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1.
Changes of community structure in response to competition usually take place on timescales that are much too short to be visible in the geological record. Here we report the notable exception of a benthic marine community in the wake of the end‐Permian mass extinction, which is associated with the microbial limestone facies of the earliest Triassic of South China. The newly reported fauna is well preserved and extraordinarily rich (30 benthic macroinvertebrate species, including the new species Astartella? stefaniae (Bivalvia) and Eucochlis obliquecostata (Gastropoda)) and stems from an environmentally stable setting providing favourable conditions for benthic organisms. Whereas changes in the taxonomic composition are negligible over the observed time interval of 10–100 ka, three ecological stages are identified, in which relative abundances of initially rare species continuously increased at the cost of previously dominant species. Concomitant with the changes of dominant species is an increase in faunal evenness and heterogeneity. In the absence of both environmental and taxonomic changes, we attribute this pattern to the long‐term effects of interspecific competition, which acted at an unusually slow pace because the number of competing species and potential immigrants was dramatically reduced by the end‐Permian mass extinction. We suggest that these non‐actualistic conditions led to decreased rates of niche differentiation and hence to the delayed rediversification of benthos that characterizes the aftermath of the greatest Phanerozoic mass extinction event. A hyperbolic diversification model is proposed, which accounts for the positive relationship between the intensity of interspecific competition and the rate of niche differentiation and resolves the conundrum of delayed rediversification at a time when niche space was largely vacated.  相似文献   

2.
Characterizing marine water bodies and defining ecological status, both present and past (pre-impacted), has become an important task for EU's Member States and their associates during the last decade due to the implementation of the Water Framework Directive (WFD). However, none of the methods used to define Ecological Quality Status (EcoQS) are able to accurately define the status for both the present-day and reference conditions at a given site (i.e., in situ). Recent studies have revealed a significant correlation between the diversity of living (stained) fossilizable benthic foraminifera (protists) and associated environmental parameters (e.g., dissolved oxygen concentration). The present study takes this relationship a step further by applying methods used to define present-day EcoQS on fossil benthic foraminiferal assemblages and, thereby, defining past EcoQS (PaleoEcoQS). This is particularly useful for defining reference condition in areas where biological- and instrumental time-series are limited or lacking. Our case study from the Oslofjord, Norway, shows that (1) the “Foraminiferal method” can define temporal developments in in situ EcoQS from reference to present-day conditions, (2) results of the “Foraminiferal method” reflect available historical biological records and hydrographic time series, (3) data (1993 and 2009) on macrofauna (traditional bio-monitoring tool) and benthic foraminifera from the same sites define the same EcoQS, and (4) the changes in foraminiferal diversity through time are due to human activity (pollution), rather than climate change. Using in situ data to define ecological reference conditions is preferable compared to modeling or comparisons with present-day supposedly similar reference conditions.  相似文献   

3.
在区域气候变暖的背景下,干旱事件引起的水位频繁波动以及流域开发导致的污染物输入均可以直接威胁水库的生态安全。本研究以云南省太平水库为对象,通过对沉积物的物理(粒度、烧失量)、化学(碳、氮元素)、生物(硅藻群落)等代用指标的分析,并结合监测数据和调查资料,重建了该水库1937—2018年的生态环境变化过程,并识别了硅藻群落的演替特征及其环境影响因子。粒度结果指示,太平水库在筑坝期间(1956—1984年)水动力逐渐增强,而后水动力条件持续减弱,与文献记录的水文调控历史基本一致。总氮、总碳和有机质含量较为一致地记录了水库建设期间水体营养水平的下降、水库建成后初级生产力和内源有机质的上升过程。硅藻群落呈现出“浮游种-底栖种-浮游种”交替占优的演化模式。驱动太平水库硅藻群落演替的主要环境因子为气候变化、水动力条件和营养水平。在气候长期变暖的背景下,严格控制外源营养输入并合理开展水文调控是维持水库水体生态健康和环境安全的重要前提。  相似文献   

