The end-Permian mass extinction was followed by the formation of an enigmatic rock layer with a distinctive macroscopic spotted or dendroid fabric. This deposit has been interpreted as microbial reef rock, digitate dendrolite, digital thrombolite, dendritic thrombolite, or bacterial deposits. Agreement has been reached in considering them as microbialites, but not in their formation. This study has revealed that the spotted and dendroid microbialites were composed of numerous fossil casts formed by the planktic cyanobacterium, Microcystis, a coccoid genus that at the present-day commonly forms blooms in modern lakes, rivers, and reservoirs. The abundance of the fossils and the diagenesis they experienced has determined the macroscopic fabric: where they abundant, the rock appears as dendroid, otherwise, it appears as spotted. The ancient Microcystis bloom might produce toxin to kill other metazoans, and be responsible for the oceanic anoxia that has puzzled so many researchers for so many years.
Our current understanding of the Cambrian carbon cycle mostly comes from geologically rapid carbon isotope excursions (CIEs) of marine carbonate. In addition to carbon cycle indicators, stable carbon isotopes of marine carbonates can be used as stratigraphic correlation tools. Previous studies have recognized 10 CIEs in the Cambrian, but only 3 of them appear to have global significance. This limits the application of CIEs in chronostratigraphic correlation of Cambrian successions with the 10 successive stages. Here, we present a continuous carbon isotope record from a stratigraphically complete Cambrian section at Xiaoerbulak, Aksu region in the Tarim Basin, Xinjiang, China. Its comparison with more than 40 sections across the world shows the carbon isotope record of the Cambrian can be divided into 10 units based on the variation patterns. Each pattern has occurrences in at least two Cambrian tectonic plates, suggesting global significance. A generalized curve consisting of the 10 patterns, Carbon Isotope Pattern Curve (CIPC) has been compiled. Based on the fossil records, the 10 patterns are correlated with the 10 stages of the chronostratigraphic system of the Cambrian. The pattern Angularianis correlated with Fortunian Stage and the lower half of Stage 2, Dampingian with the upper part of Stage 2, Boxian with Stage 3, Rabbitian with Stage 4, Chairian with the lower half of Wuliuan, Zigzagian with the upper Wuliuan Stage, Stickian with the Drumian and Guzhangian stages, Towerian with the Paibian Stage and Lower Jiangshan Stage, Shelfian with Upper Jiangshan Stage and the lower half of Stage 10, and Hillian with the upper half of Stage 10.We suggest that the application of CIPC makes the chronostratigraphy of non‐fossiliferous successions realizable.
The problematic calcareous microfossil Halysis is abundant in the Middle Ordovician Darriwilian Stage of the western edge of the Ordos Basin, North China. The rich and well-preserved specimens of Halysis in this area facilitate detailed studies for its skeletal construction and tube microstructure. Halysis differs from calcified cyanobacteria and calcareous red and green algae in morphology, skeletal construction and microstructure, as well as reproduction mode. Halysis typically consists of multiple juxtaposed parallel tubes arranged in sheets (‘multiple-tube’ type) or is just composed of one tube (‘single-tube’ type). In ‘multiple-tube’ Halysis, tube fission by bifurcation results from the insertion of a microcrystalline wall at the center of a mother tube. This study demonstrates for the first time that the tube walls of Halysis have a laminofibrous (fibronormal) microstructure, composed of fibrous calcite perpendicular to wall surface, and recognizes the ‘single-tube’ type Halysis composed of one tube; in addition, for the first time, this study finds out that ‘multiple-tube’ Halysis develops buddings from the conjunction of two tubes and ‘single-tube’ Halysis shows wide-angle Y-shaped branchings. Based on these findings, this study further compares Halysis with tabulate corals. Halysis appears stratigraphically earlier than Catenipora and Aulopora, and has a smaller tube size. ‘Multiple-tube’ Halysis resembles Catenipora and ‘single-tube’ Halysis resembles Aulopora in skeletal construction and microstructure, and in their tube walls of laminofibrous microstructure composed of fibrous calcite perpendicular to the tube wall surface. Catenipora and Halysis are both characterized by the absence of septal spines. The similarities suggest that Halysis may be the ancestor of Catenipora-like and Aulopora-like tabulate corals.
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