A TAPHONOMIC TEST OF THE BIOGENICITY OF PUTATIVE PHOSPHATIZED SPONGE EMBRYOS FROM THE NEOPROTEROZOIC DOUSHANTUO FORMATION, SOUTHWEST CHINA

Microscopic objects from the Neoproterozoic Doushantuo Formation have been interpreted as phosphatized sponge embryos (e.g., Li et al., 1998; Xiao et al., 1998). As such they provide valuable insight towards understanding the early evolution of metazoans, but the biogenicity of these spectacular fossils is still questioned. The existence of taphonomic trends indicating variable preservation of these microfossils would provide support for their origin as primary biological objects. This research begins to address this issue by performing a detailed specimen-based taphonomic analysis. A total of 206 specimens identified as embryos in 65 thin sections from the Wengan Phosphorite Member of the Doushantuo Formation were examined and their levels of organic decay were scored as taphonomic grades from 1 (well preserved) to 4 (intensely decayed). The results of this examination reveal a strong taphonomic bias towards earlier (2-cell and 4-cell) cleavage stages, which tend to be well-preserved, and away from later (8-cell and 16-cell) cleavage stages, which tend to exhibit evidence for slight to intense levels of organic decay. In addition, no evidence of more advanced (beyond 16-cell) cleavage stages or eventual adult forms was found in this study, although they have been reported elsewhere. One possible explanation for these taphonomic trends is that later cleavage stages and adult forms were more physically delicate, allowing them to be more easily damaged during burial and reworking, thereby decreasing their preservation potential. The spectacular preservation of these embryos was probably enhanced by their likely internal enrichment in phosphate-rich yolk, which could have aided their rapid phosphatization. This study supports the biogenicity of these objects as embryos because it documents trends in organic decay levels that are consistent with taphonomic processes. The importance of Doushantuo Formation phosphatized microfossils as a window into Neoproterozoic life, and potentially metazoan origins, is therefore upheld.