THE EDIACARAN DOUSHANTUO MICROBIOTA: ANIMAL, MINERAL OR VEGETABLE

Because of its paleontological riches, the Doushantuo Formation (Ediacaran, ~580 Ma) of Guizhou, southern China, has achieved iconic status enjoyed by precious few stratigraphic units. Phosphatic dolomites, cherts and shales have yielded a spectacular diversity of fossil remains. Strikingly, the shallow-water, cross-laminated grainstones at the famous Weng’an locality are entirely composed of sand-sized microfossils. Phosphatized specimens were initially studied petrographically, then with scanning electron microscopy, and recently, synchrotron tomography has been employed. Early interpretations recognized the spiny forms as acritarchs and hypothesized algal affinities for the varied multicellular spheroids. However, after the discovery of middle Cambrian apatite-replaced invertebrate embryos which we announced in 1994, a morphological similarity was perceived between these and the copious spheroids. Despite the absence of phosphatized juveniles and adults or any fragments of definitive animal tissues, these spheroids were interpreted as animal embryos, an improbable conclusion that was nevertheless widely embraced. Other speculations have since entered the literature including the identification of convoluted laminae inside some objects as bilaterian animal guts. We reject the embryo hypothesis for all the Doushantuo microfossils hitherto described plus observed in our collection of many thousands of specimens. Various algal affinities are still the most reasonable, on first pass, especially given the shallow-water depositional environment. For example, spheroids and clusters of spheroids with a unique helical feature—the first example of fixed asymmetry in the fossil record—may be separate stages of the life cycle of an algal taxon, possibly a stem group with no direct younger or extant analogs known. Abandoning the embryo interpretation does not reduce the importance of the Doushantuo biota. There are various tubular and other elements of uncertain affinity, and recently a sponge-grade object has been described. Moreover, the unique paleoenvironmental setting and diagenetic processes that led to the preservation of this biota have yet to be fully understood. The relative roles of evolution, ecology and taphonomy still need to be elucidated.