Trilobite biofacies and sequence stratigraphy: an example from the Upper Ordovician of Oklahoma

There have been surprisingly few empirical investigations of the fundamental principle that the architecture of depositional sequences exerts considerable control on observed patterns of faunal distribution and replacement. In this paper, we examine trilobite associations in two sequences of the Upper Ordovician (Sandbian) Bromide Formation of southern Oklahoma. Cluster analysis and ordination of genus abundance data identified five lithofacies‐related biofacies that are also differentiated by diversity patterns. Biofacies of the transgressive system tract (TST) of successive sequences are more similar to each other than they are to biofacies in the highstand systems tract (HST) of the same sequence. This similarity likely records dominance of large, robust convex sclerites in taphonomically degraded samples from condensed, strongly winnowed grainstone and rudstone. Horizons with articulated exoskeletons of isoteline trilobites preserved by obrution deposits occur most commonly in the early HST and record behavioural aggregations. Grainstone and rudstone of the later HST are less winnowed than those of the TST and show less fragmentation and sorting of sclerites. These changes in taphonomic conditions preserve ecological patterns more clearly. In most biofacies, rarefied alpha diversity (samples) and gamma diversity (biofacies) of middle‐ and outer‐ramp HST deposits are greater than in the TSTs, and biofacies replace each other down ramp. Diversity patterns do not agree with model predictions and other data sets that indicate low beta and high alpha diversity in the TST, likely because of taphonomic degradation. Vertical replacement of biofacies is expressed by the appearance of peritidal facies in which trilobites are rare. Biofacies shifts also characterize sequence boundaries and are most profound in the inner‐ramp successions characterized by sharp facies offsets. Comparison with bathymetrically similar deposits in the Taconic foreland basin showed similar diversity trends along environmental gradients, with some differences in shallow‐water settings attributed to taphonomic differences.     

Suspected microbial-induced sedimentary structures (MISS) in Furongian (Upper Cambrian; Jiangshanian,Sunwaptan) strata of the Upper Mississippi Valley

The Furongian (Upper Cambrian; Jiangshanian and Sunwaptan) Tunnel City Group (Lone Rock Formation and Mazomanie Formation), exposed in Wisconsin and Minnesota, represents a shallow-marine clastic environment during a time of exceptionally high sea level. Lithofacies from shoreface to transitional-offshore settings document deposition in a wave- and storm-dominated sea. Flooding of the cratonic interior was associated with formation of a condensed section and the extensive development of microbial mats. Biolamination, mat fragments, wrinkle structures, and syneresis cracks are preserved in various sandstone facies of the Lone Rock Formation, as is evidence for the cohesive behavior of sand. These microbial-induced sedimentary structures (MISS) provide unique signals of biological–physical processes that physical structures alone cannot mimic. The MISS are associated with a trilobite extinction event in the Steptoean–Sunwaptan boundary interval. This may support recent claims that Phanerozoic microbial mats were opportunistic disaster forms that flourished during periods of faunal turnover. Further investigation of stratigraphic, taphonomic, and other potential biases, however, is needed to fully test this hypothesis.     

Influence of transportation and time-averaging in fossil assemblages from the Pennsylvanian of Oklahoma

The relative effects of shell destruction, transportation, and time-averaging have been assessed in seventeen soft-bottom, benthic fossil collections from the Pennsylvanian Wewoka Formation in Hughes County, Oklahoma. Invertebrates were identified to species level, and each was subject to a taphonomic evaluation to determine whether it was in place. Species that were not deemed parautochthonous (living at the location of collection shortly before final burial) were eliminated from the faunal lists to reduce the effects of time-averaging and transportation in the fossil assemblages. Both reciprocal averaging and non-metric multidimensional scaling ordinations revealed two biofacies. These were distinguished using both presence/absence data and numerical counts of species despite an overwhelming number of taxa occurring in both biofacies. Comparison between ordinations based on entire faunal lists with those based on parautochthonous faunal lists indicates that the two biofacies faithfully reproduce differences in the living distributions of member taxa but that the two biofacies underwent different degrees of time-averaging and transportation. Time-averaging leads to accumulation of taxa over time, thereby making presence/absence composition of collections more similar. Transportation, on the other hand, homogenizes originally patchy population distributions and makes relative abundances more uniform. Based on differences in abundances and presence/absence patterns, the stratigraphically lower biofacies underwent more transportation and more time-averaging than the upper biofacies.     

