Unravelling the nature of enigmatic fossils in the Ediacaran Weng’an phosphorite: a geobiological approach

Phosphorites of the Ediacaran Doushantuo Formation at Weng’an, South China, contain abundant and extraordinarily preserved microfossils. The nature of most of these microfossils is enigmatic and controversial, including spherical microfossils that resemble animal embryos. To elucidate the nature of these specific spherical microfossils, we analysed their mineral-organic relationships and geochemical signatures using Raman spectroscopy, micro-X-ray fluorescence (µ-XRF), and electron probe microanalysis (EPMA). The co-occurrence of phosphate intraclasts and fragmented and well-preserved microfossils suggests some degree of reworking and re-deposition in the original environment, consistent with overall condensed sedimentation. The analysed microfossils exhibit ~800 µm-sized spherical shapes covered by a ~10 µm thick, two-layered envelope consisting of apatite and subordinated organic matter. A more detailed observation of the internal parts revealed the presence of a matrix compositionally similar to the envelope. The matrix encloses abundant, small (≤ 2 µm) aggregates, which show reverse proportions of apatite and organic matter (i.e., relatively more organic matter than apatite). As a whole, the observed features resemble modern microbial spheres covered by lectin-dominated extracellular polymeric substances (EPS), as exemplified by communities of anaerobic methanotrophic archaea (ANME-2) and sulphate reducing bacteria (SRB) in the euxinic Black Sea, likely as a strategy to cope with environmental stressors (e.g. varying redox conditions and substrate concentrations). Overall, our findings reinforce a microbial origin of various embryo-like microfossils preserved within phosphorites of the Ediacaran Doushantuo Formation at Weng’an, highlighting the need for further studies to assess the diversity of the palaeo-communities.