Radiation,diversity and environmental expansion of Early Ordovician ostracods: a view from the Southern Hemisphere

Tremadocian–Floian siliciclastic successions in the Argentine Cordillera Oriental are of great interest for the study of early ostracod radiation. Four genera and five species of ostracods have so far been documented in the basin from the Early–Late Tremadocian (Tr2). This contrasts with occurrences from other palaeocontinental areas, where typically only one species is described. Patterns of environmental distribution and ecological approaches of ostracod assemblages were analysed on the basis of richness, Fisher's alpha diversity, relative abundance and occupancy. Ecological patterns remained stable throughout the studied interval, mostly with one genus notably dominant over the others. At the regional scale, dominant taxa occur at a relatively large number of sites, which display high occupancy. The literature data on carbonate successions from the Baltoscandian basin show a similar pattern, with a single taxon dominant over the others in local assemblages and wide regional distribution of these dominants. This ecological pattern would therefore have persisted at least until the Dapingian, independently of the sedimentary regime. Ostracods were already present along the onshore–offshore gradient during the initial stages of radiation, showing a strong preference for deep subtidal settings during Tr2 and then spreading to shallower environments in Tr3 and to deeper ones in the Floian (Fl2–Fl3). A compilation of ostracod diversity data from several regions shows a remarkable increase during the Darriwilian. Based on the regions studied herein, this rise in diversity is decoupled from the environmental expansion of ostracods, as they were already recorded along the onshore–offshore profile early in their history on both siliciclastic and carbonate shelves. The Argentine Cordillera Oriental could be considered as a cradle of diversity, with an important role in the subsequent radiation of the group.     

Diversity patterns in upper Cambrian to Lower Ordovician trilobite communities of north‐western Argentina

The hierarchical structure of biodiversity from a regional scale analysis has received much attention as an alternative approach to unravelling the principal drivers of biodiversification. To better understand the processes that control the diversification of Cambro‐Ordovician trilobite communities from the Argentine Cordillera Oriental, we explore patterns of occupancy and diversity trajectories at the local and regional scales through seven intervals (Furongian, loTr1, upTr1, loTr2, upTr2, Tr3 and Fl2–3), and across an onshore‐offshore profile. Our results indicate: (1) a decrease in regional diversity from the upper Tr2 onwards, mainly caused by a reduction in the number of rare taxa, coupled with stable beta diversity at regional scale and a constant rise in beta diversity in deep subtidal environments; (2) a higher proportion of regional diversity allocated to the within‐habitat beta component; and (3) that changes in gamma diversity are driven primarily by changes in alpha diversity during the Furongian–Tr3, whereas in the Floian, beta diversity seems to modulate regional diversity. These trends and associated patterns indicate increasing ecological differences among taxa, shifting from metacommunities where most taxa have similar ecological preferences or ‘Hubbell type’ to metacommunities with high niche differentiation or ‘Hutchinson type’. Interestingly, the timing of this shift coincides with the regional‐scale turnover between trilobite evolutionary faunas suggesting that the rise in niche differentiation among these genera may be related to the transition. Superimposed on this general trend, particular diversity structures can be understood in the light of metacommunity dynamics, such as dispersal limitation and mass effect.     

Evenness and diversity in Upper Cambrian – Lower Ordovician trilobite communities from the Central Andean Basin (Cordillera Oriental,Argentina)

Community evenness has recently received much attention, either because it is related to ecosystem functioning or because it may affect estimation of diversity. Temporal and environmental trends in diversity and evenness of trilobite communities during the Late Cambrian – Early Ordovician of the Cordillera Oriental (north‐western Argentina) are here analysed. Richness and evenness increase through time in both deep subtidal (between fair‐weather and storm wave base) and offshore (below storm wave base) communities. Two significant patterns are superimposed on this general trend: (1) the magnitude of the increase in evenness is much more pronounced in deep than in shallower settings, and (2) richness and evenness trajectories are decoupled (while a significant rise in evenness is recorded in the middle Tremadocian (Tr2), an increase in richness is delayed until the late Tremadocian (Tr3)). In contrast to expectations, a single family (Olenidae) is dominant in samples associated with this earlier rise in evenness relative to richness. Hence, this trend is explained neither by the number of families present in the communities nor by the familial identity of the most abundant taxon. Large‐scale comparisons of the timing and geographical components of these trends are restricted to the patterns recognized in Laurentian North American studies. Results from the Cordillera Oriental mirror those of Laurentia regarding the rise in both metrics in deep marine settings. Nevertheless, the timing of this increase in richness and evenness is delayed in the Cordillera Oriental, supporting the idea that palaeogeographical regions differed in the nature and timing of ecological changes. Finally, the rise in trilobite alpha‐diversity through the Late Cambrian – Early Ordovician of the Cordillera Oriental supports the idea that trilobite alpha‐diversity did not decline worldwide, suggesting that the relative decline in trilobite alpha‐diversity is most probably caused by the dilution effect.     

