华南泥盆系法门阶浅海相四射珊瑚的组合序列

晚泥盆世弗拉期末F-F的生物灭绝事件,导致珊瑚以及许多浅海底栖生物灭绝,几乎整个法门期珊瑚群都处于残存阶段,华南地区目前只在湖南的个别地方发现少量的Smithiphyllum。直到法门期末珊瑚和其它一些造礁底栖生物开始复苏,出现了不少与典型泥盆纪珊瑚存在很大差别的新分子。华南泥盆纪最晚期(Strunian)的珊瑚可划分成上、下两个组合:上部Cystophrentis组合;下部Eocaninophyllum组合。泥盆—石炭纪之交的另一次生物灭绝事件,使新生的泥盆纪最晚期的珊瑚又遭灭绝。     

Extinction and diversification in the Devonian Brachiopoda of New York state: No correlation with sea level?

Sea level highstand is generally considered to promote high species diversities among marine organisms through habitat expansion and global climatic amelioration, and marine regression to trigger elevated extinction rates among marine benthic organisms by habitat reduction (the species‐area effect), and among both marine and terrestrial organisms by global climatic deterioration. The Devonian is unusual in that the Late Devonian mass extinction occurs during an interval of global sea level highstand. To further explore this anomaly, the potential relationship between relative sea level and evolutionary biology is analyzed here for the Brachiopoda of the Devonian Period. Successive linear modeling reveals a total lack of correlation between relative sea level and either origination rates, extinction rates, or standing diversity among the Devonian brachiopods.     

Microbes and mass extinctions: paleoenvironmental distribution of microbialites during times of biotic crisis

Widespread development of microbialites characterizes the substrate and ecological response during the aftermath of two of the 'big five' mass extinctions of the Phanerozoic. This study reviews the microbial response recorded by macroscopic microbial structures to these events to examine how extinction mechanism may be linked to the style of microbialite development. Two main styles of response are recognized: (i) the expansion of microbialites into environments not previously occupied during the pre-extinction interval and (ii) increases in microbialite abundance and attainment of ecological dominance within environments occupied prior to the extinction. The Late Devonian biotic crisis contributed toward the decimation of platform margin reef taxa and was followed by increases in microbialite abundance in Famennian and earliest Carboniferous platform interior, margin, and slope settings. The end-Permian event records the suppression of infaunal activity and an elimination of metazoan-dominated reefs. The aftermath of this mass extinction is characterized by the expansion of microbialites into new environments including offshore and nearshore ramp, platform interior, and slope settings. The mass extinctions at the end of the Triassic and Cretaceous have not yet been associated with a macroscopic microbial response, although one has been suggested for the end-Ordovician event. The case for microbialites behaving as 'disaster forms' in the aftermath of mass extinctions accurately describes the response following the Late Devonian and end-Permian events, and this may be because each is marked by the reduction of reef communities in addition to a suppression of bioturbation related to the development of shallow-water anoxia.     

新疆准噶尔盆地西北缘洪古勒楞组时代的新认识

标准地点的洪古勒楞组长期以来一直被认为是准噶尔盆地西北缘,乃至整个新疆北部以海相为主的层位上最高的晚泥盆世晚期的沉积。然而牙形类和微体脊椎动物的研究证实,该组跨弗拉阶－法门阶（Ｆ－Ｆ）界线,它至少包括了一小部分弗拉晚期和主体部分为法门早期的沉积。两个牙形类带ＬａｔｅｒｈｅｎａｎａＺｏｎｅ和ＭｉｄｄｌｅｃｒｅｐｉｄａＺｏｎｅ被识别出,它们在Ｆ－Ｆ界线附近,并分别在大绝灭事件（在ｌｉｎｇｕｉｆｏｒｍｉｓＺｏｎｅ内）之前后。另一个层位更高的牙形类带ＬａｔｅｃｒｅｐｉｄａＺｏｎｅ也可能存在。同时还证实,长期以来一直被认为“正常层序”的标准地点的洪古勒楞组的层序是倒转的。由此,含植物大化石Ｌｅｐｔｏｐｈｌｏｅｕｍｒｈｏｂｉｃｕｍ等的陆相层在下（此层可能归于该组下伏的朱鲁木特组）,而产牙形类和其它丰富的无脊椎动物的海相层在上。按牙形类年代带的年代地层时间表（Ｆｏｒｄｈａｍ,１９９２）,该组距今约３６８－３６４百万年。至于它和邻近地区和布克河组的关系,由于后者发现了更高层位的牙形类,两者还不能完全对比。     