4.
Fossil pollen as a record of past biodiversity   总被引:7,自引:0,他引:7  
Quaternary pollen records may contribute uniquely to the understanding of present plant diversity. Pollen assemblages can reflect diversity at community and landscape scales but the time resolution of most studies does not match that of modern ecological studies. Because of the complicating effects of differential pollen productivity and dispersal, pollen records do not directly reflect equitability aspects of vegetation diversity. Vegetation diversity indices other than S (the total number of taxa) are therefore not appropriate for pollen assemblages. As a measure of the species richness palynological richness is biased by the lack of taxonomic precision, by a possible interference on pollen dispersal from vegetation structure and by pollen representation. The nonlinear relationship between species richness and pollen-taxa richness may be used in attempts to estimate past floristic richness from fossil pollen assemblages. Using a hypothetical example the strong effect of cover shifts in the vegetation affecting taxa with different representation (Rrel) values on observed palynological richness is demonstrated. It is suggested that estimates of relative pollen productivity should be used to guide the pollen sum on which pollen-type richness is estimated by rarefaction techniques and this approach is illustrated using a paired site study of late Holocene diversity dynamics. The need for a modern training set relating pollen-type richness to species richness, pollen productivity and vegetation structure is emphasized.  相似文献   

5.
Paleoecology allows construction of paleoenvironmental models, faunal changes and evolutionary trends of paleontological taxa using modern analogs. However, when linking modern analogs to paleontological taxa in paleoecological reconstruction, differential taxonomic preservation in the fossil record has to be taken into account. Paleontologists have known the biased nature of the fossil record since Efremov’s publication on taphonomy in 1940, yet many ecological models of habitats associated with hominins in paleontological and archaeological sites in Africa and elsewhere barely address the complexity of the fossil record. We use randomly sampled ungulates from modern biomes in a comparative taxonomic abundance to demonstrate how the combination of modern thanatocoenoses and taphocoenoses, when used in reference to habitat-specific biocoenosis, produce better inferences of past habitats in paleontological and archaeological sites than approaches currently used.  相似文献   

6.
Climate change is threatening tropical reefs across the world, with most scientists agreeing that the current changes in climate conditions are occurring at a much faster rate than in the past and are potentially beyond the capacity of reefs to adapt and recover. Current research in tropical ecosystems focuses largely on corals and fishes, although other benthic marine invertebrates provide crucial services to reef systems, with roles in nutrient cycling, water quality regulation, and herbivory. We review available information on the effects of environmental conditions associated with climate change on noncoral tropical benthic invertebrates, including inferences from modern and fossil records. Increasing sea surface temperatures may decrease survivorship and increase the developmental rate, as well as alter the timing of gonad development, spawning, and food availability. The broad latitudinal distribution and associated temperature ranges of several pantropical taxa suggest that some reef communities may have an in‐built adaptive capacity. Tropical benthic invertebrates will also show species‐specific sublethal and lethal responses to sea‐level rise, ocean acidification, physical disturbance, runoff, turbidity, sedimentation, and changes in ocean circulation. In order to accurately predict a species' response to these stressors, we must consider the magnitude and duration of exposure to each stressor, as well as the physiology, mobility, and habitat requirements of the species. Stressors will not act independently, and many organisms will be exposed to multiple stressors concurrently, including anthropogenic stressors. Environmental changes associated with climate change are linked to larger ecological processes, including changes in larval dispersal and recruitment success, shifts in community structure and range extensions, and the establishment and spread of invasive species. Loss of some species will trigger economic losses and negative effects on ecosystem function. Our review is intended to create a framework with which to predict the vulnerability of benthic invertebrates to the stressors associated with climate change, as well as their adaptive capacity. We anticipate that this review will assist scientists, managers, and policy‐makers to better develop and implement regional research and management strategies, based on observed and predicted changes in environmental conditions.  相似文献   

7.
星云湖硅藻群落响应近现代人类活动与气候变化的过程   总被引:4,自引:0,他引:4  
随着人类活动的增强与全球气候变暖的持续,近年来云南湖泊的生态系统功能持续退化,而目前对云南湖泊生态系统的研究还主要集中于单一环境压力的生态效应。以星云湖为研究对象,通过沉积物记录与现代监测资料,识别在湖泊富营养化、气候变化以及人类强烈干扰下硅藻群落结构响应的过程,并甄别驱动群落变化的主要环境压力及其强度。结果显示随着湖泊生产力水平(如沉积物叶绿素a浓度)的增加,硅藻物种组成发生了明显的变化,主成分分析表明了水体富营养化是驱动群落变化的主要环境因子(r=-0.63,P0.001)。简约模型与方差分解的结果表明近200年来(钻孔长度38cm),湖泊营养水平和水动力是驱动星云湖硅藻群落变化的主要环境因子,分别解释了群落变化的18.8%和2.9%;而1951年以后,湖泊营养水平和温度分别解释了硅藻群落结构变化的31.4%和26.8%。研究结果表明了硅藻群落长期变化的主控因子是湖泊营养水平,而人类活动及气候变化等可以通过改变湖泊水动力及湖水温度来驱动硅藻群落的演替,同时抚仙湖-星云湖的连通性也对硅藻群落的演替产生了一定影响。  相似文献   