Stratigraphical distribution of the Ordovician graptolite Azygograptus Nicholson & Lapworth in the Central Andean Basin (northwestern Argentina and southern Bolivia)

We analyzed new occurrences of Azygograptus lapworthi from the Cordillera Oriental, Argentina. The bearer sandstones levels, corresponding to the Acoite Formation, are overlying the deposits, in which the Didymograptellus bifidus Biozone (Lower Ordovician, upper Floian, Fl3) was previously recognized, and are overlain by younger pelitic levels yielding Xiphograptus lofuensis (Middle Ordovician, early Dapingian, Dp2). Previous records from the Central Andean Basin are also reviewed in detail and accurately correlated, allowing us to conclude that the Azygograptus lapworthi Biozone corresponds to the Middle Ordovician (lower Dapingian, Dp1). This biostratigraphic framework documents that the transition between the Lower and Middle Ordovician deposits occurs in the uppermost levels of the Acoite Formation in the Argentine Cordillera Oriental. It is additionally integrated with up to date conodont records to establish a high-resolution regional correlation, with equivalent deposits from the Puna of northwestern Argentina and Cordillera Oriental of Bolivia, and to discuss new insights for global correlation.     

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.     

Conodontophorid biodiversification during the Ordovician in South China

Wu, R., Stouge, S. & Wang, Z. 2012: Conodontophorid biodiversification during the Ordovician in South China. Lethaia, Vol. 45, pp. 432–442. Analysis of the Ordovician conodontophorid diversity pattern for South China using normalized and total diversity measures reveals that diversity peaks occurred in the mid‐Tremadocian, mid‐late Floian, early Dapingian and mid‐Darriwilian periods. The conodontophorids radiated during the Floian, maintaining relatively high diversity into the early part of the Middle Ordovician until a significant diversity decrease occurred in the late Dapingian. A relatively low diversity level prevailed in the Late Ordovician. Three diversification intervals based on origination, extinction and turnover rates have been identified i.e. (1) Tremadocian to mid‐late Floian, (2) early Dapingian and (3) late Dapingian to early Darriwilian. Diversity curves for conodontophorids, brachiopods, graptolites, acritarchs and trilobites from South China are comparable during the Early Ordovician, although differences are apparent in the Middle and Late Ordovician. In South China, conodontophorid diversity reacted primarily to sea‐level changes during the Early and Middle Ordovician, when the peak of this biodiversification generally coincided with a transgression. Climate changes – especially the global cooling that occurred during the Late Ordovician glaciation – and sea‐water chemistry were also important controlling factors. □Biodiversification, conodonts, Ordovician, South China.     

Local–regional species richness relationships are linear at very small to large scales in west-central Pacific corals

If local communities are saturated with species, the relationship between local and regional species richness [the local species richness (LSR)–regional species richness (RSR) relationship] is predicted to become increasingly curvilinear at more local spatial scales. This study tested whether the LSR–RSR relationship for coral species was linear or curvilinear at three local scales across the west-central Pacific Ocean, along a regional biodiversity gradient that includes the world’s most diverse coral assemblages. The local scales comprised transects 10^0–2 m apart, sites 10^3–4 m apart and islands 10^4–6 m apart. The LSR–RSR relationship was never significantly different from linear at any scale. When the Chao1 estimator was used to predict true RSR and LSR, all relationships were also strongly linear. We conclude that local assemblages are open to regional influences even when the local scale is very small relative to the regional scale, and even in extraordinarily rich regions.     

