Re-investigation of Girvanella from the Cambrian of China: preservation modes, diversity, spatio-temporal distribution, and its geological significance
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Abstract:
Cyanobacteria, as ancient autotrophic groups, have played vital roles in the evolution of life, environmental changes, and the formation of microbialites on Earth. Girvanella is one of the common form genera of cyanobacteria. In this paper, we compiled and analyzed relevant data on Girvanella from the Cambrian of China to better understand its preservation modes, diversity, and spatio-temporal distribution patterns. The results indicate that: (1) Cambrian Girvanella fossils from China are predominantly preserved through calcification, mostly in microbialites, with secondary preservation through phosphatization and iron mineralization in phosphorites and ferruginous stromatolites, respectively. (2) Four species of Girvanella have been identified; The species diversity of Girvanella remained stable, while its geographic distribution expanded from the Terreneuvian to Epoch 2, peaking during the Miaolingian Epoch; However, both species diversity and geographic distribution declined during the Furongian Epoch. Further investigation reveals that Girvanella belongs to the Oscillatoriaceae, and that factors such as seawater carbonate saturation, Ca2+ concentration, pH values, and atmospheric CO2 concentration influence its fossilization. Calcified fossils of Girvanella are likely resulted from filamentous cyanobacteria employing carbon-concentrating mechanisms that induced sheath calcification. Phosphatized fossils of Girvanella reflect secondary phosphatization, while iron–mineralized specimens may have formed through absorbing and binding flocculated sediments by filamentous cyanobacteria, either via the cells themselves or extracellular polymeric substances. Girvanella contributes to the formation of microbialites through the complicated calcification process of both themselves and microbial mats, as well as trapping and binding microbial mats that are dominated by these organisms. Additionally, also plays a significant role in constructing and reinforcing the structural stability of metazoan reefs. The fossil record of Girvanella also serves as an important indicator of long-term changes in the composition of the ancient oceans and the atmosphere.