Facies of a Lower Ordovician carbonate shelf (Mungok Formation: Taebaeksan Basin,Korea)

Summary The lithologic associations within the Lower Ordovician Mungok Formation in Korea define four depositional facies that formed across a continental margin fringing the Sino-Korean block: these facies represent lagoonal/restricted marine, shoal, inner shelf, and outer shelf environments. The stacking pattern of these facies reveals two systems tracts composed of five depositional sequences. The lower highstand systems tract consists of the lagoonal/restricted marine and shoal facies, whereas the upper lowstand systems tract comprises, in ascending order, inner shelf, outer shelf, and inner shelf facies. Three trilobite biofacies are recognized in the Mungok Formation: i.e.,Yosimuraspis, Kainella, andShumardia biofacies in ascending order. TheYosimuraspis Biofacies is dominated byYosimuraspis but also containsJujuyaspis andElkanaspis. The predominance of the endemic eponymous taxon suggests a lagoonal/restricted marine environment. The nearly monotaxicKainella Biofacies, which comprises pandemic genera such asKainella and occasionallyLeiostegium, may represent a less restricted environment than theYosimuraspis Biofacies. TheShumardia Biofacies occurs in the marlstone/shale lithofacies through relatively thick stratigraphic interval and is dominated by cosmopolitan trilobite taxa with some endemic species. The lithofacies and cosmopolitan trilobite assemblage of theShumardia Biofacies indicate that it occupied an outer shelf environment. The vertical succession of lithofacies and trilobite biofacies in the Mungok Formation records in general a shift from a restricted, shallow water environment to deeper-water environment.     

湘西晚寒武世多节类三叶虫新种及Erixanium属首次发现

文中描记 3个多节类三叶虫新种 :Hardyoidesdamaensissp .nov .,Meringaspisdamaensissp .nov .,andRhyssometopus (Rostrifinis)nitidussp .nov .,均产自湘西凤凰花桥组中部。同时描记首次在江南斜坡带中发现的Erixanium一属的保留命名种Erixaniumcf.E .sentum。Erixanium是全球性分布的多节类三叶虫 。     

Taphonomic pathways and comparative biofacies and taphofacies in a Recent intertidal/shallow shelf environment

The development of taphonomic approaches to facies analysis requires a foundation in facies-based actualistic studies. Modern intertidal and shallow shelf environments at Provincetown Harbor. northern Cape Cod, Massachusetts (USA) provide an opportunity to compare pattcrns and controlling factors in molluscan biofacies and taphofacies distributions. Variation in faunal composition, ecologic variables, and taphonomic attributes of molluscan death assemblages produce distinct patterns of environmental zonation: (1) Faunal composition (biofacies) primarily tracks variation in substrate type among environments (sand, rock, peat, and Zostera marina beds). (2) Ecologic variables (equitability, infauna: epifauna ratio, gastropod:bivalve ratio, and predation on M. mercenaria) appear to reflect tidal exposure time. (3) Taphononic attributes (fragmentation, abrasionm, corrosion, bioerosion, and encrustation) of the common bivalve M. mercenaria track environmental energy, in particular its effects on the stability and reworking of hardparts at the sediment surface. Shells in different environments proceed along different taphonomic pathways - the order of acquisition of taphonomic features by hardparts. An encrustation/bioerosion-dominated pathway characterizes low energy environments; the upper intertidal and deeper subtidal. An abrasion-dominated pathway characterizes the high energy lower intertidal and shallow subtidal. Contrasting pathways produce distinct proportions of taphonomic attributes in time-averaged samples; proportions that delineate taphofacies. Integrated taxonomic, ecologic and taphonomic data provide a more complete picture of environmental processes than any approach alone. Taphonomic data not only furnish information not readily provided by other approaches, but free paleoecology from the constraints of taxonomic uniformitarianism. □Taphonomy, comparative taphonomy, taphofacies, biofacies, cluster analysis, multidimensional scaling, taphonomic pathways, Recent, actualism, intertidal, molluses.     