Tempo and mode in the replacement of trilobite evolutionary faunas from the Cordillera Oriental basin (Northwestern Argentina)

Major ecological transitions among evolutionary faunas are matters of continual debate. Our study analyses the timing and mode of replacement of Ibex I and II Trilobite Evolutionary Faunas (TEF) in Lower Ordovician successions of northwestern Argentina, with special emphasis on olenids and asaphids as key groups of the Ibex I and Ibex II faunas, respectively. We explore richness by using the sample rarefaction method, and occupancy based on a presence–absence dataset. The late Tremadocian 2 (Tr2) represents an inflexion point in richness and occupancy trajectories of asaphids and olenids. While olenids diminish their generic richness through time, asaphids became the richest family since the Tr3 predating the global trend recognized for the expansion of the Ibex II Fauna. Asaphids gained diversity at the expense of other trilobites as well, their rise in diversity lags behind the increase in occupancy suggesting a displacement scenario as the probable mode of replacement between the Ibex I and II TEFs. Apparently, the processes of replacement variate among regions across the globe, configuring a complex global mosaic of regional patterns.     

Are local patterns of anthropoid primate diversity related to patterns of diversity at a larger scale?

Aims (1) To determine the relationship between local and regional anthropoid primate species richness. (2) To establish the spatial and temporal scale at which the ultimate processes influencing patterns of primate species coexistence operate. Location Continental landmasses of Africa, South America and Asia (India to China, and all islands as far south as New Guinea). Methods The local–regional species richness relationship for anthropoid primates is estimated by regressing local richness against regional richness (independent variable). Local richness is estimated in small, replicate local assemblages sampled in regions that vary in total species richness. A strong linear relationship is taken as evidence that local assemblages are unsaturated and local richness results from proportional sampling of the regional pool. An asymptotic curvilinear relationship is interpreted to reflect saturated communities, where strong biotic interactions limit local richness and local processes structure the species assemblage. As a further test of the assumption of local assemblage saturation, we looked for density compensation in high‐density local primate assemblages. Results The local–regional species richness relationship was linear for Africa and South America, and the slope of the relationship did not differ between the two continents. For Asia, curvilinearity best described the relationship between local and regional richness. Asian primate assemblages appear to be saturated and this is confirmed by density compensation among Asian primates. However, density compensation was also observed among African primates. The apparent assemblage saturation in Asia is not a species–area phenomenon related to the small size of the isolated islands and their forest blocks, since similar low local species richness occurs in large forests on mainland and/or peninsular Asia. Main conclusions In Africa and South America local primate assemblage composition appears to reflect the influence of biogeographic processes operating on regional spatial scales and historical time scales. In Asia the composition of primate assemblages are by‐and‐large subject to ecological constraint operating over a relatively small spatial and temporal scale. The possible local influence of the El Niño Southern Oscillations on the evolution and selection of life‐history characteristics among Asian primates, and in determining local patterns of primate species coexistence, warrants closer inspection.     

Hierarchical multi-scale occupancy estimation for monitoring wildlife populations

Occupancy estimation is an effective analytic framework, but requires repeated surveys of a sample unit to estimate the probability of detection. Detection rates can be estimated from spatially replicated rather than temporally replicated surveys, but this may violate the closure assumption and result in biased estimates of occupancy. We present a new application of a multi-scale occupancy model that permits the simultaneous use of presence–absence data collected at 2 spatial scales and uses a removal design to estimate the probability of detection. Occupancy at the small scale corresponds to local territory occupancy, whereas occupancy at the large scale corresponds to regional occupancy of the sample units. Small-scale occupancy also corresponds to a spatial availability or coverage parameter where a species may be unavailable for sampling at a fraction of the survey stations. We applied the multi-scale occupancy model to a hierarchical sample design for 2 bird species in the Black Hills National Forest: brown creeper (Certhia americana) and lark sparrow (Chondestes grammacus). Our application of the multi-scale occupancy model is particularly well suited for hierarchical sample designs, such as spatially replicated survey stations within sample units that are typical of avian monitoring programs. The model appropriately accounts for the non-independence of the spatially replicated survey stations, addresses the closure assumption for the spatially replicated survey stations, and is useful for decomposing the observation process into detection and availability parameters. This analytic approach is likely to be useful for monitoring at local and regional scales, modeling multi-scale habitat relationships, and estimating population state variables for rare species of conservation concern. © 2011 The Wildlife Society.     

