SPHEROIDAL MICROFOSSILS FROM THE EDIACARAN DOUSHANTUO FORMATION: ANIMAL EMBRYOS OR GIANT SULFIDE OXIDIZING BACTERIA?

Spheroidal microfossils enclosed in smooth, sculptured, and grooved envelopes from the Doushantuo Formation have been interpreted as animal eggs and embryos. Recently, Bailey et al. reinterpreted the smooth-walled microfossils as gigantic Thiomargarita-like sulfide oxidizing bacteria, while implicitly acknowledging the sculptured fossils may be animal eggs and embryos. They also argue that spiral grooves of certain Doushantuo fossils may be pyrite trails. The most important evidence against a bacterial interpretation is the observation that sculptured envelopes of Doushantuo fossils may be partially or completely lost so that poorly preserved specimens are wrapped within the smooth innermost envelope. The co-occurrence and similar cleavage pattern of smooth and sculptured spheroidal fossils suggest that they represent taphonomic variations of the same taxon or a few phylogenetically related taxa. The sculptured envelope finds no comparison in bacterial sheath, but is indistinguishable from some modern animal egg cases and implies a diapause stage. Furthermore, the spiral grooves always consist of three clockwise coils (evidence for biological chirality) and are distinct from pyrite trails. Additionally, a bacterial interpretation is inconsistent with organelle-like subcellular structures, spatial distribution of vacuole-like structures, and large cell numbers (>100 in some Doushantuo fossils vs. typically <16 in Thiomargarita) present in Doushantuo fossils.

Bailey et al. argue that a bacterial interpretation better explains the phosphatization and abundance of Doushantuo microfossils, because Thiomargarita concentrates phosphate in pore water, its slow cell division allows a longer preservation window between cell divisions, and it is abundant in modern sediments. However, modern Thiomargarita does not self-entomb in phosphate; if it did, it would be ecologically unsustainable. The preservation of Doushantuo fossils is determined by the competing degradation and mineralization processes, not by cell division rate, because phosphatization occurs after death, not between cell divisions. Abrasion, rounding, and inclusion in intraclasts clearly suggest intrabasinal reworking and winnowing have concentrated Doushantuo microfossils in phosphatic grainstone.