Filtration models, guilds, and biofacies: Crinoid paleoecology of the Stanton Formation (Upper Pennsylvanian), midcontinent, North America

Filtration models make several predictions concerning the distribution of crinoids among benthic habitats: (1) generally, higher energy shoreward settings should contain crinoid assemblages dominated by taxa with dense mesh filtration fans; (2) generally, lower energy offshore settings should be dominated by crinoids with open mesh filtration fans; (3) diversity should be highest in nearshore settings, which generally contain low energy microhabitats within high energy settings where both dense fan and open fan forms can co-occur, whereas offshore assemblages should be dominated by open fan taxa; thus, open fan forms should be more eurytopic than dense fan forms. Also, previous empirical models for crinoid distribution patterns note that nearshore assemblages are dominated by large forms whereas offshore assemblages are dominated by small forms. Filtration fan morphology and body size interact significantly to determine how and where crinoids feed and thus are used here as criteria for distinguishing late Paleozoic crinoid guilds. The distribution patterns of crinoid taxa and guilds among the benthic paleoenvironments of the Upper Pennsylvanian (Missourian) Stanton Formation were used to test the predictions of the filtration models.

The results of quantitative analyses show that large, dense fan crinoids are significantly more abundant in shoreward assemblages than smaller, open fan crinoids, which are more abundant in offshore assemblages; these results are consistent with the aforementioned models. However, diversity patterns are not consistent with the filtration models, indicating that mid-depth and aerobic offshore assemblages are more taxonomically and trophically diverse than nearshore and dysaerobic offshore assemblages, ostensibly due to the overlapping ranges of characteristically nearshore and offshore taxa. Analysis of guild distribution patterns also indicates some contradictions to predicted patterns of stenotopy and eurytopy, indicating additional complexities to late Paleozoic crinoid paleoecology.

Five recurrent crinoid assemblages, or biofacies, have been identified from the Stanton Formation of midcontinent North America. The guild structure and diversity of these biofacies are characterized here as: (1) a moderate diversity nearshore biofacies dominated by large calyx, dense fan guilds; (2) a high diversity midshelf biofacies characterized by equitable contributions of many taxa and guilds; (3) a moderate diversity offshore biofacies dominated by non-pinnulate and small calyx, open fan pinnulate guilds; (4) a low diversity offshore dysaerobic biofacies dominated by small calyx pinnulate guilds; (5) a depauperate offshore biofacies dominated by a unique stalkless species, Paragassizocrinus tarri. Except for Biofacies 5, the number of guilds appears to increase offshore; however, the equitability of taxa among and within the guilds generally decreases offshore. This pattern indicates that the heterogeneity of resources, or the recognition by taxa of different classes of resources, as reflected by the variety of functional types, increased from nearshore to offshore, but that the abundance or accessibility of these resources, and/or the relaxation of competition within guilds, increased from offshore to nearshore.     

Benthic diversity patterns in oxygenation gradients: an example from the Middle Jurassic of Switzerland

Etter, W. 1995 11 30 Benthic diversity patterns in oxygenation gradients: an example from the Middle Jurassic of Switzerland.
A high-resolution study of the lower Opalinum Clay (Aalenian, Middle Jurassic) of northern Switzerland revealed a pattern of macrobenthic diversity and abundance which does not conform with the dysaerobic-biofacies models currently in use. The upper dysoxic zone contains an association of moderate diversity and moderate abundance which, with increasing oxygen depletion, is replaced by an association dominated by a few peak opportunists (tiny epibenthic bivalves and presumably small annelids) at high abundances but very low diversity. In the lower dysoxic zone, abundance drops to very low levels, but diversity rises again to quite high values. The rather high diversity of the lower dysaerobic biofacies is explained in part by the presence of specialized chemosymbiontic species. The pattern documented in the lower Opalinum Clay has been calibrated by sedimentologic, taphonomic, ichnologic, and microfaunal evidence. It contradicts equilibrium models, which postulate gradual or even linear decreases in both abundance and diversity with increasing oxygen-depletion of the bottom water. The equation of rising diversity with improved oxygenation can be misleading and can yield erroneous results in the reconstruction of ancient bottom-water oxygenation. □ Dysaerobic biofacies, high-resolution study, diversity, chemosymbionts, equilibrium and non-equilibrium models .     

