Late Ordovician extinction of North American and British crinoids

After reaching a diversity peak in the Caradocian, North American Ordovician crinoids underwent a gradual decline to a nadir in the early Ashgillian (Maysvillian). This interval, recording extinction of the Cleiocrinidae, Merocrinidae, Ottawacrinidae, Hybocystitidae, and several lineages of camerate crinoids, was apparently caused by major environmental shifts in seas of eastern North America resulting from a westward-prograding wedge of terrigenous clastics derived from the Taconic Highlands, possibly coupled with a marine transgression in the Maysvillian that allowed colder water slope biofacies to invade the craton. Crinoids suffered a major episode of extinction in the late Ashgillian (late Richmondian/Rawtheyan). This event, preceding the end of the Ordovician by at least one stage or 2 to 4 million years, resulted in extinction of 12 families of crinoids including the Xenocrinidae, Tanaocrinidae, Reteocrinidae, Archaeocrinidae, Anthracocrinidae, Cincinnaticrinidae, Iocrinidae, Anomalocrinidae, Carabocrinidae, Cupulocrinidae, Porocrinidae, and Hybocrinidae. Glacio-eustatic lowering of sea level may have triggered this crisis by partially draining the North American craton, resulting in changes in oceanic circulation, salinity, and temperature. Latest Ordovician (Hirnantian) carbonates of the North American mid-continent region contain pelmatozoan assemblages from which Silurian crinoids radiated. These taxa were largely unaffected by a minor extinction event at the Ordovician/Silurian boundary.     

Ecological disruption close to the Ordovician‐Silurian boundary

Following a long period of stability there was substantial ecological disruption in the late Ordovician, accompanied by widespread extinction. The ecological upheaval was spread over a period of time (1 My?) and varied in intensity between different groups.

The greatest change in brachiopod faunas occurred in the tropics, where there were widespread extinctions. In temperate regions the community structure, although disrupted, was not destroyed. The deep shelf associations (Benthic Assemblages 4–5) of the Rawtheyan persisted in an attenuated form into the Hirnantian when they were supplemented by genera of the ”Hirnantia fauna” and then became strongly re‐established in the Llandovery.

Trilobites, in contrast to brachiopods, suffered a worldwide extinction in the late Ashgill. Approximately 40% of trilobite genera became extinct at, or near, the Rawtheyan‐Hirnantian boundary.

Analysis of the distribution of Rawtheyan trilobite and brachiopod genera supports a correlation between geographical distribution and survival. Global range charts for brachiopod and trilobite genera, incorporating ecological and palaeogeographical data, have been compiled for this study.     

The relation of Late Ordovician glaciation to the Ordovician-Silurian changeover in North American brachiopod faunas

Sheehan, P. M.: The relation of Late Ordovician glaciation to the Ordovician-Silurian changeover in North American brachiopod faunas.
The Ordovician-Silurian changeover of brachiopod faunas in North American epicontinental seas involved the abrupt extinction of endemic Late Ordovician stocks and subsequent repopulation of North American seas by Old World taxa. The Late Ordovician Gondwanaland glaciation may have lowered sea levels sufficiently to place severe stress on the widespread shallow marine faunas in North America, resulting in their eventual extinction. The Late Ordovician depositional history in North America is not well enough known to establish the presence of a latest Ordovician regression, but the earliest Silurian was an interval of off-lap in North America. Therefore, the glacial lowering of sea level is considered to be the most likely cause of the faunal changeover.     