The fate of the homoctenids (Tentaculitoidea) during the Frasnian–Famennian mass extinction (Late Devonian)

The homoctenids (Tentaculitoidea) are small, conical‐shelled marine animals that are among the most abundant and widespread of all Late Devonian fossils. They were a principal casualty of the Frasnian–Famennian (F‐F, Late Devonian) mass extinction, and thus provide an insight into the extinction dynamics. Despite their abundance during the Late Devonian, they have been largely neglected by extinction studies. A number of Frasnian–Famennian boundary sections have been studied, in Poland, Germany, France, and the USA. These sections have yielded homoctenids, which allow precise recognition of the timing of the mass extinction. It is clear that the homoctenids almost disappear from the fossil record during the latest Frasnian ‘Upper Kellwasser Event’. The coincident extinction of this pelagic group, and the widespread development of intense marine anoxia within the water column, provides a causal link between anoxia and the F‐F extinction. Most notable is the sudden demise of a group, which had been present in rock‐forming densities, during this anoxic event. One new species, belonging to Homoctenus is described, but is not formally named here.     

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

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

Fungal abundance spike and the Permian-Triassic boundary in the Karoo Supergroup (South Africa)

The most severe mass extinction of marine species and terrestrial vertebrates and plants is associated with the Permian-Triassic boundary (∼251 Ma). The extinction interval is also marked by the disappearance of most Late Permian gymnosperm palynomorphs at a layer containing solely the abundant remains of fungi. This ‘fungal spike’ apparently represents widespread devastation of arboreous vegetation. Stratigraphic and palynological study of the Carlton Heights section in the southern Karoo Basin of South Africa revealed a 1-m-thick fungal spike zone that occurs simultaneously with the last appearance of typically Late Permian gymnosperm pollen. The plant extinction and fungal spike zone are found above the last occurrence of Late Permian mammal-like reptiles of the Dicynodont Zone at other Karoo sections. Using the fungal event as a time line in marine and non-marine sections allows placement of the marine extinctions and the extinction of terrestrial plants and reptiles within a brief crisis interval of less than about 40?000 years at the end of the Permian.     

Paleolatitudes in the Devonian of Brazil and the Frasnian-Famennian mass extinction

Differences in faunal composition of contemporary Devonian marine communities in Brazilian sedimentary basins suggest variation because of climatic gradients. Temperatures may have ranged from subarctic conditions in the Parana Basin, a type of Devonian Hudson's Bay, to temperate in the Amazon and adjoining basins, analogous to a modern “north Atlantic” climate. These gradients were paralleled in other parts of South America. The Malvinokaffric cold climate fauna was dominated by groups oforganisms that survived the worldwide late Devonian mass extinctions. Invertebrate groups absent in the Malvinokaffric regions of Brazil, Bolivia and Argentina (i.e. those restricted to Devonian equatorial belts of North America, Eurasia and Australia) were decimated in the late Devonian. These two parallel observations suggest that late Devonian extinction may have been the result of drastic cold spells that killed off reefal and peri-reefal life. By elimination of crucial benthic community components, the trophic structure of the shallow marine ecosystem of the time was upset. Subsequent repopulation of the Carboniferous seas was accomplished by hardy, eurythermal invertebrate taxa present in cold as well as tropical regions. Cold spells and mass mortality in very shallow waters may go some way to explain the production of the widespread black shale environments so typical of marine regressive phases in the late Devonian of the western hemisphere.     