Distribution of recent benthic foraminifera on the Sunda Shelf (South China Sea)

In this study we investigate the species composition and spatial distribution patterns of Rose Bengal stained and unstained benthic foraminifera from the central part of the Sunda Shelf in the south-western South China Sea in relation to environmental factors. The uppermost centimetre of the surface sediments (> 150 μm) from 45 sites from inner (60 m) to outer shelf (226 m) water depths revealed 584 species including 443 stained species.The univariate analyses of individual species abundances and community parameters and next canonical correspondence analysis were used to relate the faunal data to a set of measured environmental parameters. Four biofacies recognised on the Sunda Shelf are most strongly correlated to water depth, primary production and sediment type of the habitat. The inner shelf biofacies (CCA cluster A), defined by Ammomassilina alveoliniformis and Asterorotalia pulchella, occurs in fine grained sediments classified as modern terrigenous mud in the region with the highest primary production values. The high-energy inner shelf biofacies (CCA cluster B), defined by Heterolepa dutemplei and Textularia lythostrota, occurs in modern terrigenous sand and silt dominated sediments, northeast from the Natuna Island. The high-energy outer shelf biofacies (CCA cluster C), defined by Cibicidoides pachyderma and Textularia bocki, is sandwiched between assemblages of biofacies D. It occurs in the region characterised by neritic relict sand. In the shallow-waters on the Sunda Shelf the relationship of benthic foraminiferal faunal composition to grain size of sediments indirectly signals the prevailing bottom hydrodynamic conditions. The dominance of the epibenthic foraminifera attached to bigger particles (e.g. Cibicides lobatulus, Planulina arimiensis) and much higher abundances of empty tests suggest greater current velocities northeast of Natuna Island. The outer shelf biofacies (CCA cluster D) is defined by Facetocochlea pulchra and Bulimina marginata. It occurs in an area covered with modern terrigenous silt and mud and is characterised by lower annual primary production, but seasonally influenced by weak upwelling.     

Trace fossils from arenig flysch sediments of eire and their bearing on the early colonisation of the deep seas

A sequence of Lower Ordovician (Arenig) turbidites in Co. Wexford, Eire, has yielded one of the earliest diverse ichnofaunas yet recorded from deep water sediments comprising: Chondrites, Glockerichnus, Gordia, Helminthopsis, Lorenzinia, Neonereites, Palaeophycus, Paleodictyon, Planolites, Sublorenzinia, Taenidium, Taphrhelminthopsis, Teichichnus and Tomaculum. This ichnofauna is critical in any analysis of the colonisation of the deep seas by trace fossil‐producing animals.

A world‐wide review shows that the earliest trace fossils are mainly from Late Precambrian shelf sea environments, but many more evolved during very rapid diversification in the pre‐trilobite Lower Cambrian.

There was little increase in diversity in shallow water after the Lower Cambrian but a progressive colonisation of the deep ocean took place and this accelerated during the Ordovician, when the main lineages of deep sea trace fossils were established there. Rosetted, patterned, meandering and simple spiral forms evolved in shallow water in the Upper Precambrian and pre‐trilobite Lower Cambrian and only later migrated into the deep sea, whereas complex, closely programmed, spiral traces may have evolved there.     

Diversity,Abundance and Community Structure of Benthic Macro- and Megafauna on the Beaufort Shelf and Slope

Diversity and community patterns of macro- and megafauna were compared on the Canadian Beaufort shelf and slope. Faunal sampling collected 247 taxa from 48 stations with box core and trawl gear over the summers of 2009–2011 between 50 and 1,000 m in depth. Of the 80 macrofaunal and 167 megafaunal taxa, 23% were uniques, present at only one station. Rare taxa were found to increase proportional to total taxa richness and differ between the shelf ( 100 m) where they tended to be sparse and the slope where they were relatively abundant. The macrofauna principally comprised polychaetes with nephtyid polychaetes dominant on the shelf and maldanid polychaetes (up to 92% in relative abundance/station) dominant on the slope. The megafauna principally comprised echinoderms with Ophiocten sp. (up to 90% in relative abundance/station) dominant on the shelf and Ophiopleura sp. dominant on the slope. Macro- and megafauna had divergent patterns of abundance, taxa richness ( diversity) and diversity. A greater degree of macrofaunal than megafaunal variation in abundance, richness and diversity was explained by confounding factors: location (east-west), sampling year and the timing of sampling with respect to sea-ice conditions. Change in megafaunal abundance, richness and diversity was greatest across the depth gradient, with total abundance and richness elevated on the shelf compared to the slope. We conclude that megafaunal slope taxa were differentiated from shelf taxa, as faunal replacement not nestedness appears to be the main driver of megafaunal diversity across the depth gradient.     