Bryozoan provinces and patterns of generic evolution and extinction in the Late Ordovician of North America

The Late Ordovician bryozoan genera of central and southeastern North America are geographically distributed in three biotic provinces, separated by boundaries reflecting major lithofacies differences. The central Cincinnati Province contains most of the North American endemic genera, and represents a narrow ecological zone separating the clastic wedges of the marginal Reedsville-Lorraine Province from the cratonic carbonate platform of the Red River-Stony Mountain Province. The provinces provided major life zones, or biomes, for each of the five bryozoan orders. Genera comprising the provinces differed as well in morphologic complexity, geochronologic survivorship, tiering, endemism and eurytopy. Regions on either side of the Cincinnati Province were dominated by inferred immigrants from Baltoscandia. Al-logenic provincial succession produced time-averaged mixed faunas in regions near the provincial boundaries. Although most generic originations took place within the Cincinnati Province, evolutionary novelties are associated with the Reedsville-Lorraine Province. The loss of the diverse Cincinnati Province, connected with global cooling and a eustatic lowering of sea level, may have been a chief factor in the Late Ordovician extinction of bryozoan genera. Genera from the Red River-Stony Mountain Province differentially survived into the Silurian.     

Biogeographic associations of Silurian bryozoan genera in North America, Baltica and Siberia

Six distinct bryozoan paleobiogeographical provinces existed during the Silurian of North America, Baltica and Siberia. These provinces are based on parsimony analysis of endemicity (PAE), each supported by a number of endemic genera only present in that assemblage. The area cladogram displays a nested pattern, permitting the clustering of assemblages into larger units that might be indicative of their areas of origin. Paired-group cluster analysis supports the existence of provincial assemblages. Relationships indicate the action of both vicariance and geodispersal. Geographic boundaries between the provinces include land, deep sea, and climate zone boundaries, including barriers created by the Iapetus Ocean and the Tornquist Sea.     

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.     

Extinctions and migrations of Devonian rugose corals in the Eastern Americas Realm

Oliver, W. A., Jr. 1990 04 15: Extinctions and migrations of Devonian rugose corals in The Eastern Americas Realm. Lethaia , Vol. 23 , pp. 167–178. Oslo. ISSN 0024–1164.
Detailed plotting of the stratigraphic ranges of Devonian rugose coral genera within the Eastern Americas Realm reveals new information about the extinctions and migrations of this largely endemic fauna. There were significant faunal turnovers in the Lochkovian, middle Eifelian and late Givetian. as well as in the often discussed late Frasnian. The late Givetian turnover was nearly as great as the Frasnian one. Inward migration was principally from western North America, and the greatest influxes were during the carly Givetian and Frasnian. It seems likely that there were several separate incursions and that some genera were introduced to the east two, or even three times.     

Were Phanerozoic mass extinctions among brachiopod superfamilies selective by taxa longevity?

All mass extinctions are characterized by certain kind of selectivity. An analysis of stratigraphic ranges of 112 brachiopod superfamilies implies that some Phanerozoic mass extinctions (Late Ordovician, Frasnian/Famennian and Devonian/Carboniferous, Early Jurassic, and Cretaceous/Paleogene) were selective by taxa longevity. They preferentially affected relatively old superfamilies and favoured a survival of relatively young superfamilies. No explanation of this selectivity as an apparent phenomenon is fully satisfactory. The Permian/Triassic mass extinction did not favour a survival of “young” superfamilies because of abnormally low rate of origination established since the Pennsylvanian and the absence of these “young” taxa. This study confirms tentatively a difference between Paleozoic and post-Paleozoic times by the importance of post-extinction recovery intervals for taxa longevity.     

Phylogenetic Clustering of Origination and Extinction across the Late Ordovician Mass Extinction

