals)。更新统厚215.6 m, 产21属石珊瑚(Scleractinia)和1属八射珊瑚(Octocorallia)。珊瑚是营底栖固着生活的动物, 在地层断代方面它没有能像一些浮游动物(例如浮游有孔虫等)那么“精准”。但也有一些珊瑚的地质历程相对比较短暂的, 它们在地层划分和对比方面还是能起到一定的重要作用。根据本文的研究在琛科二井井深364–878.3 m的这一段中新世地层中就鉴定出有好几个只见于中新世或者常见于中新世的珊瑚, 如: 星日珊瑚(Astrhelia)、安的列斯珊瑚(Antillophyllia)和大安的列斯珊瑚(Antillia)等。另外在井深21.4–237 m的更新世地层中也发现了好几个只限于第四纪的珊瑚, 如: 叶状珊瑚(Lobophyllia)、合叶珊瑚(Symphyllia)、轮沙珊瑚(Trochopsammia)和苍珊瑚(Heliopora)等。上述那些珊瑚的地质分布与有孔虫(Foraminifera)生物地层鉴定的结果以及古地磁(Palaeomagnetism)和同位素(Isotope)年代地层测定的数据完全吻合。此外, 珊瑚除了在地层的划分对比方面(subdivision and correlation of the stratigraphy)能够起到一些关键的作用外, 更重要的是它在恢复和解释古地理(Palaeogeography)、古气候(Palaeoclimatology)和古生态(Palaeoecology)等方面也都能起到非常重要的作用。     

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.     

Global diversity and endemism in Early Silurian (Aeronian) brachiopods

Silurian brachiopods of Middle Llandovery (Aeronian) ages are reviewed, and 215 genera are identified here, compared with 109 in the preceding Early Llandovery (Rhuddanian), indicating a recovery‐radiation interval after the end‐Ordovician mass extinction. The chief regions in which they are found are the continents of South China, Avalonia‐Baltica, Laurentia and Siberia, which were all at tropical latitudes with the exception of Avalonia. In addition, the very large superterrane of Gondwana, although with patchy brachiopod distribution, included temperate faunas, as well as subtropical faunas (in Iran and Afghanistan). Aeronian brachiopods greatly increased in diversity, with dominance of four major groups: orthides and strophomenides (which had flourished previously in the Rhuddanian), pentamerides and atrypides (which became dominant in the Aeronian), and many newly evolved taxa, and occupied deeper water and wider ecological niches (level bottom and reef) than those in the Rhuddanian. Each of the continents has some endemic genera, but there is a greater proportion of them in South China, where some groups (such as the pentamerides and atrypides) are more diverse and others, such as the orthides, are much less common than elsewhere. Affinity indices (AI) show that two megaunits can be recognized: South China and Avalonia‐Baltica‐Laurentia (ABL); Siberia seems to have been loosely connected with ABL and even more loosely to South China presumably because of its geographical separation in the Northern Hemisphere. The separation of South China from the other megaunits is further supported by cluster analysis.