Ranked species occupancy curves reveal common patterns among diverse metacommunities

Aim Community ecologists often compare assemblages. Alternatively, one may compare species distributions among assemblages for macroecological comparisons of species niche traits and dispersal abilities, which are consistent with metacommunity theory and a regional community concept. The aim of this meta‐analysis is to use regressions of ranked species occupancy curves (RSOCs) among diverse metacommunities and to consider the common patterns observed. Location Diverse data sets from four continents are analysed. Methods Six regression models were translated from traditional occupancy frequency distributions (OFDs) and are distributed among four equation families. Each regression model was fitted to each of 24 data sets and compared using the Akaike information criterion. The analysed data sets encompass a wide range of spatial scales (5 cm–50 km grain, 2–7000 km extent), study scales (11–590 species, 6–5114 sites) and taxa. Observed RSOC regressions were tested for the differences in scale and taxa. Results Three RSOC models within two equation families (exponential and sigmoidal) are required to describe the very different data sets. This result is generally consistent with OFD research, but unlike OFD‐based expectations the simple RSOC patterns are not related to spatial scale or other factors. Species occupancy in diverse metacommunities is efficiently summarized with RSOCs, and multi‐model inference reliably distinguishes among alternative RSOCs. Main conclusions RSOCs are simple to generate and analyse and clearly identified surprisingly similar patterns among very different metacommunities. Species‐specific hypotheses (e.g. niche‐based factors and dispersal abilities) that depend on spatial scale may not translate to diverse metacommunities that sample regional communities. A novel set of three metacommunity succession and disturbance hypotheses potentially explain RSOC patterns and should be tested in subsequent research. RSOCs are an operational approach to the regional community concept and should be useful in macroecology and metacommunity ecology.     

Evolution of shallow benthic communities during the Late Paleocene–earliest Eocene transition in the Northern Tethys (SW Slovenia)

A paleoecological and sedimentological study was carried out on shallow-water carbonates of the Kras Plateau (SW Slovenia) with the goal of reconstructing paleoenvironmental conditions and evolution of foraminiferal communities on the northwestern Adriatic Carbonate Platform (AdCP) during the Late Paleocene–earliest Eocene. Three facies have been recognized and summarized in a carbonate ramp model. Within these facies, six foraminiferal assemblages, representing different ramp sub-environments, have been defined: during the Late Paleocene sedimentation took place in a protected innermost ramp with (1) smaller miliolids- and (2) small benthic foraminifera-dominated assemblages thriving on partly vegetated, soft substrates. In the Uppermost Paleocene, sedimentation primarily occurred along a mid ramp. The upper mid-ramp was sporadically influenced by storms/currents and occupied by (3) Assilina-dominated assemblage occurring on a soft sandy substrate. The deeper mid-ramp was characterized by (4) ‘bioconstructors’- and (5) orthophragminids-dominated assemblages, colonizing biotopes with substrates of different nature. During the earliest Eocene, deposition occurred in an inner-ramp setting with (6) alveolinids-nummulitids assemblage thriving on muddy and sandy substrate, partly covered or close to seagrass beds. The Late Paleocene–earliest Eocene environmental conditions, coupled with the long-term evolution of larger benthic foraminifera (LBF), seem to have favored this low-light dependent group as common sediment contributors. By comparing the evolution of the shallow-water biota from the Adriatic area with data from the Pyrenees and Egypt, a general latitudinal trend can be recognized. However, on a smaller geographical scale, local conditions are likely to have played a pivotal role in promoting the evolution of biota characterized by suites of unique features.     