Mass extinctions can have dramatic effects on the trajectory of life, but in some cases the effects can be relatively small even when extinction rates are high. For example, the Late Ordovician mass extinction is the second most severe in terms of the proportion of genera eliminated, yet is noted for the lack of ecological consequences and shifts in clade dominance. By comparison, the end-Cretaceous mass extinction was less severe but eliminated several major clades while some rare surviving clades diversified in the Paleogene. This disconnect may be better understood by incorporating the phylogenetic relatedness of taxa into studies of mass extinctions, as the factors driving extinction and recovery are thought to be phylogenetically conserved and should therefore promote both origination and extinction of closely related taxa. Here, we test whether there was phylogenetic selectivity in extinction and origination using brachiopod genera from the Middle Ordovician through the Devonian. Using an index of taxonomic clustering (R_CL) as a proxy for phylogenetic clustering, we find that A) both extinctions and originations shift from taxonomically random or weakly clustered within families in the Ordovician to strongly clustered in the Silurian and Devonian, beginning with the recovery following the Late Ordovician mass extinction, and B) the Late Ordovician mass extinction was itself only weakly clustered. Both results stand in stark contrast to Cretaceous-Cenozoic bivalves, which showed significant levels of taxonomic clustering of extinctions in the Cretaceous, including strong clustering in the mass extinction, but taxonomically random extinctions in the Cenozoic. The contrasting patterns between the Late Ordovician and end-Cretaceous events suggest a complex relationship between the phylogenetic selectivity of mass extinctions and the long-term phylogenetic signal in origination and extinction patterns.     

A second varanopseid skull from the Upper Permian of South Africa: implications for Late Permian 'pelycosaur' evolution

Late Permian terrestrial faunas of South Africa and Russia are dominated taxonomically and ecologically by therapsid synapsids. On the basis of a single specimen from the Upper Permian of South Africa, the varanopseid Elliotsmithia longiceps is the sole basal synapsid ('pelycosaur') known from Gondwana. Recent fieldwork in the Upper Permian of South Africa has produced a second varanopseid specimen that is referrable to Elliotsmithia . Data from both this specimen and the holotype suggest that Elliotsmithia forms a clade with Mycterosaurus from the Lower Permian of North America and Mesenosaurus from the Upper Permian of Eastern Europe. That postulate is supported by the three most parsimonious trees discovered in a new analysis of varanopseid phylogeny. However, the available data cannot resolve the interrelationships of these three genera. The new phylogenetic results contrast with earlier work identifying Elliotsmithia as the basal member of a clade that includes the North American taxa Aerosaurus , Varanops , and Varanodon . The new trees reduce the stratigraphic debt required by the latter scenario, and the one with the least stratigraphic debt identifies Elliotsmithia and Mesenosaurus as sister taxa. Two new taxa are erected, Mycterosaurinae and Varanodontinae, for the two varanopseid subclades.     

The Gondwanan and South American Episodes: Two Major and Unrelated Moments in the History of the South American Mammals

