Doushantuo Formation Microfossils (~600 Ma): Giant Sulfur Bacteria, Embryos, or Both?  

Certain microfossils of the Neoproterozoic Doushantuo Formation (c. 600 Ma), including structures interpreted as the oldest known metazoan eggs and embryos, display features similar to giant vacuolate sulfur bacteria.  Colourless sulfur bacteria of the genus Thiomargarita have sizes and morphologies similar to many Doushantuo Formation globular microfossils, including symmetrical cell clusters that result from multiple stages of reductive division in three planes; a multilayered cell ultrastructure; and associations with complex epibiont communities.  The bacterial hypothesis does not invalidate the possibility that some Doushantuo Formation globular microfossils are indeed animal eggs and embryos, because the two hypotheses are not mutually exclusive.  Indeed, recently collected samples from the oxygen minimum zone along the Costa Rica Margin contain benthic accumulations of Thiomargarita mixed with spiny globular protists and invertebrate eggs, providing an interesting analogue for Doushantuo Formation biostratinomy. 

One of the most unusual attributes of the Doushantuo Formation assemblage is the phosphatization of the microfossils.  As an important and commonly limiting nutrient, large accumulations of phosphorous are somewhat unusual in the geologic record—why did the phosphorite form at this particular time?  Thiomargarita is known to mediate phosphate accumulation in modern sediments, and we have found sulfur bacteria fossils related to Thiomargarita (putative Beggiatoa and Thioploca) in younger phosphorites.  The interpretation of certain Doushantuo microfossils as Thiomargarita-like bacteria provides an explanation for the phosphorite precipitation that preserved the fossils.  Furthermore, Thiomargarita lives at the interface between oxic conditions above and sulfidic conditions below.  It may not be coincidence that Thiomargarita-like fossils accompanied by phosphatization appear in the rock record nearly coincident with proposed benthic oxygenation in Neoproterozoic time.