Overdispersion of body size in Australian desert lizard communities at local scales only: no evidence for the Narcissus effect

Both local and regional processes may contribute to community diversity and structure at local scales. Although many studies have investigated patterns of local or regional community structure, few have addressed the extent to which local community structure influences patterns within regional species pools. Here we investigate the role of body size in community assembly at local and regional scales in Ctenotus lizards from arid Australia. Ctenotus has long been noted for its exceptional species diversity in the Australian arid-zone, and previous studies have attempted to elucidate the processes underlying species coexistence within communities of these lizards. However, no consensus has emerged on the role of interspecific competition in the assembly and maintenance of Ctenotus communities. We studied Ctenotus communities at several hundred sites in the arid interior of Australia to test the hypothesis that body sizes within local and regional Ctenotus assemblages should be overdispersed relative to null models of community assembly, and we explored the relationship between body size dispersion at local and regional scales. Results indicate a striking pattern of community-wide overdispersion of body size at local scales, as measured by the variance in size ratios among co-occurring species. However, we find no evidence for body size overdispersion within regional species pools, suggesting a lack of correspondence between processes influencing the distribution of species phenotypes at local and regional scales. We suggest that size ratio constancy in Ctenotus communities may have resulted from contemporary ecological interactions among species or ecological character displacement, and we discuss alternative explanations for the observed patterns. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reproductive traits associated with species turnover of amphibians in Amazonia and its Andean slopes

Assembly of ecological communities is important for the conservation of ecosystems, predicting perturbation impacts, and understanding the origin and loss of biodiversity. We tested how amphibian communities are assembled by neutral and niche‐based mechanisms, such as habitat filtering. Species richness, β‐diversities, and reproductive traits of amphibians were evaluated at local scale in seven habitats at different elevation and disturbance levels in Wisui Biological Station, Morona‐Santiago, Ecuador, on the foothills of the Cordillera del Kutukú; and at regional scale using 109 localities across evergreen forests of Amazonia and its Andean slopes (0–3,900 m a.s.l.). At local scale, species composition showed strong differences among habitats, explained mainly by turnover. Reproductive modes occurred differently across habitats (e.g., prevalence of direct developers at high elevation, where breeding in ground level water disappears). At regional scale, elevation was the most important factor explaining the changes in species richness, reproductive trait occurrences, and biotic dissimilarities. Species number in all groups decreased with elevation except for those with lotic tadpoles and terrestrial reproduction stages. Seasonality, annual precipitation, and relative humidity partially explained the occurrence of some reproductive traits. Biotic dissimilarities were also mostly caused by turnover rather than nestedness and were particularly high in montane and foothill sites. Within lowlands, geographic distance explained more variability than elevation. Habitat filtering was supported by the different occurrence of reproductive traits according to elevation, water availability, and breeding microhabitats at both scales, as well as other assembly mechanisms based in biotic interactions at local scale. Human‐generated land use changes in Amazonia and its Andean slopes reduce local amphibian biodiversity by alteration of primary forests and loss of their microhabitats and the interaction network that maintains their unique amphibian assemblages with different reproductive strategies.     

Not everything is everywhere: the distance decay of similarity in a marine host–parasite system

Aim We test the similarity–distance decay hypothesis on a marine host–parasite system, inferring the relationships from abundance data gathered at the lowest scale of parasite community organization (i.e. that of the individual host). Location Twenty‐two seasonal samples of the bogue Boops boops (Teleostei: Sparidae) were collected at seven localities along a coastal positional gradient from the northern North‐East Atlantic to the northern Mediterranean coast of Spain. Methods We used our own, taxonomically consistent, data on parasite communities. The variations in parasite composition and structure with geographical and regional distance were examined at two spatial scales, namely local parasite faunas and component communities, using both presence–absence (neighbour joining distance) and abundance (Mahalanobis distance) data. The influence of geographical and regional distance on faunal/community divergence was assessed through the permutation of distance matrices. Results Our results revealed that: (1) geographical and regional distances do not affect the species composition in the system under study at the higher scales; (2) geographical distance between localities contributes significantly to the decay of similarity estimated from parasite abundance at the lowest scale (i.e. the individual host); (3) the structured spatial patterns are consistent in time but not across seasons; and (4) a restricted clade of species (the ‘core’ species of the bogue parasite fauna) contributes substantially to the observed patterns of both community homogenization and differentiation owing to the strong relationship between local abundance and regional distribution of species. Main conclusions The main factors that tend to homogenize the composition of parasite communities of bogue at higher regional scales are related to the dispersal of parasite colonizers across host populations, which we denote as horizontal neighbourhood colonization. In contrast, the spatial structure detectable in quantitative comparisons only, is related to a vertical neighbourhood colonization associated with larval dispersal on a local level. The stronger decline with distance in the spatial synchrony of the assemblages of the ‘core’ species indicates a close‐echoing environmental synchrony that declines with distance. Our results emphasize the importance of the parasite supracommunity (i.e. parasites that exploit all hosts in the ecosystem) to the decay of similarity with distance.     