8.
多重环境压力下大屯海硅藻群落结构的长期变化   总被引:1,自引:1,他引:0  
云南亚热带高原湖泊在过去几十年内面临着工业污染、富营养化、滩地围垦、极端干旱等多重环境压力的影响.本研究以大屯海为研究对象,结合沉积物硅藻、粒度、碳氮同位素以及年代序列等数据进行了多指标分析,重建并识别了大屯海近百年来生态环境变化的历史和硅藻群落变化的特征.结果表明: 近百年来硅藻群落组合出现了较大的转变,优势种由连接脆杆藻转变为极细微曲壳藻.结合多指标环境记录(如碳氮同位素)和现代监测记录,采用排序分析以及方差分解发现,工业污染和湖泊营养盐富集是大屯海硅藻群落结构长期变化的主要驱动因子.此外,沉积物粒度分析结果与气象数据显示,由于大屯海受到修建大坝以及多次干旱事件的影响,湖泊水动力与水体交换能力减弱,从而使沉积物硅藻群落结构也发生了相应的变化.  相似文献   

9.
10.
Monitoring changes in population levels of a wide range of species in biodiversity research and conservation requires practical, easy-to-use and efficient assessment and monitoring methods. Dragonflies (Insecta: Odonata) are a valuable tool for assessing aquatic systems and have been used as indicators of ecological health, ecological integrity, and environmental change, including climatic change, as well as indicators of habitat recovery. We field-tested a freshwater ecological integrity index, the Dragonfly Biotic Index (DBI), based on dragonfly assemblages at the local scale, and compared the DBI to a biodiversity index (average taxonomic distinctness, AvTD) as well as to a standard freshwater benthic macroinvertebrate-based freshwater health index (South African Scoring System, using Average Score Per Taxon, ASPT). We sampled 20 river sites, selected a priori. Adult dragonflies and benthic macroinvertebrates were collected using standardized methods. Environmental variables were collected in situ, and water samples taken. Temperature and pH were the most important physical environmental variables in explaining the assemblage structure, and we found significant abiotic–biotic relationships, as well as biotic–biotic relationships. Overall, dragonflies were more sensitive to changes in river condition than were macroinvertebrates, in part because they were responding at the species rather than higher taxonomic level. AvTD scores did not show any significant relationship with changes in river condition. Furthermore, sites with low biotic scores (indicating disturbance) had high AvTD values. In contrast, DBI site value and ASPT scores were highly significantly correlated. We conclude that dragonfly assemblages in the form of a DBI are an excellent tool for environmental assessment and monitoring freshwater biodiversity, with the potential to replace labour-intensive benthic macroinvertebrate-based freshwater quality assessments, such as SASS.  相似文献   

11.
树轮稳定同位素比率能有效地记录树木生长过程中气候环境变化信息及树木的生理响应机制。年内高分辨率树轮稳定同位素比率则能够提供更为详细的气候环境信息,揭示树木对季节尺度气候环境变化的生理生态响应机制,在古气候和全球变化生态学研究方面显示出巨大的潜力。本文收集了1990年以来发表的树轮年内高分辨率稳定同位素比率研究论文,从样品剥离方法、α-纤维素化学提取方法以及应用等方面综述了其研究进展,展望了年内高分辨率树轮稳定同位素记录研究的潜力和未来发展方向。  相似文献   

