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Available data on the temporal and geographical distribution of Middle–early Late Ordovician trilobites in South China were reviewed under a high-resolution biostratigraphical framework to investigate the diversity dynamics and explain the faunal turnover and radiation pattern of trilobites during the Middle Ordovician. Three macroevolutionary phases are recognized on the basis of the diversity changes at taxonomic levels. Cluster analysis shows three trilobite groups, two distinct trilobite faunas and three faunal turnovers through nine time slices. Three trilobite groups (Groups A, B and C) are coupled with three macroevolutionary phases, and characterized by different proportions of the Whiterock Fauna. Two distinct trilobite faunas (Early–Middle Ordovician and Late Ordovician Trilobite Faunas) and three faunal turnovers could be recognized at different similarity coefficients, their replacements were mainly affected by changes of sea level and substrate conditions. The trilobite radiation was manifested by the development, expansion and domination of the Whiterock Fauna (part of the Paleozoic Evolutionary Fauna) through the Middle Ordovician. The first diversity acme of trilobites occurred during the Dapingian–early Darriwilian, while the radiation centre gradually shifted from inner platform to outer platform environments. From the end Darriwilian onwards, elements of the Whiterock Fauna had further developed and dominated all environmental settings from the inner platform to the lower slope, coinciding with the second and more extensive radiation of trilobites in South China. Two phases of radiation may have been directly triggered by two major transgressions during the late Dapingian–early Darriwilian and the late Darriwilian–early Sandbian respectively. In addition, climate cooling and various substrate conditions, as well as the increased oxygen levels in the atmosphere and marine water, nutrients and tectonic movements during the Middle Ordovician may have provided essential conditions for the trilobite radiation. • Three macroevolutionary phases of South China trilobites based on diversity changes. • Heatmap of trilobites based on cluster analysis shows trilobite groups and faunal turnovers. • Development, expansion and domination of the Whiterock Fauna along environmental gradients. • Status of the Whiterock Fauna in the macroevolution of the Ordovician trilobite and Paleozoic Fauna. • Trilobite radiations of South China were closely related to sea level changes. [ABSTRACT FROM AUTHOR]

The decimeter-scale 3-D network structure of the Upper Ordovician Pagoda Formation in South China has been an enigmatic sedimentary feature and a case of non-uniformitarian carbonate depositional system, without modern analog. The network bands are developed pervasively throughout the entire Pagoda Formation, superficially resembling desiccation or syneresis cracks, especially in bedding-plane view, with decimeter-scale sub-polygonal to irregularly sinuous patterns. Detailed petrographic analysis, however, shows that the crack-like network bands have the same carbonate matrix, skeletal grains, and overall fabrics as the rock they surround. In this study, we demonstrate that the network structure is not generated by shrinkage cracks but is rather a time-specific lithofacies. It originated from prolonged growth of nodular carbonate, controlled by the convergence of several geological and paleoclimatic factors, including a) a protracted sea-level highstand lasting for about 5 Myr, b) an interval of tectonic quiescence favoring the dominance of carbonate production, with minimal siliciclastic input, and c) location of the South China paleoplate in the hurricane-free paleoequatorial zone devoid of severe storm disturbances between the fairweather and maximum storm wave bases (normally 15–120 m depths), favoring long-term, uninterrupted growth of CaCO 3 meganodules on a mid-shelf substrate. This study corroborates recent paleogeographic reconstructions that position the South China paleoplate on the Equator during the Late Ordovician. [ABSTRACT FROM AUTHOR]

During the Early-Middle Ordovician, the Yangtze Platform in South China was drowned, as indicated by the replacement of pure grey grainstones and packstones of the Hunghuayuan Formation by purple red, argillaceous limestone of the Zitai Formation and correlative units, which are mixed siliciclastic and carbonate deposits. The Xiangshuidong Section, located in Songzi City, southwestern Hubei Province, was selected to study the process in detail. Cluster analysis of point-count groups of the Hunghuayuan and the Zitai formations, indicates that the former contains six microfacies: MF-1 peloidal grainstone, MF-2 thrombolitic grainstone, MF-3 oolitic grainstone, MF-4 bioclastic grainstone, MF-5 lithoclastic grainstone, MF-6 interreef bioclastic grain-packstone, indicating a shelf setting before drowning, while the latter contains seven microfacies: MF-7 bioclastic grainstone, MF-8 bioclastic grain- to packstone, MF-9 bioclastic packstone, MF-10 bioclastic wackestone, MF-11 argillaceous bioclastic wackestone, MF-12 open-marine bioclastic wackestone, MF-13 calcimudstone, indicating the ramp setting during and after the drowning. The drowning was gradual and fluctuating, starting as a marginal shoal, through the inner ramp, and becoming a stable ramp setting. This drowning is mainly attributed to a global sea-level rise, and an increase of terrigenous supply that resulted from local tectonic movement (i.e. the initiation of collision between the South China and Cathaysia blocks). After drowning, red offshore marine facies were well developed along the platform margin, indicating an oxic sea bottom environment during the first acme of GOBE in South China. The changes in environments associated with this transition may have provided new evolutionary opportunities that contributed to the Middle Ordovician diversification. [ABSTRACT FROM AUTHOR]

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]