Ecological relationships between phytoplankton communities and different spatial scales in European reservoirs: implications at catchment level monitoring programmes

Phytoplankton communities are structured by factors acting over temporal and spatial scales. Identifying which factors are driving spatial patterns in aquatic communities is the central aim of ecology. In this study, data sets of phytoplankton communities and environmental data of two Portuguese reservoirs types (lowland “riverine reservoirs” and higher altitude “artificial lake reservoirs”) were used to determine the importance of environmental variables at different spatial (geographical, regional and local) and time scales (seasons, years) on the community structure. In all the data sets, the multivariate ordination technique Canonical Correspondence Analysis (CCA) showed that regional and local scales explained the majority (9–18% and 13–19%, respectively) of the taxa variance. However, for “riverine reservoirs”, time variables were more important, explaining 27% of the variability in phytoplankton assemblages. Variance partitioning was used to assess the individual importance of the three spatial scales and time for the community structure of the two reservoir types. The majority of among-site variability (5.9–21.4%) was accounted for by time variables, with local, regional, and geographical scale variables accounting for 3.3–5.6%, 3.7–4.5% and 2.6–2.9%, respectively. The effects of different spatial scales on phytoplankton communities were clearly interrelated; thus, implying that phytoplankton assemblages are capable of detecting stress from catchment to site scales. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: J. Padisak     

Top predator fish assemblages in Northern Patagonia,Argentina. What factors regulate their patterns of distribution and abundance?

According to assemblage theory, three factors regulate fish biogeography: restriction of dispersion, environmental restrictions and biotic interactions. The first two factors act on a regional scale and delimit the area of action of the third, which operates on a local scale. Salmonid introductions began in Patagonia in 1904, and this has led to a restructuring of trophic webs and an increase in the number of top predators. This situation allowed us to evaluate, in a natural setting, how communities are formed on different geographic scales. We studied two large basins in Patagonia, situated close to each other but with different assemblages of top predatory fish. We hypothesized that differences in the structuring of the top predator assemblages between and within the basins are due to 1) environmental factors and dispersion processes facilitated by connectivity on a regional scale; and 2) biotic interaction (internal dynamics) between native perch and salmonids, the former acting as a modulator of the top predator assemblages on a local scale. To test these hypotheses, we analysed the top predator assemblages of 16 lakes and one reservoir, as well as their environmental characteristics. We performed a cluster analysis and related the resulting assembly groups to environmental factors by means of a tree model. We also analysed fish diets, using a similarities test to study biotic interactions. On regional and local scales, water basin, degree of connectivity, area, temperature and Zoogeographic integrity coefficient (ZIC) were important factors in the structuring of top predator assemblages. On a local scale, creole perch modulates the salmonid populations through feeding and the consequent distribution of resources. Our work showed that the structure of top predator assemblages was determined by a combination of local and regional factors acting in synergy, as postulated by the assemblage theory.     

Early Ordovician reefs in South China (Chenjiahe section, Hubei Province): deciphering the early evolution of skeletal-dominated reefs

