Phosphatized microbial fossils from the lowest Cambrian of South China and their ecological and environmental implications for the Kuanchuanpu biota

Abstract

The early Cambrian Fortunian-age Kuanchuanpu Formation exposed in the northwestern margin of South China is well known for recording unique aspects of the “Cambrian explosion”, notably the coexistence of Small Shelly Faunas and soft-bodied meiofaunas. Although microorganisms, both primary producers and decomposers, are important biotic components of marine ecosystems in which they play a crucial ecological role in matter cycling and energy flow, Cambrian microbial records have not been brought into attention so much as their intimately associated faunas. To extend these records here we report a diverse assemblage of cellularly preserved microbial fossils from cherts of the Kuanchuanpu Formation. Nine species comprising six genera and two undescribed taxa have been identified. Among taxa interpreted as primary producers, are Gloeodiniopsis and unnamed form B that are morphologically analogous to coccoidal chroococcacean cyanobacteria, and Megathrix and Siphonophycus to filamentous oscillatoriacean cyanobacteria. In contrast, co-occurring members of endobiotic Endoconchia ? are inferred to be biodegraders. The high preservational fidelity of this microbial assemblage is here documented by use of optical microscopy, scanning electron microscopy, energy dispersive spectrometry, back-scattered electron microscopy and Raman spectroscopy. These analyses results demonstrate that the microbial fossils were organic remains (Kerogen) permineralized by pre-decay phosphatization and then sealed within the encompassing siliceous matrix. This mode of fossilization differs from typical chert-embedded silica-permineralized Proterozoic microfossils in the presence of apatite permineralization, which is regarded as a response to the phosphogenic event during the Neoproterozoic-Cambrian transition.