12.
The available taxonomic expertise and knowledge of species is still inadequate to cope with the urgent need for cost‐effective methods to quantifying community response to natural and anthropogenic drivers of change. So far, the mainstream approach to overcome these impediments has focused on using higher taxa as surrogates for species. However, the use of such taxonomic surrogates often limits inferences about the causality of community patterns, which in turn is essential for effective environmental management strategies. Here, we propose an alternative approach to species surrogacy, the “Best Practicable Aggregation of Species” (BestAgg), in which surrogates exulate from fixed taxonomic schemes. The approach uses null models from random aggregations of species to minimizing the number of surrogates without causing significant losses of information on community patterns. Surrogate types are then selected in order to maximize ecological information. We applied the approach to real case studies on natural and human‐driven gradients from marine benthic communities. Outcomes from BestAgg were also compared with those obtained using classic taxonomic surrogates. Results showed that BestAgg surrogates are effective in detecting community changes. In contrast to classic taxonomic surrogates, BestAgg surrogates allow retaining significantly higher information on species‐level community patterns than what is expected to occur by chance and a potential time saving during sample processing up to 25% higher. Our findings showed that BestAgg surrogates from a pilot study could be used successfully in similar environmental investigations in the same area, or for subsequent long‐term monitoring programs. BestAgg is virtually applicable to any environmental context, allowing exploiting multiple surrogacy schemes beyond stagnant perspectives strictly relying on taxonomic relatedness among species. This prerogative is crucial to extend the concept of species surrogacy to ecological traits of species, thus leading to ecologically meaningful surrogates that, while cost effective in reflecting community patterns, may also contribute to unveil underlying processes. A specific R code for BestAgg is provided.  相似文献   

13.
The analysis of sedimentary chironomid assemblages is an approach that has been widely adopted for inferring past environmental conditions. However, there is an ongoing discussion in the literature about whether this approach could become more sensitive at detecting past environmental changes if paleolimnologists conducted finer taxonomic analyses of their specimens. To advance this discussion, we conducted comparative analyses of larval chironomid assemblages resolved to two levels of taxonomic resolution. For this exercise, we chose to use live assemblages (as opposed to sub-fossil assemblages) because fine taxonomic resolution of live assemblages is more easily obtained. Our specific aims were to (i) determine if finely resolved taxa comprising a coarsely resolved group have similar ecological niches, (ii) determine if different environmental predictors of community composition are identified when two different levels of taxonomic resolution are applied and (iii) evaluate whether the variance explained by environmental variables differs substantially between levels of taxonomic resolution. We found that there was substantial dispersion among finely resolved taxa belonging to a single coarse group, which suggests that the merging of these taxa results in the loss of ecological information, and therefore warrants higher taxonomic precision. However, the identification of significant environmental predictors and the proportion of variance explained by these did not differ greatly between our two levels of taxonomic resolution. Overall our results show that coarse-resolution analyses may be adequate for some applications, but if the aim is to infer subtle environmental changes (as is the case in most paleolimnological studies) we recommend the highest possible level of taxonomic resolution.  相似文献   

14.
Environmental conditions and productivity changes in the southeastern Okhotsk Sea have been reconstructed for the last 20 ka using planktonic and benthic foraminiferal oxygen isotope records and calcium carbonate, organic carbon and opal content data from two sediment cores. Species variability in benthic foraminiferal and diatom assemblages provides additional palaeoceanographic evidence. AMS radiocarbon dating of the sediments and oxygen isotope stratigraphy serve as the basis for the age models of the cores for the last 20 14C kyr and for correlation between environmental variations in the Okhotsk Sea, and regional and global climate changes. Benthic foraminiferal assemblages in the two cores (depth 1590 and 1175 m) varied with time, so that we could recognise seven zones with different species composition. Changes in the benthic foraminiferal assemblages parallel major environmental and productivity variations. During the last glaciation, fluxes of organic matter to the sea floor showed strong seasonal variations, indicated by the presence of abundant A. weddellensis and infaunal Uvigerina spp. Benthic foraminiferal assemblages changed with warming at 12.5–11 and 10–8 14C kyr BP, when productivity blooms and high organic fluxes were coeval with global meltwater pulses 1A and 1B. Younger Dryas cooling caused a decline in productivity (11–10 kyr BP) affecting the benthic faunal community. Subsequent warming triggered intensive diatom production, opal accumulation and a strong oxygen deficiency, causing significant changes in benthic fauna assemblages from 5.26–4.4 kyr BP to present time.  相似文献   