The first steps in the history of South American mammals took place ca. 130 Ma., when the South American plate, still connected to the Antarctic Peninsula, began to drift away from the African-Indian plate. Most of the Mesozoic history of South American mammals is still unknown, and we only have a few enigmatic taxa (i.e., a Jurassic Australosphenida and an Early Cretaceous Prototribosphenida) that pose more evolutionary and biogeographic questions than answers. The best-known Mesozoic, South American land-mammal fossils are from Late Cretaceous Patagonian beds. These fossils represent the last survivors of non- and pre-tribosphenic Pangaean lineages, all of them with varying endemic features: some with few advanced features (e.g., ?Eutriconodonta and “Symmetrodonta”), some very diversified as endemic groups (e.g., ?Docodonta Reigitheriidae), and others representing vicariant types of well known Laurasian Mesozoic lineages (e.g., Gondwanatheria as vicariant of Multituberculata). These endemic mammals lived as relicts (although advanced) of pangeic lineages when a primordial South American continent was still connected to the Antarctic Peninsula and, at the northern extreme, near the North American Plate. By the beginning of the Late Cretaceous, the volcanic and diastrophic processes that finally led to the differentiation of the Caribbean region and Central America built up transient geographic connections that permitted the initiation of an overland inter-American exchange that included, for example, dinosaurian titanosaurs from South America and hadrosaurs from North America. The immigration of other vertebrates followed the same route, for example, polydolopimorphian marsupials. These marsupials were assumed to have differentiated in South America prior to new discoveries from the North American Late Cretaceous. The complete extinction of endemic South American Mesozoic mammals by the Late Cretaceous-Early Paleocene, and the subsequent and in part coetaneous immigration of North American therians, respectively, represent two major moments in the history of South American mammals: a Gondwanan Episode and a South American Episode. The Gondwanan Episode was characterized by non- and pre-tribosphenic mammal lineages that descended from the Pangeic South American stage (but already with a pronounced Gondwanan accent, and wholly extinguished during the Late Cretaceous-Early Paleocene span). The South American Episode, in turn, was characterized only by therian mammals, mostly emigrated from the North American continent and already with a South American accent obtained through isolation. The southernmost extreme of South America (Patagonia) remained connected to the present Antarctic Peninsula at least up until about 30 Ma., and both provided the substratum where the primordial cladogenesis of “South American” mammals occurred. The resulting cladogenesis of South American therian mammals followed Gould's motto: early experimentation, later standardization. That is to say, early cladogenesis engendered a great variety of taxa with scarce morphological differentiation. After this early cladogenesis (Late Eocene-Early Oligocene), the variety of taxa became reduced, but each lineage became clearly recognizable distinctive by a constant morphologic pattern. At the same time, those mammals that underwent the “early experimentation” were part of communities dominated by archaic lineages (e.g., brachydont types among the native “ungulates”), whereas the subsequent communities were dominated by mammals of markedly “modern” stamp (e.g., protohypsodont types among the native “ungulates”). The Gondwanan and South American Episodes were separated by a critical latest Cretaceous-earliest Paleocene hiatus, it is as unknown as it is important in which South American land-mammal communities must have experienced extinction of the Gondwanan mammals and the arrival and radiation of the North American marsupials and placentals (with the probable exception of the xenarthrans, whose biogeographic origin is still unclear).     

华南与缅甸奥陶纪末赫南特贝动物群中的无铰类腕足动物

在奥陶纪末生物大灭绝的两幕之间,海洋底域繁盛着赫南特贝腕足动物群（Hirnantia Fauna）。它数量丰富,分布广泛,历程短暂。以往国内外学者研究这个动物群时,常把有铰类腕足动物作为重点,而无铰类只被简单描述或列出名单,整体面貌不明。无铰类化石尽管材料有限、研究基础薄弱,但只就属的数目而言,占据了全球赫南特贝动物群总属数的近1/5,其群落、演化和环境意义不可小觑。文中专门记述上扬子区（华南古板块）观音桥层（赫南特早中期）与缅甸曼德勒地区（滇缅马苏古地体）Hwe Mawng紫色页岩段（赫南特中期）所产赫南特贝动物群的无铰类化石,计有3目、4超科[Linguloidea（舌形贝超科）、Discinoidea（平圆贝超科）、Craniopsoidea（似髑髅贝超科）和Cranioidea（髑髅贝超科）]的10属、12种,包括5个命名属种[Plectoglossa cf. davidsoni (Barrande),Schizotretinia cf. euxina (Havlí?ek), Pseudopholidops partibilis (Rong),Petrocrania cribrum (Temple),Xenocrania haimei (Reed)]和7个未定种（Trematis sp.,Paracraniops sp.,Acanthocrania sp.,Petrocrania? sp. 1,Petrocrania? sp. 2, Pseudolingula? sp. 和Orbiculoidea sp.）,其中,Pseudolingula? sp.和Orbiculoidea sp.两个未定名种因标本不佳,未正式描述。其中,以Pseudopholidops最为常见,其次是Xenocrania和Petrocrania。前两属是华南、滇缅马苏、波罗的卡、阿瓦隆尼亚、佩鲁尼卡等古板块或地体赫南特贝动物群的常见分子。它们已知限于南、北纬30°之间的低纬度地区,这可能与化石采集和研究程度有关,但更可能反映奥陶纪末全球气候仍存在分异现象。研究识别了奥陶纪晚期无铰类腕足动物7个目,可归为3个类群：灭绝目（仅神父贝目Paterinida）、消减目（多样性与丰度大幅衰减,尤其是三分贝目Trimerellida和乳孔贝目Acrotretida在Hirnantian销声匿迹）和延续目（适应能力强、忍耐阈值高,成功穿越大灾难的首幕）。这些分类单元对研究奥陶纪-志留纪交界期腕足动物群的多样性、群落生态、生物地理及宏演化等有重要意义。     

