DISSECTING DOUSHANTUO EMBRYOS: MICROCHEMICAL, MICROSTRUCTURAL, AND TAPHONOMIC INVESTIGATION OF PROPOSED SUBCELLULAR STRUCTURES

The phosphatized Ediacaran microfossils Megasphaera and Parapandorina from the Doushantuo Formation (635–551 Ma) of South China have been the subjects of extensive research, and their interpretations as metazoan resting eggs and blastular embryos have been the focus of much scrutiny. As an alternative interpretation, Megasphaera and Parapandorina have been described as massive vacuolate bacteria akin to Thiomargarita namibiensis. The presence of petrographically and tomographically observable subcellular structures (analogous to coated yolk granules, cytoplasmic vesicles, or membrane-bound organelles), together with the presence of an enclosing envelope with complex ornaments, provides a critical test of the alternative interpretations. However, to truly uncover the identity of these subcellular structures, we must first understand how they are preserved. Using a combination of petrographic microscopy, x-ray computed tomography, scanning electron microscopy, and energy dispersive x-ray spectroscopy on numerous Doushantuo microfossils, we have begun to explore taphonomic details of various features, including the subcellular inclusions. During the examination of the subcellular inclusions, we found striking microchemical, microstructural, and taphonomic similarities (in terms of apatitic crystal growth and orientation) to the degradation and preservation of Doushantuo cell membranes. These similarities imply that the subcellular structures likely represent true membrane-bound organelles.