The Lower Ordovician (late Tremadocian–early Floian) Fenhsiang and the overlying Hunghuayuan Formations at the Chenjiahe section in the Three Gorges area of Hubei Province, South China, include four types of reef: microbe-dominated (lithistid sponge–stromatolite and lithistid sponge–calcimicrobial) reefs, and skeletal-dominated (lithistid sponge–bryozoan and bryozoan–pelmatozoan) reefs. The microbe-dominated reefs are characterized by the dominance of microbial sediments that encrusted and bound the surfaces of sponges to reinforce the reef frameworks. In contrast, the skeletal-dominated reefs are distinguished by bryozoans that encrusted frame-building sponges and pelmatozoans, and that grew downward to fill the open spaces available within the frameworks. A series of these reefs shows a temporal succession in reef type, with a decline in the lithistid sponge–stromatolite reefs and an increase in the lithistid sponges and receptaculitids within the lithistid sponge–calcimicrobial reefs in the Hunghuayuan Formation; the lithistid sponge–bryozoan reefs are common in both the Fenhsiang and Hunghuayuan Formations. These features of the Chenjiahe reefs are in marked contrast to other coeval reefs on the Yangtze Platform and elsewhere. Skeletal-dominated reefs first developed in the Three Gorges and adjacent areas, located on the central part of the platform. Likewise, lithistid sponges and receptaculitids first developed in the Three Gorges area and then expanded their range. In contrast, stromatolites declined over time, but remained abundant on a marginal part of the platform. The spatial–temporal distributions of these reefs on the Yangtze Platform reflect the initiation of the Great Ordovician Biodiversification Event and its consequences, although influenced by local environmental conditions. The Three Gorges area was a center for the development of skeletal-dominated reefs, which were established earlier here than elsewhere in the world. These reef types and their spatial–temporal successions provide invaluable clues to the earliest evolution of skeletal-dominated reefs and their ensuing development during the Middle–Late Ordovician.     

New insights into the paleobiogeography of the Early Ordovician graptolite fauna of northwestern Argentina

A multivariate analysis was used to determine the faunistic relationships of the northwestern Argentinean graptolite faunas of Floian age to faunas from Baltica, Avalonia, Laurentia and SW China. A statistical analysis at the species level of the five geographic regions for the Lower Floian was performed with the classic Jaccard's index. The resulting affinity dendrogram shows stronger relationships between Early Floian graptolite faunas of northwestern Argentina and those from Baltica, with less obvious similarities to Great Britain (Avalonia) faunas and only weak affinities to North American (Laurentia) and Chinese (SW China) graptolite faunas. The statistical analysis confirms the paleobiogeographic relationships previously observed in other areas of the Cordillera Oriental, and supports the hypothesis that during the Early Ordovician, northwestern Argentina was located at middle to high latitudes, corresponding to the Atlantic Faunal Realm of cold water graptolite biofacies. The studied graptolite material from the Quinilicán and Agua Chica sections is preserved in shales and siltstones interbedded with fine to medium-grained sandstones corresponding to the lower part of the Acoite Formation. The biostratigraphic implications of the associated graptolites are briefly discussed, and Trichograptus dilaceratus (Herrmann), Acrograptus gracilis (Törnquist), Expansograptus latus (T.S. Hall) and Corymbograptus v-fractus tullbergi (Monsen) are described from the Argentine Cordillera Oriental for the first time.     

Modern vegetation–pollen–climate relationships for the Pampa grasslands of Argentina

Aim To analyse the relationships between potential natural vegetation, pollen and climate in order to improve the interpretation of fossil pollen records and provide the background for future quantitative palaeoclimatic reconstructions. Location Pampa grasslands of Argentina, between 33–41° S and 56–67° W. Methods Modern pollen data were obtained from a pollen data base developed by the Grupo de Investigación de Paleoecología y Palinología, Universidad Nacional de Mar del Plata, Argentina (143 surface samples and 17 pollen types). Analysis of pollen and climate data involved multivariate statistics (cluster analysis and principal components analysis), scatter diagrams, Pearson’s correlation and isopoll mapping. Results Vegetation patterns at regional scales (grasslands and xerophytic woodlands) and local scales (edaphic communities) were identified by cluster analysis of pollen surface samples. The main climatic variables that appear to constrain the vegetation distribution and abundance of taxa are mean annual precipitation, annual effective precipitation and summer temperature. Individual pollen types such as Chenopodiaceae, Apiaceae, Cyperaceae, Prosopis, Schinus, Condalia microphylla and other xerophytic taxa are good indicators of moisture regime. Many pollen types are significantly correlated with summer temperature. The modern vegetation–pollen–climate relationships vary in a broadly predictable manner, supporting the contention that fossil pollen assemblages can be related to particular climatic characteristics. Main conclusions An expanded suite of modern analogues facilitated new insights into vegetation–pollen–climate relationships at the regional scale in Pampa grasslands. Relationships between individual pollen types and climate are appraised at a regional scale and new modern analogues are presented. The results provide the basis for improved vegetation and climate reconstruction from fossil records of the study area.