Evolutionary and biogeographical shifts in response to the Late Ordovician mass extinction

The Late Ordovician mass extinction was an interval of high extinction with inferred low ecological selectivity, resulting in little change in community structure after the event. In contrast, the mass extinction may have fundamentally changed evolutionary dynamics in the surviving groups. We investigated the phylogenetic relationships among strophomenoid brachiopods, a diverse brachiopod superfamily that was a primary component of Ordovician ecosystems. Four Ordovician families/subfamilies sampled in the analysis (Rafinesquinidae, Strophomeninae, Glyptomenidae and Furcitellinae) were reconstructed as monophyletic groups, and the base of the strophomenoid clade that dominated the Silurian recovery was reconstructed as diversifying alongside these families during the Middle Ordovician. We time‐calibrated the phylogeny and used geographical occurrences to investigate biogeographical changes in the strophomenoids through time with the R package BiogeoBEARS . Our results indicate that extinction was higher in taxa whose ranges were constrained to tropical or subtropical regions. Furthermore, our results suggest important shifts in the diversification patterns of these brachiopods after the mass extinction. While most of the strophomenoid families survived the Late Ordovician event, ecologically abundant taxonomic groups during the Ordovician were either driven to extinction, reduced in diversity, or slowly died off during the Silurian. The new abundant strophomenoid taxa derived from one clade (consisting of Silurian–Devonian groups such as Douvillinidae, Strophodontidae and Amphistrophiidae) that diversified during the post‐extinction radiation. Our results suggest the selective diversification during the Silurian radiation, rather than selective extinction in the Late Ordovician, had a greater impact on the evolutionary history of strophomenoid brachiopods.     

Stepwise and large-magnitude negative shift in δ13Ccarb preceded the main marine mass extinction of the Permian–Triassic crisis interval

Large perturbations to the global carbon cycle occurred during the Permian–Triassic boundary mass extinction, the largest extinction event of the Phanerozoic Eon (542 Ma to present). Controversy concerning the pattern and mechanism of variations in the marine carbonate carbon isotope record of the Permian–Triassic crisis interval (PTCI) and their relationship to the marine mass extinction has not been resolved to date. Herein, high-resolution carbonate carbon isotope profiles (δ¹³C_carb), accompanied by lithofacies, were generated for four sections with microbialite (Taiping, Zuodeng, Cili, and Chongyang) in South China to better constrain patterns and controls on δ¹³C_carb variation in the PTCI and to test hypotheses about the temporal relationship between perturbations to the global carbon cycle and the marine mass extinction event. All four study sections exhibit a stepwise negative shift in δ¹³C_carb during the Late Permian–Early Triassic, with the shift preceding the end-Permian crisis being larger (> 3‰) than that following it (1–2‰). The pre-crisis shifts in δ¹³C_carb are widely correlatable and, hence, represent perturbations to the global carbon cycle. The comparatively smaller shifts following the crisis demonstrate that the marine mass extinction event itself had at most limited influence on the global carbon cycle, and that both Late Permian δ¹³C_carb shifts and the mass extinction must be attributed to some other cause. Their origin cannot be uniquely determined from C-isotopic data alone but appears to be most compatible with a mechanism based on episodic volcanism in combination with collapse of terrestrial ecosystems and soil erosion.     

A stable and productive marine microbial community was sustained through the end‐Devonian Hangenberg Crisis within the Cleveland Shale of the Appalachian Basin,United States

The end‐Devonian Hangenberg Crisis constituted one of the greatest ecological and environmental perturbations of the Paleozoic Era. To date, however, it has been difficult to precisely constrain the occurrence of the Hangenberg Crisis in the Appalachian Basin of the United States and thus to directly assess the effects of this crisis on marine microbial communities and paleoenvironmental conditions. Here, we integrate organic and inorganic chemostratigraphic records compiled from two discrete outcrop locations to characterize the onset and paleoenvironmental transitions associated with the Hangenberg Crisis within the Cleveland Shale member of the Ohio Shale. The upper Cleveland Shale records both positive carbon (δ¹³C_org) and nitrogen (δ¹⁵N_total) isotopic excursions, and replenished trace metal inventories with links to eustatic rise. These dual but apparently temporally offset isotope excursions may be useful for stratigraphic correlation with other productive end‐Devonian epeiric marine locations. Deposition of the black shale succession occurred locally beneath a redox‐stratified water column with euxinic zones, with signs of strengthening denitrification during the Hangenberg Crisis interval, but with an otherwise stable and algal‐rich marine microbial community structure sustained in the surface mixed layer as ascertained by lipid biomarker assemblages. Discernible trace fossil signals in some horizons suggest, however, that bioturbation and seafloor oxygenation occurred episodically throughout this succession and highlight that geochemical proxies often fail to capture these rapid and sporadic redox fluctuations in ancient black shales. The paleoenvironmental conditions, source biota, and accumulations of black shale are consistent with expressions of the Hangenberg Crisis globally, suggesting this event is likely captured within the uppermost strata of the Cleveland Shale in North America.     

