PRESERVATION OF NEOPROTEROZOIC DOUSHANTUO PHOSPHATIZED MICROFOSSILS FROM SOUTHWESTERN CHINA

Doushantuo phosphatized microfossils have received considerable attention with the realization that they may retain evidence of the developmental forms of the earliest animals. Although the mechanisms of phosphatization still remain controversial, phosphatization is known to preserve originally organic structures in exquisite detail, which is also the case for Doushantuo microfossils with cellular and even subcellular structures preserved. Much work has been done on the preservation of Doushantuo microfossils including fossil observation of thin sections, acid residues, acetate peels under the microscope, SEM and CONFOCAL, as well as phosphatization experiments on animal embryos.

Analysis of phosphatization factors indicates organisms in Doushantuo Formation were preferentially phosphatized through phosphatic encrustation and impregnation, the latter consisting of bulk phosphate infilling and matrix replacement. Microfossils were subject to as many as 7 taphonomic phases during preservation. Generally the relatively refractory structural-organic layers surrounding organisms such as algal cell walls, acritarch organic walls, and embryo envelopes are preserved as intact organic residue and partly replaced with euhedral crystalline apatite, commonly with fibrous apatite crystals encrusted perpendicularly on one or both sides of these layers. Phosphatization of internal tissues took place through phosphate infilling and replacement in course of which replacement is increasing in preservation from algal cell bioplasms, acritarch vesicles to embryo blastomere interiors. Algal cell lumens are usually infilled with structureless collophane while embryo blastomere interiors are preserved with subcellular organelles including nuclei and yolk granules. Bulk phosphate and carbon infilling is normal in primary internal voids of organisms as well as secondary cavities left from shrinkage and decay of soft tissues.