Petrographic analysis of new specimens of the putative microfossil Vernanimalcula guizhouena (Doushantuo Formation, South China)

Abstract

The Neoproterozoic Doushantuo Formation contains some of the earliest evidence for animals and animal embryos, including the putative bilaterian Vernanimalcula guizhouena. Samples of the black facies of the Weng'an Phosphate Member of the Doushantuo Formation were disaggregated in a weak, buffered formic acid solution in order to concentrate microfossils, and mounted in epoxy. The resulting grain mounts were made into thin sections and examined petrographically. Five specimens with morphology matching what was described as Vernanimalcula were found in a single sample from the black facies. Up to five individual, repeated structures are found in each specimen. Measurements of lengths and widths of morphological features were made on the new specimens. These measurements are found to be consistently similar across the new specimens, and within the range reported for the original specimens. A diagenetic sequence is constructed using the available evidence. This sequence consists of: The early phosphatization of a biological structure; a second phase of internal sedimentation, compaction and dewatering; and a later phase of phosphatic cementation. While this study does not constitute the final word on the biogenicity of these morphologies, it seems unlikely that phosphatization alone could be responsible for a large number of specimens with the same repeated form with the same dimensions. The strikingly similar sizes and morphologies, as well as the observed diagenetic sequence, lead us to conclude that these specimens are likely biogenic in nature.

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