Invasive species and biodiversity crises: testing the link in the late devonian

During the Late Devonian Biodiversity Crisis, the primary driver of biodiversity decline was the dramatic reduction in speciation rates, not elevated extinction rates; however, the causes of speciation decline have been previously unstudied. Speciation, the formation of new species from ancestral populations, occurs by two primary allopatric mechanisms: vicariance, where the ancestral population is passively divided into two large subpopulations that later diverge and form two daughter species, and dispersal, in which a small subset of the ancestral population actively migrates then diverges to form a new species. Studies of modern and fossil clades typically document speciation by vicariance in much higher frequencies than speciation by dispersal. To assess the mechanism behind Late Devonian speciation reduction, speciation rates were calculated within stratigraphically constrained species-level phylogenetic hypotheses for three representative clades and mode of speciation at cladogenetic events was assessed across four clades in three phyla: Arthropoda, Brachiopoda, and Mollusca. In all cases, Devonian taxa exhibited a congruent reduction in speciation rate between the Middle Devonian pre-crisis interval and the Late Devonian crisis interval. Furthermore, speciation via vicariance is almost entirely absent during the crisis interval; most episodes of speciation during this time were due to dispersal. The shutdown of speciation by vicariance during this interval was related to widespread interbasinal species invasions. The lack of Late Devonian vicariance is diametrically opposed to the pattern observed in other geologic intervals, which suggests the loss of vicariant speciation attributable to species invasions during the Late Devonian was a causal factor in the biodiversity crisis. Similarly, modern ecosystems, in which invasive species are rampant, may be expected to exhibit similar shutdown of speciation by vicariance as an outcome of the modern biodiversity crisis.     

Testing reduced evolutionary rates during the Late Palaeozoic Ice Age using the crinoid fossil record

In 2003, Stanley & Powell reported depressed rates of origination and extinction in marine invertebrates during the Late Palaeozoic Ice Age (LPIA). Using a database of crinoid genera, rates of origination, extinction and genus duration were calculated at the stage level from the Early Devonian to the Late Permian. This 165 m.y. time span includes non‐glacial intervals before and after the LPIA, which spanned the Serpukhovian to Sakmarian, providing background rates for comparison. Data generated on crinoid evolutionary rates during the Middle to Late Palaeozoic were analysed and compared to Stanley & Powell's data to determine whether crinoid evolutionary patterns support their findings or suggest an alternative hypothesis. Rates of origination and extinction in all crinoid clades were reduced during the LPIA compared to the combined background intervals before and after the LPIA. However, crinoid diversity was higher during the LPIA than the surrounding time intervals. The difference in diversity trends between crinoids and other marine invertebrates is due to the advanced cladids clade. Unstable, fluctuating environmental conditions during the LPIA may have created habitats suitable for opportunistic crinoid genera that reduced both the probability of origination and extinction. The increased diversity of the advanced cladids is likely due to their unique adaptation of muscular arm articulations, which allowed them to thrive in marine settings with increased siliciclastic influx brought on by the Alleghenian orogeny. Despite the advanced cladids’ departure from the expected diversity count, the results of analyses performed on the updated crinoid database provide independent confirmation of Stanley & Powell's original hypothesis of depressed evolutionary rates in marine invertebrates during the LPIA.     

Volcanic perturbations of the marine environment in South China preceding the latest Permian mass extinction and their biotic effects

