Evidence of giant sulphur bacteria in Neoproterozoic phosphorites

They might be giants The globular microfossils from the Neoproterozoic Doushantuo Formation in China are arguably one of the most significant fossil finds in the past decade. They were thought to be animal embryos, based on size and the presence of reductive cell division. If the attribution is correct, 600-million-year-old fossilized cells would provide an important window into early animal evolution. But they may not be embryos at all. The recent discovery of reductive cell division in a modern sulphur bacterium, and their direct association with phosphate minerals, find exact parallels in with the Doushantuo microfossils. Such an accumulation of animal embryos has always been seen as problematic, and no plausible phosphatization mechanism has been offered. The simplest explanation, therefore, is that these are the fossils of giant sulphur bacteria. In situ phosphatization 1 and reductive cell division 2 have recently been discovered within the vacuolate sulphur-oxidizing bacteria. Here we show that certain Neoproterozoic Doushantuo Formation (about 600 million years bp ) microfossils, including structures previously interpreted as the oldest known metazoan eggs and embryos 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , can be interpreted as giant vacuolate sulphur bacteria. Sulphur bacteria of the genus Thiomargarita have sizes and morphologies similar to those of many Doushantuo microfossils, including symmetrical cell clusters that result from multiple stages of reductive division in three planes. We also propose that Doushantuo phosphorite precipitation was mediated by these bacteria, as shown in modern Thiomargarita -associated phosphogenic sites, thus providing the taphonomic conditions that preserved other fossils known from the Doushantuo Formation.