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Stromatolites just after the end-Permian extinction are exceptionally well developed in the Chongyang area of Hubei Province, South China. The stromatolites include endolithic coccoidal microbes and exhibit three growth forms: columnar-layered, pseudo-columnar, and columnar, each located in different stratigraphic horizons. The columnar type is the most conspicuous and provides key information on the mode of their construction. In this paper, we describe the mega- to microscopic features of these stromatolites together with interpretations of their construction. The columnar type typically comprises alternations of lighter and darker laminae with intercalations of dense micritic laminae. The lighter laminae are composed of fine to coarse calcite cement and fine dolomite, whereas the darker laminae are composed of micrite and microspar. Endolithic coccoids are sporadic in these laminae. In contrast, the dense micritic laminae consist of micrite with abundant endolithic coccoids, accentuating the laminae of stromatolites. The columnar stromatolites were formed under deteriorating conditions after the end-Permian mass extinction as follows: (1) the alternating development of lighter and darker laminae due to the colonization of different microbial communities in lower-energy, shallow- to deep-subtidal settings; (2) carbonate precipitation within each microbial community through microbial metabolic activity and degradation of extracellular polymeric substances (EPS), leading to the alternations of lighter and darker laminae; (3) episodic interruption of the growth of the lighter and darker laminae by endolithic coccoidal activity; and (4) high levels of endolithic activity, accompanied by the precipitation of carbonate within boreholes to produce the dense micritic laminae. In contrast, the columnar-layered and pseudo-columnar types comprise alternations of lighter and darker laminae, composed mainly of fine to coarse calcite cement, fine dolomite, micrite, peloids, and microspar with sporadic endolithic coccoids. The dense micritic laminae are absent or rare in these stromatolites. These types were essentially produced by the repetition of processes (1) and (2) documented in the columnar type. The finely alternating laminae of these stromatolites are therefore the product of the repeated initiation and cessation of growth of relevant microbial communities in response to changing environmental conditions. The stromatolites were formed immediately after the end-Permian extinction and persisted intermittently throughout the Early Triassic. The Chongyang stromatolites are undoubtedly some of the best-developed stromatolites near the Permian–Triassic boundary and retain a high-resolution record of microbial successions and palaeo-oceanic perturbations immediately following the end-Permian extinction. [ABSTRACT FROM AUTHOR]

Abstract: The archaeocyath-bearing reefs that first appeared in South China in the Atdabanian were last seen in the lower Toyonian (e.g., in the Tianheban and Jindingshan formations). The upper Toyonian–lower Amgan Qingxudong Formation of Hunan Province examined in this study includes large-sized reefs formed solely by calcimicrobes of Epiphyton, Kordephyton, Girvanella, Hedstroemia, and Renalcis. During the middle and upper Cambrian, small-scale stromatolite reefs were formed. The Qingxudong reefs are therefore interpreted to be the most well-developed microbial reefs of the post archaeocyath-bearing reefs in the late early Cambrian. The transition from archaeocyath-bearing to purely microbial reefs was caused by a decline in the diversity of metazoans, and was accompanied by a reduction in the abundance of reefs, possibly related in part to late early Cambrian regression. The late early Cambrian reefs of South China contain a well-preserved record of reef succession, and provide important insights into the factors that led to the demise of archaeocyath-bearing reefs and the subsequent long-lasting hindrances to the development of skeletal-dominated reefs during the middle and late Cambrian. [Copyright &y& Elsevier]

Abstract: The Lower Ordovician records a transition from microbial- to metazoan-dominant reefs immediately prior to the Great Ordovician Biodiversification Event. The Hunghuayuan Formation (late Tremadocian-early Floian) of the Jianxin section in Anhui Province, South China, includes well-preserved reefs, which consist of a combination of microbial boundstones and microbial-lithistid sponge-receptaculitid boundstones. The microbial boundstones are characteristically made up of nodular or columnar stromatolites produced by an irregular alternation of Girvanella-rich layers and thick micrite layers in which the filamentous calcimicrobe Girvanella and bioclasts occur sporadically. Microbial biofilms, including the filamentous microbes, may have trapped and bound lime mud and/or precipitated micrite during their growth and metabolic activities. These activities contemporaneously contributed to substrate stabilisation, encrustation, and the construction of microbialite frameworks (stromatolites). In contrast, the microbial-lithistid sponge-receptaculitid boundstones are characterised by more micritic constituents than skeletal reef-building constituents. Thin and/or domal crusts produced by peloid and dense micrite, together with various proportions of Girvanella, occur upon bioclastic sediments and reef-building skeletal organisms. These micritic crusts with Girvanella therefore played roles as binders and stabilisers, and as consolidators by encrusting reef-building skeletal organisms. In the boundstones, it is remarkable that the degradation of lithistid sponges, due to microbial decay, produced micrite in the sponge bodies. As a result, lithistid sponges played an apparently inconspicuous but potentially significant role as micrite producers as well as automicrite framework constructors. These Lower Ordovician reefs, in which microbialites (e.g., stromatolites) dominate but coexist with metazoans (e.g., sponges), were widespread at the time. They provide excellent examples for understanding reefal palaeoecology during the transition from microbe-dominant reefs to reefs enriched in metazoans. [Copyright &y& Elsevier]