Unusual trilobite biofacies from the Lower Ordovician of the Argentine Cordillera Oriental: new insights into olenid palaeoecology

Balseiro, D., Waisfeld, B.G. & Buatois, L.A. 2010: Unusual trilobite biofacies from the Lower Ordovician of the Argentine Cordillera Oriental: new insights into olenid palaeoecology. Lethaia, Vol. 44, pp. 58–75. The study of biofacies has proven to be relevant in the understanding of trilobite palaeoecology, palaeobiogeography and macroevolution. The widespread Olenid biofacies is one of the best known, and is usually interpreted as occuring in dysoxic environments. Tremadocian successions of the Argentinian Cordillera Oriental bear a diverse and long‐studied olenid‐dominated fauna. Based on cluster analysis, five distinct biofacies are defined for the middle Tremadocian (Tr2 stage slice), distributed from shelf (below storm wave base) to lower‐shoreface settings (above fair‐weather wave base). Ordination shows biofacies along two gradients, a bathymetrical one and another related to oxygen content. All of them are dominated both taxonomically and ecologically by olenids. This detailed quantitative palaeoecological study challenges current views suggesting instead that the Olenidae dominated a broad range of environments, from oxygenated shallow‐marine to dysoxic deep‐marine. Comparisons with largely coeval trilobite records from geodynamically and palaeobiogeographically disparate sites suggest that siliciclastic sedimentation appears as the most influential controlling environmental factor upon olenid distribution and dominance. Further comparisons across different climatic belts show that siliciclastic input controlled trilobite diversity gradients, even more than latitude. From an autoecological viewpoint distribution of traditional olenid morphotypes shows no relation to depth or to oxygen content, and at least some members of the group appear to have had the possibility of coping with low oxygen content, rather than being restricted to oxygen‐deficient environments. The analysis performed herein, together with recent research on the group, demonstrate that factors controlling olenid distribution are more complex than previously envisaged. □Biofacies, diversity, Olenidae, palaeoecology, Tremadocian, trilobite.     

Cambrian (Series 2 to Miaolingian) platform facies from central Sonora,Mexico and the regional correlation

This study provides new insights about depositional paleoenvironments through siliciclastic microfacies, carbonate microfacies, and biofacies analysis from sedimentary formations of the lower and middle Cambrian (Stage 4–Wuliuan), exposed in central Sonora, northern Mexico. Results of the petrographic analysis of 48 samples revealed the following lithologies: quartzarenite, oncolytic rudstone, grainstone-packstone, wackestone, mudstone, and to a lesser extent sandy limestone. Two siliciclastic microfacies were identified: (A) quartzarenite with cross-bedded and horizontal stratification deposited in an intertidal and supratidal environment; and (B) massive quartzarenite with Skolithos ichnofacies deposited in subtidal and intertidal environments. Four carbonate microfacies were identified: microfacies 1 is a sandy limestone with trilobite fragments; microfacies 2 is a grainstone with intraclasts, salterellids, hyolithids, trilobites, and echinoderms plates; microfacies 3 is an oncolytic rudstone consisting of microbes and abundant echinoderms plates; and microfacies 4 is a packstone-grainstone with abundant ooids, trilobite fragments, and echinoderm plates. Two biofacies were identified: Agnostid-polymeroid biofacies with predominance of the trilobites Pentagnostus, Bathyuriscus, Oryctocephalites, and Elrathina; and Pagetia biofacies with abundant trilobites of the genera Pentagnostus, Pagetia and Elrathina. It is concluded by the sedimentation model that changes in sea level is the most important parameter in determining the siliciclastic microfacies, carbonate microfacies and biofacies; as well as the depositional environments that vary from the coastline (subtidal to supratidal) to shallow-water open circulation marine platform with low and high energy waters. The Cambrian deposits of northern Mexico are correlated with the deposits of California and Nevada (USA), as well as to the Precordillera (Argentina), where the species in common show a strong affinity.     