15.
《Marine Micropaleontology》2006,60(2):157-166
Deep-sea benthic foraminifera have been quantitatively analyzed in samples (> 125 μm size fraction) from Ocean Drilling Program (ODP) Site 238, to understand paleoceanographic changes in the Central Indian Basin over the past 1.9 Myr. Factor and cluster analyses of the 25 highest-ranked species made it possible to identify five biofacies, characterizing distinct deep-sea environmental settings. The environmental interpretation of each biofacies is based on the ecology of recent deep-sea benthic foraminifera. The benthic faunal record indicates fluctuating deep-sea conditions in environmental parameters including oxygenation, surface productivity and organic food supply. These changes appear to be linked to Indian summer monsoon variability, the main climatic feature of the Indian Ocean region. The benthic assemblages show a major shift at ∼ 0.7 to 0.6 Ma, marked by major turnovers in the relative abundances of species, coinciding with an increased amplitude of glacial cycles. These cycles appear to have influenced low latitude monsoonal climate as well as deep-sea conditions in the Central Indian Ocean Basin.  相似文献   

16.
1. Sedimentary remains of aquatic plants, both vegetative (turions, leaves, spines) and reproductive (fruits, seeds, pollen), may provide a record of temporal changes in the submerged vegetation of lakes. An independent assessment of the degree to which these remains reflect past floristic change is, however, rarely possible. 2. By exploiting an extensive series of historical plant records for a small shallow lake we compare plant macrofossil (three cores) and pollen (one core) profiles with the documented sequence of submerged vegetation change since c. 1750 AD. The data set is based on 146 site visits with 658 observations including 42 taxa classified as aquatic, spanning 250 years. 3. Approximately 40% of the historically recorded aquatic taxa were represented by macro‐remains. In general macrofossils underestimated past species diversity, with pondweeds (three of eight historically recorded Potamogeton species were found) particularly poorly represented. Nonetheless, several taxa not reported from historical surveys (e.g. Myriophyllum alterniflorum and Characeae) were present in the sediment record. 4. The pollen record revealed taxa which left no macro‐remains (e.g. Littorella uniflora), and the macrofossil record provided improved taxonomic resolution for some taxa (e.g. Potamogeton) and a more reliable record of persistence, appearance and loss of others (e.g. Myriophyllum spp. and Nymphaeaceae). 5. Detrended correspondence analysis indicated that changes in the community composition evidenced by the palaeolimnological and historical records were synchronous and of a similar magnitude. Both records pointed to a major change at around 1800, with the historical record suggesting a more abrupt change than the sedimentary data. There was good agreement on a subsequent change c. 1930. 6. The palaeolimnological data did not provide a complete inventory of historically recorded species. Nevertheless, these results suggest that combined macrofossil and pollen records provide a reliable indication of temporal change in the dominant components of the submerged and floating‐leaved aquatic vegetation of shallow lakes. As such palaeolimnology may provide a useful tool for establishing community dynamics and successions of plants over decadal to centennial timescales.  相似文献   

17.
Caribbean coral reefs have transformed into algal-dominated habitats over the past half-century, but the role of specific anthropogenic drivers is unresolved due to the lack of ecosystem-level data predating human disturbance. To better understand the extent and causes of long-term Caribbean reef declines, we produced a continuous 3000-yr record of the ecosystem state of three reefs in Bocas del Toro, Caribbean Panama. From fossils and sediments obtained from reef matrix cores, we tracked changes in reef accretion rates and the taxonomic and functional group composition of fish, coral, urchin, bivalve and benthic foraminifera. This dataset provided a comprehensive picture of reef community and environmental change. At all sites, reefs shifted from systems with greater relative abundance of herbivorous fish, epifaunal suspension feeding bivalves and Diadema urchins to systems with greater relative abundance of micropredator fish, infaunal bivalves and Echinometra urchins. These transitions were initiated a millennium ago at two less-degraded reefs fringing offshore islands and ~250 yr ago at a degraded patch reef near the continental coast. Ecosystem shifts were accompanied by a decline in reef accretion rates, and at the patch reef, a decline in water quality since the 18th century. Within all cores, synchronous increases in infaunal bivalves and declines in herbivorous fish regardless of water quality suggest a loss of hard substrate and increasingly hypoxic sediment conditions related to herbivore loss. While the early timing of ecosystem transitions at the fringing reefs implicates large-scale hydrological change, the more recent timing of change and loss of water quality at the patch reef implicates terrigenous runoff from land-clearing. Our whole-ecosystem reconstruction reveals that reef ecosystem deterioration appears to follow a predictable trajectory whether driven by natural or anthropogenic disturbances and that historical local human activities have quickly unraveled reefs at a scale similar to longer-term natural environmental change.  相似文献   