The Dongpan section in southern Guangxi Province records the influence of local volcanic activity on marine sedimentation at intermediate water depths (~200-500 m) in the Nanpanjiang Basin (South China) during the late Permian crisis. We analyzed ~100 samples over a 12-m-thick interval, generating palynological, paleobiological, and geochemical datasets to investigate the nature and causes of environmental changes. The section records at least two major volcanic episodes that culminated in deposition of approximately 25- to 35-cm-thick ash layers (bentonites) and that had profound effects on conditions in both the Dongpan marine environment and adjacent land areas. Intensification of eruptive activity during each volcanic cycle resulted in a shift toward conifer forests, increased wildfire intensity, and elevated subaerial weathering fluxes. The resulting increase in nutrient fluxes stimulated marine productivity in the short term but led to a negative feedback on productivity in the longer term as the OMZ of the Nanpanjiang Basin expanded, putting both phytoplankton and zooplankton communities under severe stress. Radiolarians exhibit large declines in diversity and abundance well before the global mass extinction horizon, demonstrating the diachroneity of the marine biotic crisis. The latest Permian crisis, which was probably triggered by the Siberian Traps flood basalts, intensified the destructive effects of the earlier local eruptions on terrestrial and marine ecosystems of the South China craton.     

川东北二叠纪长兴晚期微生物丘:大灭绝事件过程的征兆?

虽然二叠系-三叠系界线附近的微生物岩是研究生态环境转折期海洋环境的热点素材,但以往所发现的实例都赋存于显生宙最大灭绝事件界线之上,还未报道过位于灭绝界线之下的微生物岩。川东北地区长兴组上部发育骨架礁层位之上的微生物丘,其中以尖山微生物丘最为典型,具三个生长旋回。根据微生物丘地层中所含牙形刺,和有孔虫,其时代为长兴晚期,处于大灭绝界线之下,其顶部距Hindeodus parvus带7.5m。碳同位素及微量元素分析揭示,从晚二叠世晚期开始,海洋生态环境条件的幕式不稳定性就已经显现并持续发展,导致生物危机也呈多阶段性特征。川东北晚二叠世微生物丘记录见证了重大地质转折期将至时生物与环境的相互作用关系。     

Composition and dynamics of the great Phanerozoic Evolutionary Floras

Factor analysis of a data set representing the global distribution of vascular plant families through time shows the broad pattern of vegetation history can be explained in terms of five Evolutionary Floras. The Rhyniophytic (=Eotrachyophytic) Flora represents the very earliest (Silurian and earliest Devonian) vascular plants, notably the Rhyniophytopsida. The Eophytic Flora represents the early (Early–Middle Devonian) mainly homosporous land plants, notably the Zosterophyllopsida, Trimerophytopsida and early Lycopsida. The Palaeophytic Flora represents the Late Devonian and Carboniferous vegetation, which saw the introduction of heterospory among the spore producing plants and of early gymnosperms. The Mesophytic Flora first appeared in the Late Carboniferous and Permian macrofossil record, although there is palynological evidence of these plants having grown earlier in extra‐basinal habitats and was dominated by gymnosperms with more modern affinities. The Cenophytic Flora that first appeared during Cretaceous times was overwhelmingly dominated by angiosperms. The end‐Devonian, end‐Triassic and end‐Cretaceous mass‐extinction events recognized in the marine fossil record had little impact on the diversity dynamics of these Evolutionary Floras. Rather, the changes between floras mainly reflect key evolutionary innovations such as heterospory, ovules and angiospermy.     

Onshore expansion of benthic communities after the Late Devonian mass extinction

A detailed ichnological analysis of the Upper Devonian–Lower Mississippian Bakken Formation of sub‐surface Saskatchewan and the partially coeval Exshaw Formation of Alberta indicates the presence of an anomalous ichnofacies gradient. The distal Cruziana Ichnofacies, which in rocks of other ages is restricted to lower‐offshore facies, here ranges from this setting to the lower shoreface. No archetypal Cruziana Ichnofacies is present in these deposits. This pattern is interpreted as resulting from the differential effects of the Late Devonian mass extinction in shallow‐water ecosystems. The onshore expansion evidenced by ichnological data is consistent with the pattern displayed by the body‐fossil record, which indicates a re‐invasion of shallow‐water environments by the Palaeozoic evolutionary fauna during the Late Devonian and into the Early Carboniferous. The ichnofauna studied is overwhelmingly dominated by deposit feeders, with suspension feeders being notably absent, further underscoring the importance of trophic type as a selectivity trait during mass extinctions.     