Assembly rules of reef corals are flexible along a steep climatic gradient

Coral reefs, one of the world's most complex and vulnerable ecosystems, face an uncertain future in coming decades as they continue to respond to anthropogenic climate change, overfishing, pollution, and other human impacts [1, 2]. Traditionally, marine macroecology is based on presence/absence data from taxonomic checklists or geographic ranges, providing a qualitative overview of spatial shifts in species richness that treats rare and common species equally [3, 4]. As a consequence, regional and long-term shifts in relative abundances of individual taxa are poorly understood. Here we apply a more rigorous quantitative approach to examine large-scale spatial variation in the species composition and abundance of corals on midshelf reefs along the length of Australia's Great Barrier Reef, a biogeographic region where species richness is high and relatively homogeneous [5]. We demonstrate that important functional components of coral assemblages "sample" space differently at 132 sites separated by up to 1740 km, leading to complex latitudinal shifts in patterns of absolute and relative abundance. The flexibility in community composition that we document along latitudinal environmental gradients indicates that climate change is likely to result in a reassortment of coral reef taxa rather than wholesale loss of entire reef ecosystems.     

Microstratigraphy and taphonomy of rudist shell concentrations in Upper Cretaceous limestones,Cilento area (central-southern Italy)

Rudist bed type and distribution has been investigated in Upper Cretaceous limestones cropping out in the northern Cilento area (southern Italy). These limestones are dominated by fine-grained, peloidal, silty packstone in which rudist-rich beds are intercalated. An inner shelf environment may be inferred on the basis of the recognized sedimentary and taphonomic features. The rudist shell beds are characterized by low species diversity, with slight differences in abundance of a few species belonging to the Durania, Bournonia, Sauvagesia, Gorjanovicia and Biradiolites genera, which usually form oligo- or monospecific congregations. The internal fabric of these levels (i.e. orientation, arrangement, packing and sorting of the skeletal elements; internal microstratigraphy) has permitted us to distinguish two broad shell bed categories: (a) shell beds considered as “Primary Shell Concentration”, in which the shell concentration is essentially created by the behaviour of local shell producers, preserved in situ and in growth position; (b) shell beds considered as “Hydraulic Shell Concentration”, which were deposited under the influence of hydraulic processes and/or input of surrounding bioclastic sediments. The taphonomic analyses allowed us to highlight the role of some of the biotic and abiotic factors that controlled the distribution of the rudists in the various habitats. The increase of physical disturbance (especially hydrodynamism) is the primary difference between these shell bed categories. The establishment and development of the densest rudist congregations appear to be related to the accommodation space made available by means of relative sea level rise. The lowering of the sea level was often accompanied by the increased influence of waves and/or currents on the seabed and the consequent sediment disturbance and demise of the rudist lithosome, although other factors cannot be excluded.     

Compostional variations and patterns of condont reworking in Late Devonian and early carboniferous calciturbidites (Moravia, Czech Republic)

Summary Compositional variations and grain-size properties of both carbonate constituents and conodonts as an alternative component group were used for interpreting the processes governing the deposition of upper Famennian and middle Tournaisian calciturbidites in Moravia, Czech Republic. Both the composition and grain-size properties of conodont element associations showed to be markedly dependant on facies type of their host sediment. Upper Devonian calciturbidite successions deposited on flanks of wide, Moravian-Silesian carbonate platform are composed mainly of echinoderm-and peloid-rich wacke/packstones and intraclastic float/rudstones (fine-grained calciturbidites, “normal” calciturbidites with Tab Bouma sequences, debris-flow breccias) with abundance of shelf-and shelf margin conodont taxa and epipelagic and “mesopelagic” conodonts. Upper Devonian calciturbidites deposited on slopes of volcanic sea-mounts are composed of echinoderm-and peloid-rich wacke/packstones and float/rudstones with increased proportion of intraclasts and volcanigenic lithoclasts (fine-grained calciturbidites, normal calciturbidites), yeilding abundant conodont associations with higher proportion of “mesopelagic” taxa compared to the platform-flank examples. Middle Tournaisian calciturbidite succession composed of crinoid-, peloid-, intraclast-and lithoclast-rich lime mudstones, wacke/packstones and float/rudstones (normal calciturbidites and debris-flow breccias) yielded conodont element associations rich in shelt-and shelf-margin taxa, “mesopelagic” conodonts and reworked Middle-and Upper Devonian conodonts. In general, the ratio of shelf-and shelf margin conodont taxa to “mesopelagic” taxa is distinctly lower in finegrained calciturbidites than it is in normal calciturbidites and debris-flow breccias. Grain-size properties (mean grain size and sorting) and percentage of fragmented conodont elements, too, are markedly dependant on the facies type: in fine-grained calciturbidites the values of mean grain-size and fragmentation are low and the sorting is good to very good whereas in normal calciturbidites and debris-flow breccias the values of mean grain-size and fragmentation are distinctly higher and the sorting is poorer. The interdependence of facies type and composition and grain-size properties of conodont element associations in gravity-flow deposits is explained as resultant from hydrodynamic sorting during turbidity current flow and final deposition of the bed. Compositional variations observed in our sections may thus be attributed to facies variability (coarsening-and thickening-upward trends) rather than to sea-level fluctuations (highstand shedding of carbonate platforms). On the other hand, significant enrichment in reworked conodont taxa in middle Tournaisian normal calciturbidites compared to scarcity and/or absence of such conodonts in essentially identical facies of upper Famennian age indicate sea-level to be the major control governing such compositional variations, with low relative sea-level stand in middle Tournaisian and high relative sea-level stand in upper Famennian. Thorough analysis of conodont evolution, palaeoecology and taphonomy, with emphasis on understanding the processes of deposition of their host rock, are recommended for any biostratigraphic and biofacies study to be done in carbonate sediments deposited under strong hydrodynamic regimes, such as calciturbidites, temperstites, debris-flow deposits, shelf-edge oolitic sands, tidal-channel facies etc.     