西藏南部聂拉木地区晚泥盆世—早石炭世的介形类

本文系统描述西藏南部聂拉木地区晚泥盆世–早石炭世介形类共计15属24种。根据介形类的组合面貌及孢子、牙形刺的研究结果推断,亚里剖面上的亚里组时代为晚泥盆世–早石炭世,泥盆系-石炭系界线(D-CB)大致位于亚里东沟段顶部至沟陇日段。亚里组产出的介形类生态组合属于光滑速足目集群或艾菲尔生态型,指示亚里组在沉积时为远岸浅水沉积环境,且在上部腊扎蒲段水深变浅。亚里组介形类面貌与华南泥盆纪介形类相似程度高,与俄罗斯和欧美地区也有相似的属种,说明泥盆纪-石炭纪之交介形类的地理隔离并不显著,在各个板块之间有属种交流的可能。     

Mississippian crinoid biodiversity,biogeography and macroevolution

Abstract: The biodiversity and biogeography of 217 genera of Mississippian crinoids from North America and the British Isles shed light on the macroevolutionary turnover between the Middle Palaeozoic and Late Palaeozoic Crinoid Evolutionary Faunas. This turnover resulted from steady differential extinction among clades during the middle Mississippian after crinoids reached their Phanerozoic peak of generic richness during the early Mississippian. This peak richness was primarily a function of Mississippian originations rather than Devonian–holdover taxa. North America had 100 per cent higher generic richness than the British Isles, but rarefaction analysis adjusts the difference to only 37 per cent higher. Rarefaction demonstrated that North America had increased biodiversity, compared to the British Isles, almost entirely among monobathrid camerates, disparids and primitive cladids. In contrast, diplobathrid camerates, advanced cladids and flexibles had the same generic biodiversity between regions, when compared using rarefaction. The early Mississippian radiation resulted from two primary causes: (1) the expansion of Tournaisian carbonate ramps following the Frasnian mass extinction of reef faunas and (2) the predatory release in the Tournaisian following the end‐Famennian Hangenberg extinction of durophagous fishes. A majority of crinoid genera from the British Isles are cosmopolitan. When combined with rarefaction analysis and evidence for more first occurrences in North America, this suggests higher origination rates in North America, especially when carbonate ramps were widespread. With the gradual reduction in the area of carbonate ramps from the early to late Mississippian, in conjunction with the radiation of new durophagous fishes, camerate crinoids in particular experienced continuous background extinction, without replacement, beginning during the earliest Viséan (late Osagean). By middle Viséan time (late Meramecian) advanced cladids were dominant in all settings. This resulted in the transition from the Middle Palaeozoic to the Late Palaeozoic Crinoid Macroevolutionary Fauna.     

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.     

HIRNANTIAN (LATEST ORDOVICIAN) GRAPTOLITES FROM THE UPPER YANGTZE REGION, CHINA

Abstract:  The Upper Yangtze region yields a Hirnantian (latest Ordovician) graptolite fauna that includes 41 species assigned to 13 genera. This fauna is particularly important for understanding the Late Ordovician mass extinction event because it is the most diverse known from this interval. In addition, it records the survival, well into the Hirnantian, of many taxa of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) fauna, which was previously regarded as having gone extinct at the beginning of the Hirnantian. Taxa exhibiting six different astogenetic patterns, including taxa with reclined stipes, scandent, biserial, full-periderm and 'archiretiolitid' rhabdosome forms occur in the lower Normalograptus extraordinarius-N. ojsuensis Biozone. In contrast, in the upper N. persculptus Biozone only four genera remain, all but one of which are Normalograptidae: scandent and biserial taxa with Pattern H astogeny. Normalograptids are the dominant form of the succeeding, lower Rhuddanian, faunas. The Yangtze faunas also document the early expansion of normalograptids coeval with the decline of the DDO fauna. Many previously identified species considered endemic to China have been synonymized; 24 of the 41 species recorded here have been recognized elsewhere. No new taxa are described.     