18.
Paleopathology has revealed much about disease in the past but is usually limited to conditions with osteological manifestations; this often excludes acute soft tissue infections and causes of death for most individuals in the past and present. Our understanding of the evolution of disease is essential for contextualizing and predicting the epidemiological shifts that are happening in modern society, as high rates of infectious disease coexist alongside high rates of chronic disease in rates unlike those observed previously in human history. Moreover, many physiological states not previously classified as “disease” (obesity) have become pathologized, influencing our conception of disease and what defines health. By using the Galler Collection, a pre‐antibiotic and pre‐chemotherapeutic osteological series with modern autopsy records, our research quantifies disease burden of the past using the Charlson Index (CI), a modern comorbidity index of disease severity. Galler Collection remains and autopsy records were scored with the Charlson Index to correlate bone findings with soft tissue findings, and statistical analysis was performed for cumulative scores and absolute diagnosis counts, with patients stratified by sex and cause of death (pneumonia or cancer). Osteological diagnosis counts were more predictive of soft‐tissue autopsy disease counts than were associated cumulative CI scores. Diagnosis counts and CI scores for osteological data were more closely related to associated soft tissue data for cancer patients than for pneumonia patients. This research indicates how interdisciplinary paleopathological analysis assists in making more reliable assessments of health and mortality in the past, with implications for trending and predicting future epidemiological shifts. Am J Phys Anthropol 154:215–221, 2014. © 2014 Wiley Periodicals, Inc.  相似文献   

19.
Terrestrial ecosystems originate when particular plant species attain dominance at specific locations under specific environmental regimes. Ecosystems terminate, gradually or abruptly, when the dominant species or functional types are replaced by others, usually owing to environmental change or severe and irreversible disturbance. Assessing whether current ecosystems are sustainable in the face of future environmental change can be aided by examining the range of environmental variation those ecosystems have experienced in the past, and by determining the environmental conditions under which those ecosystems arose. The range of environmental variation depends on the time scale at which it is assessed. A narrow time span (e.g. 200–300 years) may underestimate the range of variation within which an ecosystem is sustainable, and it may also underestimate the risk of major transformation or disruption of that ecosystem by environmental change. Longer time spans (e.g. 1000–2000 years) increase the range of variation, by encompassing a larger sample of natural variability as well as non‐stationary variability in the earth system. Most modern ecosystems disappear when the time span is expanded to 10000–15 000 years owing to secular changes in earth's climate system. Paleo‐ecological records can pinpoint the time of origination of specific ecosystems, and paleo‐environmental records can reveal the specific environmental changes that led to development of those ecosystems and the range of environmental variation under which those ecosystems have maintained themselves in the past. This information can help identify critical environmental thresholds beyond which specific modern ecosystems can no longer be sustained.  相似文献   

20.
Taxonomic and ecological recovery from the Cretaceous–Palaeogene (K–Pg) mass extinction 66 million years ago shaped the composition and structure of modern ecosystems. The timing and nature of recovery has been linked to many factors including palaeolatitude, geographical range, the ecology of survivors, incumbency and palaeoenvironmental setting. Using a temporally constrained fossil dataset from one of the most expanded K–Pg successions in the world, integrated with palaeoenvironmental information, we provide the most detailed examination of the patterns and timing of recovery from the K–Pg mass extinction event in the high southern latitudes of Antarctica. The timing of biotic recovery was influenced by global stabilization of the wider Earth system following severe environmental perturbations, apparently regardless of latitude or local environment. Extinction intensity and ecological change were decoupled, with community scale ecological change less distinct compared to other locations, even if the taxonomic severity of the extinction was the same as at lower latitudes. This is consistent with a degree of geographical heterogeneity in the recovery from the K–Pg mass extinction. Recovery in Antarctica was influenced by local factors (such as water depth changes, local volcanism, and possibly incumbency and pre‐adaptation to seasonality of the local benthic molluscan population), and also showed global signals, for example the radiation of the Neogastropoda within the first million years of the Danian, and a shift in dominance between bivalves and gastropods.  相似文献   

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