The role of amorphous silica coating on apatite nanocrystals in the exceptional preservation of phosphatized embryo-like microfossils from the Ediacaran Doushantuo Formation

Abstract The Ediacaran Doushantuo Formation (ca. 600 Ma) in South China is renowned for its exceptionally preserved phosphatized microfossils, including globular structures interpreted as metazoan embryos. To preserve such fine cellular and intracellular structures, the stability of apatite nanocrystals against dissolution and recrystallization is essential. Once preserved, these structures exhibit a level of detail defined by the nanocrystals, analogous to fine pixels in a high-resolution image. However, the mechanisms responsible for the stability of apatite nanocrystals in these microfossils are still unknown. This study investigates the nanoscale features of apatite nanocrystals in these fossilized embryos using high-resolution transmission electron microscopy (HRTEM) to elucidate the mechanisms responsible for their remarkable preservation. X-ray diffraction and Rietveld refinement confirmed that the fossils are composed of crystalline fluorapatite. HRTEM imaging revealed that the fossilized embryos consist of ordered arrays of slightly elongated fluorapatite nanocrystals, typically measuring ~90 nm in width and ~180 nm in length. A key finding is the presence of a thin (~1 nm), conformal coating of amorphous silica intimately associated with crystallographic facets of the fluorapatite nanocrystals. The silica sheathing may have contributed to the long-term preservation of the embryos by helping to protect the nanocrystals from recrystallization and potentially enhancing structural stability during processes such as diagenesis and low-temperature alteration. Thus, these nanocrystals are insulated from dissolution or recrystallization overgrowth, maintaining the fine “pixel” size in phosphatized microfossils. These findings offer valuable insights into the processes responsible for exceptional fossil preservation, paving the way for future research on early animal evolution and the preservation of delicate biological structures in the geological record.