A dramatic shift in microbial reefs occurred around the Ediacaran–Cambrian boundary. Here we describe changes in the composition, construction, and texture of microbial reefs in the Zavkhan Terrane of Gobi-Altai Province, western Mongolia, during the late Ediacaran and early Cambrian. Stromatolites consisting of peloids, micritic clots, and homogeneous lime mud without calcified microbes (calcimicrobes) are characteristic of the upper Ediacaran (units 9 and 16A of the Zuun-Arts Formation). In contrast, abundant thrombolites with stromatolites occur in the lowest Terreneuvian (units 17A and 17 of the Bayan Gol Formation). These Cambrian microbial reefs are made up of micritic clots and homogeneous lime mud in close association with calcimicrobes including Korilophyton , Renalcis , and Tarthinia. The thrombolites and calcimicrobial reefs studied herein occur directly stratigraphically above strata that record a strong negative shift in δ13C values and are dominated by small shelly fossils; these are the earliest known calcimicrobial reef representatives of the Phanerozoic. These microbial reefs changed almost simultaneously with drastic fluctuations in environmental conditions (e.g., seawater chemistry, Ca concentration, carbonate saturation, and oxygen level). These changes would have been influenced by the evolution of calcimicrobes and skeletal metazoans across the Ediacaran–Cambrian boundary. The present work provides crucial geobiological information on substantial shifts at the Ediacaran–Cambrian boundary and how calcimicrobes and related textures appeared in tandem with the innovation of biomineralisation. • Stromatolites without calcimicrobes occur in the late Ediacaran. • Calcimicrobe-bearing thrombolites first appeared in the earliest Cambrian. • Calcimicrobial reefs formed after a strong negative shift in δ13C values and SSFs. • Microbial reefs changed almost synchronously with fluctuations in marine conditions. [ABSTRACT FROM AUTHOR]

• Late Ediacaran Boxonia -bearing stromatolites occur in the Zuun-Arts Formation of western Mongolia. • Stromatolites comprise mainly micritic components, including peloids, clot-1 and clot-2, and homogeneous lime mud. • Varied micritic components have originated from in situ precipitation through microbial activities. • These stromatolites may provide clues to the variation in microbial metabolic activities and degradation processes. A couplet comprising a Boxonia -bearing stromatolite unit and phosphatic layers occurs widely at the base of the Zuun-Arts Formation in the Zavkhan Terrane of Gobi-Altay Province, western Mongolia. The stromatolite unit is late Ediacaran in age and forms bioherms of several kilometres in lateral extent. The stromatolites consist of two parts: the lower columnar Boxonia stromatolites (ca. 7 m thick) change abruptly into the upper domed stromatolites (ca. 4.5 m thick) in the Bayan Gol Gorge. The columnar stromatolites are made up of columnar structures (2–6 cm in diameter), formed by the accumulation of upward-convex laminae, and partly protrude laterally to form bridges between neighbouring columns. In contrast, the domed stromatolites (30–60 cm in width) are composed of accumulations of low convex laminae. Both types of stromatolite are characterized by alternating darker and lighter laminae, which consist of peloids, two types of micritic clot, homogeneous lime mud, and spar-filled fenestral fabrics. These stromatolites are inferred to have been deposited in a subtidal setting below the wave base, with no evidence of sediment displacement by strong currents. The stratigraphic transition from columnar to domed growth forms reflects deepening of the basin. The change in column diameter observed within the columnar stromatolites might reflect fluctuations in microbial activity. The appearance of inter-columnar bridges in the columnar stromatolites might indicate more active colonization of microbial mats, which expanded into the inter-columnar areas. Neither type of stromatolite includes microbial remains, such as filamentous or coccoidal cells. However, the various micritic components of the stromatolites (clots, peloids, and homogeneous lime mud) originated from in situ precipitation through microbial activity, and reflected differences in the timing and intensity of microbial calcification and degradation. These stromatolites, including the variety of micritic components, may provide clues to the variations in microbial metabolic activity and degradation processes that were associated with stromatolite formation. [ABSTRACT FROM AUTHOR]