Late Early Triassic climate change: Insights from carbonate carbon isotopes,sedimentary evolution and ammonoid paleobiogeography

The late Early Triassic sedimentary–facies evolution and carbonate carbon-isotope marine record (δ¹³C_carb) of ammonoid-rich, outer platform settings show striking similarities between the South China Block (SCB) and the widely distant Northern Indian Margin (NIM). The studied sections are located within the Triassic Tethys Himalayan belt (Losar section, Himachal Pradesh, India) and the Nanpanjiang Basin in the South China Block (Jinya section, Guangxi Province), respectively. Carbon isotopes from the studied sections confirm the previously observed carbon cycle perturbations at a time of major paleoceanographic changes in the wake of the end-Permian biotic crisis. This study documents the coincidence between a sharp increase in the carbon isotope composition and the worldwide ammonoid evolutionary turnover (extinction followed by a radiation) occurring around the Smithian–Spathian boundary.Based on recent modeling studies on ammonoid paleobiogeography and taxonomic diversity, we demonstrate that the late Early Triassic (Smithian and Spathian) was a time of a major climate change. More precisely, the end Smithian climate can be characterized by a warm and equable climate underlined by a flat, pole-to-equator, sea surface temperature (SST) gradient, while the steep Spathian SST gradient suggests latitudinally differentiated climatic conditions. Moreover, sedimentary evidence suggests a transition from a humid and hot climate during the Smithian to a dryer climate from the Spathian onwards. By analogy with comparable carbon isotope perturbations in the Late Devonian, Jurassic and Cretaceous we propose that high atmospheric CO₂ levels could have been responsible for the observed carbon cycle disturbance at the Smithian–Spathian boundary. We suggest that the end Smithian ammonoid extinction has been essentially caused by a warm and equable climate related to an increased CO₂ flux possibly originating from a short eruptive event of the Siberian igneous province. This increase in atmospheric CO₂ concentrations could have additionally reduced the marine calcium carbonate oversaturation and weakened the calcification potential of marine organisms, including ammonoids, in late Smithian oceans.     

The late Devonian trilobite crises

Mid‐Devonian to end‐Late Devonian trilobites of different taxonomic categories are updated as to their actual stratigraphical range with respect to the internationally defined stage boundaries. The main palaeogeographical and ecological occurrences are summarized. Numerical analyses emphasize the clear relationship between fluctuations in diversity and global eustatic events. Already declining in diversity from the early mid‐Devonian, shallow‐water communities became most restricted during the mid‐Givetian Taghanic transgression. After a phase of adaptive radiation, off‐shore trilobite communities were severely affected during the mid‐ and end‐Late Devonian crises. From an initial 5 orders 3 were lost at the end‐Frasnian Kellwasser crisis while only 1 from the remaining 2 orders survived the Devonian‐Carboniferous boundary Hangenberg event. In both cases extinction was preceded by a unidirectional evolutionary trend in eye reduction accompanied by impoverishment of lower rank taxa. This phenomenon is obviously a result of selective adaptation under constant long‐lasting environmental influences. Specialization to obligate epi‐ or even endo‐benthic life habit, however, led fatally to extinction when stable conditions became substantially perturbed. Sudden sea‐level changes with subsequent break in the REDOX‐equilibrium took place at the Kellwasser and Hangenberg events, which were most probably responsible for trilobite mass extinctions.     

Palaeoecology and diversity of endosymbionts in Palaeozoic marine invertebrates: Trace fossil evidence

Endosymbionts are organisms that live within the growing skeleton of a live host organism, producing a cavity called a bioclaustration. The endosymbiont lives inside the bioclaustration, which it forms by locally inhibiting the normal skeletal growth of the host, a behaviour given the new ethological category, impedichnia. As trace fossils, bioclaustrations are direct evidence of past symbioses and are first recognized from the Late Ordovician (Caradoc). Bioclaustrations have a wide geographic distribution and occur in various skeletal marine invertebrates, including tabulate and rugose corals, calcareous sponges, bryozoans, brachiopods, and crinoids. Ten bioclaustration ichnogenera are recognized and occur preferentially in particular host taxa, suggesting host-specificity among Palaeozoic endosymbionts. The diversity of bioclaustrations increased during the Silurian and reached a climax by the late Middle Devonian (Givetian). A collapse in bioclaustration diversity and abundance during the Late Devonian is most significant among endosymbionts of host coral and calcareous sponge taxa that were in decline leading up to the Frasnian-Famennian mass extinction.