The earliest calcified cyanobacteria from the Cambrian Terreneuvian at the western margin of the Yangtze Plate, China

The fossil records of cyanobacteria with calcified sheaths are sparse in the Precambrian, with their first prevalence appearing in the early Cambrian. Despite diverse calcified cyanobacteria being well documented from the Cambrian Terreneuvian in the Siberian Plate, their records in the adjacent Yangtze Plate are rare. The eastern Yunnan (western margin of Yangtze Plate) is a hotspot for studying the Cambrian Explosion, with abundant small shelly fossils (SSFs) known in the Cambrian Terreneuvian. Yet, contemporaneous calcified cyanobacteria have never been described within this region. Here, we describe two calcified cyanobacteria genera and four species from the phosphorous dolomite at the base of the Yu’anshan Formation in eastern Yunnan, China. These include Girvanella problematica, G. wetheredii, G. kasakiensis, and Subtifloria delicata. We further investigate the preservational pathways of these diverse calcified cyanobacteria show that all cyanobacteria from the Yu’anshan Formation are preserved due to their original calcification and later replacement by diagenetic mineralization. Subtifloria was mainly phosphatized, whereas Girvanella might have undergone both phosphatization and aluminosilicatization. Our work confirms the presence of the Terreneuvian calcified cyanobacteria in eastern Yunnan and expands the geographical range of calcified cyanobacteria in Yangtze Plate. Compared to Siberia, the calcified cyanobacteria in Yangtze Plate are much lower in species- and genus-level biodiversity and are distributed in a quite narrow range. This disparity might be attributed to the narrow suitable ecological environment and the small volume and limited distribution of carbonate rock therein.