New insights on the paleobiology, biostratigraphy and paleogeography of the pre-Sturtian microfossil index taxon Cerebrosphaera

•Morphometry confirms that Cerebrosphaera is monospecific.•Two types of complex multilayered walls are revealed by TEM.•The wall biopolymer is highly aromatic, with short/highly branched aliphatic chains.•Combination of morphology, ultrastructure and chemistry evidence eukaryotic affinity.•Comparison to modern analogues suggests an affinity to stem metazoan egg.