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The Frasnian-Famennian transition of the Late Devonian was one of the most critical intervals in the Phanerozoic. Sulphur isotopic pairs of carbonate-associated sulphate and pyrite sulphide from coeval sections in South China and Poland reveal frequent perturbations of sulphur cycling during this time interval. These data suggest a sudden oceanic overturn during a rapid sea-level fall probably induced by jerky block tilting in the latest Frasnian. This event was followed by long-lasting photic-zone euxinia during a rapid sea-level rise in the earliest Famennian. Large increases in continental nutrient fluxes, and subsequent primary productivity and organic burial, could have greatly enhanced bacterial sulphate reduction, producing excessive sulphide through the water columns owing to iron depletion. Subsequently, rapid ventilation of oceanic basins occurred, during which direct aerobic oxidation of sulphide into sulphate predominated in bottom waters and even surface sediments with minimal fractionation. This oxygenation was probably induced by intensive climatic cooling and/or large-scale sea-level fall. The temporal coincidence of two extinction phases with the oceanic overturn and succeeding photic-zone euxinia suggests that these extreme oceanic events played an important role in the severe biotic crisis. Furthermore, photic-zone euxinia coupled with subsequent climatic cooling may have delayed post-extinction recovery of some taxa.

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New specimens representing species of Cyrtiopsis Grabau, 1923, described by Grabau (1931a) were recovered in samples from the Late Devonian deposits of the Changlongjie Member of the Xikuangshan Formation in central Hunan Province of South China. Associated Famennian age conodonts and morphotypes of the rhynchonellid brachiopod Yunnanellina lineage fix the ranges of species of Cyrtiopsis and related taxa in the Xikuangshan Formation within the interval of the Middle triangularis Zone through the Lower crepida Zone (early Famennian) in central Hunan Province.Detailed study and comparison of external and internal shell structures of the type species of Cyrtiopsis Grabau, 1923, and related species described in Grabau (1931a) shows that Cyrtiopsis from South China includes the type species, C. davidsoni (embodied by the holotype), and C graciosa. Both of these species are shown to lack a delthyrial plate. Grabau's paratypes of C. davidsoni are considered to belong in Cyrtiopsis intermedia Grabau, 1931. The species C. kayseri Grabau, 1931, and C. shensiensis Grabau, 1931, are also deemed synonyms of C. davidsoni Grabau, 1923 (as restricted above). “Cyrtiopsis” intermedia differs from other species of Cyrtiopsis by development of a short delthyrial plate and its systematic position remains uncertain.The genus Pseudocyrtiopsis is proposed with Cyrtiopsis spiriferoides Grabau, 1931, as the type. Pseudocyrtiopsis new genus is distinguished from Cyrtiopsis Grabau, 1923, by its alate hinge and development of a delthyrial plate. Comparisons of species of Cyrtiopsis from South China with cyrtospiriferid brachiopods identified as Cyrtiopsis by Crickmay (1952) demonstrate that none of the early Famennian species from western Canada can be assigned to the genus as revised here, although C. mimetes Crickmay, 1952, is closely related to “C.” intermedia Grabau, 1931.