Species richness in plant genera disjunct between temperate eastern Asia and North America

The species richness of 109 amphi-Pacific disjunct genera was examined in eastern Asia and North America. Although the entire flora of eastern Asia contains approximately one-third more species than that of North America, the difference in species richness among disjunct taxa is less. When woody and herbaceous genera are considered separately, the former exhibit a strong diversity bias favouring eastern Asia whereas there is no significant difference in diversity between continents among herbaceous genera. This result is not due to habitat differences between woody and herbaceous genera, because the disjunct herbs inhabit primarily moist forests and woodlands. This result is also not related to relative phylogenetic advancement, even though older major lineages of plants tend to have a predominance of woody taxa. Woody genera are distributed in lower latitudes than herbaceous genera on both continents, and both woody and herbaceous genera are distributed in lower latitudes in eastern Asia than in North America. The North American temperate flora is primarily a relict of a flora form 7 more widespread throughout the Northern Hemisphere. Contemporary patterns of diversity suggest that the effects of climate changes in the late Tertiary were less severe in eastern Asia and promoted diversification, but were more severe in North America and may have caused widespread extinction. The difference in the effect of climate change on diversity in herbaceous and woody lineages reflects the different ecological relationships of species having these contrasting life forms. Clearly, the contemporary floras of eastern Asia and North America bear the imprint of history and emphasize the important interface between ecological relationships and evolutionary responses.     

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.     

Non-selectivity in extinction of bivalves in the Late Cretaceous of the Atlantic and Gulf Coastal Plain of North America

It has been suggested that the biogeographic and clade-level characteristics of marine invertebrate groups important in survival through mass extinctions are different from those important in survival during normal times. The role played by ecologically important characters in survival across mass extinctions, however, has not been well-studied. We obtained information from the literature about the feeding mode and morphology, burrowing habits, size and depth below sea-level inhabited, temperature range, shell thickness, species richness and abundance of bivalve genera present in the Late Cretaceous of the Atlantic and Gulf Coastal Plain of North America. Non-parametric analysis revealed that there were no significant associations between any of these characteristics and survival across the Cretaceous-Tertiary boundary. This lack of selectivity may be due to varying local conditions, which favor different ecological characteristics in each area and produce no overall pattern of selectivity. It might also be indicative of the severity of this extinction on bivalves — differences in ecological habits may have been virtually irrelevant to survivorship through this event.     

The Messinian marine molluscs record and the dawn of the eastern Atlantic biogeography

The biological impact of the Messinian Salinity Crisis (MSC) and its bearing on the Pliocene Mediterranean marine molluscan fauna has been analyzed on the basis of the biogeographic and stratigraphic distributions of the taxa of 16 early Messinian outcrops. The extinction of the last paleoendemic Proto-Mediterranean taxa is historically significant but it appears to be numerically less important than the extinction of the Tortonian and Messinian neoendemic taxa. The available data suggest that the MSC caused a regional mass disappearance but only a limited number of extinctions. It is also emphasized that the Late Pliocene (Monegatti et al., 2002) extinctions were far heavier than those caused by the MSC. We suggest that the greatest Messinian extinctions were triggered, during the salinity crisis, in the Atlantic “sanctuary” by the Messinian glacial events TG22, TG20, TG14, and TG12, of Shackleton et al. (1994), dated between 5.79 and 5.55 Ma by Krijgsman et al. (2004). A comparison between the Mediterranean Messinian and the Redonian molluscan faunas was also carried out. Finally, the possible latitudinal stability of the climatic thresholds, despite the shifting of the climatic zones throughout the Neogene along the European coast, is pointed out.