The production and preservation of trilobite resting and furrowing traces

The production and preservation of trilobite resting and furrowing traces is discussed with special reference to specimens from Upper Cambrian strata in North Walcs, Spain, and Poland. Resting excavations are attributed largely or completely to animal activity, and do not normally rcsult from currcnt scouring. Resting excavations, furrows, and many other trace fossils are formed at the sediment-water interface and not bencath a thin sand layer or by burrowing between beds. The factors which allow the initial production of the traces on the ocean floor muds (e.g. cohesion, shear strength, low pore water pressures) will also favour their preservation during deposition of the casting sand.     

Eustatic trends in conodont diversity across the Frasnian–Famennian boundary in the stratotype area, Montagne Noire, Southern France

Trends in generic diversity of successive conodont communities are analysed in sections of different environmental settings across the Frasnian–Famennian (F–F) boundary in the stratotype area, Montagne Noire, France. The evolution of conodont biofacies and abundances matches the overall pattern already observed in many sections elsewhere in the world and supports the interpretation of an important eustatic sea-level fall during the Upper Kellwasser event. The change from late Frasnian deep-water palmatolepid–polygnathid biofacies to shallower-water polygnathid-icriodid biofacies during the Upper Kellwasser event occurred in all sections studied. The shallowing trend culminated at the end of the Kellwasser Event as indicated by the substantial increase of formerly poorly represented icriodids, whereas palmatolepids concomi-tantly diminished. This event occurred earlier on oxygenated outer platform submarine rises than in oxygen-depleted depressions. The sudden sea-level fall prior to the Frasnian–Famennian boundary was followed, at the beginning of the Famennian, by a deepening trend when palmatolepids dominated again. These changes in conodont generic associations and abundances occurred rapidly and synchronously. As a result, the stratigraphic resolution obtained with the evolution of biofacies is higher: it permits not only a more accurate location of the base of the Upper Kellwasser event in environments where it cannot be distinguished lithologically, but it also allows the recognition of intrazonal gaps.     

Clustered trilobite assemblages in the Middle Devonian Hamilton Group

Clusters of well-preserved trilobites occur throughout lower and middle Paleozoic strata. This phenomenon, however, remains poorly understood. Several thin, regionally persistent beds in the Middle Devonian (Givetian) Hamilton Group of New York State display taphonomic and sedimentary features which indicate rapid burial by storm-generated clouds of fine-grained sediment. Furthermore, components in these units display little or no post-mortem disturbance and, therefore, reflect in faunal associations. These beds contain two types of clustered assemblages of the trilobites Phacops (Green), Greenops boothi (Green) and Dechenella rowi (Green). Body clusters comprise dense assemblages of three or more complete trilobite corpses. Moult clusters , in contrast, are composed of three more moult ensembles; i.e., articulated thoracopygidia in close proximity to corresponding cephala. Clusters are species-segregated and contain only a small portion of the total size range exhibited by trilobite species. Moreover, body clusters and moult clusters are morphometrically undifferentiated indicating that these two cluster modes represent related phenomena. Analogy with modern marine arthropods suggests that trilobites assembled into monospecific, age-segregated clusters and moulted prior to en masse copulation.