Abstract

The c.570 Ma Doushantuo biota of South China has yielded important fossils that include the oldest widely accepted record for the establishment of the animal evolutionary lineage, as well as a suite of specimens with alleged bilaterian affinity. However, the interpretations of all of these fossils have been criticized on the basis that interpretations of affinity are contingent on the presence of key biological structures that may be more readily interpreted as artefacts of diagenetic mineralization. Furthermore, these fossils are limited to the earliest stages of embryonic development and the absence of equivalent adults has led to the suggestion that these simple clusters of cells may rather represent clusters of vegetatively dividing bacterial cells. We attempted to discriminate among these competing hypotheses by first characterizing the mineral phases that replicate original biological structure versus later diagenetic void filling using BSE , EPMA, EBSD , SRXTM, demonstrating that those structures interpreted hitherto as evidence for derived animals are characteristic of void filling mineralization long after the original biological structures have decayed away.

To discriminate between animal versus bacterial interpretative models we undertook decay experiments on modern animal embryos and giant bacteria. The taphonomy of the Doushantuo fossils is compatible with animal embryos, but not bacteria. Furthermore, the fossilised remains of nuclei within the Doushantuo allow us to reject the bacterial model. It does not follow, however, that the animal model is correct, and new developmental stages in the Doushantuo assemblage provide compelling evidence against their interpretation as animal embryos. Thus, while these fossils may no longer be considered direct evidence of Ediacaran animals, they may represent a stage in the deep evolutionary origins of animal grade multicellularity.