Well-preserved Stanieria-like coccoid cyanobacteria and pyrites in microbi-alites from the bottom of the Yelang Formation at the Xiejiacao section of Guang’an, Sichuan
Article
Figures
Metrics
Preview PDF
Reference
Related
Cited by
Materials
Abstract:
Cyanobacteria are hard to be well preserved as intact fossils through calcification because they easily suffer from microbial degradation after death. Sometimes, only calcified sheaths in some genera like Girvanella can be preserved in rocks. Most of the well-preserved cyanobacteria in geologic time are found in some special sedi-mentary rocks such as cherts. The coccoid cyanobacteria are rarely found as well-preserved fossils in microbialites that are widespread in South China. The expansion of cyanobacteria and the wide occurrence of microbialites are one of the features related to microbial communities in the aftermath of the Permian-Triassic mass extinction. Here in this paper, well preserved unicellular Stanieria-like coccoid cyanobacterial fossils are reported from the bottom of the Yelang Formation at the Xiejiacao section of Guang’an, Sichuan. These fossils are mainly composed of likely daughter cells and sheaths. The colonial structure of the Stanieria-like cyanobacteria is resulted from the type of re-production and the mucilaginous sheaths. Reproduction of the Stanieria-like coccoid cyanobacteria is characterized by simultaneous multiple fission (baeocytes division). The preservation of the Stanieria-like coccoid cyanobacterial fossils is ascribed to the special depositional environments of the Yelang Formation. These environments, including restricted marine platform, pressure level of atmospheric CO2 and carbonate saturation state of seawater, enabled the calcification of the cyanobacteria. The discovery of these fossils is significant for us to understand the Permian-Triassic marine paleoenvironments and paleoecology. Pyrites in microbialites at the bottom of Yelang Formation have a variety of morphology including framboids, stellate, patchy and striated forms. SEM observations show that patchy and striated pyrites often have the appearances comparable to coccoid cyanobacteria. It can be inferred that they inherited their morphology from the coccoid cyanobacteria through pyritization.
