In search of the first animals

The earliest proven fossils of multicellular animals are accepted to be those of the Ediacaran biota (ca 580 - 541 Ma), although some pre-Ediacaran samples from the Rasthof Formation of the Congo craton, Namibia, and from the Balcanoona Formation in the Northern Flinders Ranges, South Australia, have been claimed to contain fossils of the proto-sponges. The current study assesses samples from the above localities for the thermal maturity of the organic matter and the presence of sponge biomarkers. A combination of SEM, Raman and FTIR spectroscopy, and GC-MS techniques shows that the organic matter in the studied samples is over-mature for oil generation, with an equivalent vitrinite reflectance (R0) ≥ 1.4%. The Namibian samples (three dolostones from the Rasthof and Berg Aukas Formations) are more mature than the Australian samples (two siltstones from the Tapley Hill Formation and a dolomitic limestone from the Balcanoona Formation).The former are of chlorite zone metamorphic grade, while the latter exhibit characteristics of both pre-greenschist and greenschist (chlorite zone) metamorphism. No samples yielded sterane biomarkers specific to sponges, or any other biomarkers,including the normally ubiquitous hopanes. The absence of intact biomarkers can be attributed to their thermal destruction during late catagenesis and early metamorphism.