Soft-tissue fossils themselves are rare. Its Cambrian soft animals, preserved in oxygen-free mud, made the Burgess shale familiar to every fossil buff. In phosphatic fossils, the preservation is so fine that even some cellular structure has been preserved. The phosphatic microfossils of the Doushantuo Formation (q.v.), a fossil-rich lagerstätte of the Ediacaran period, about 590 - 565 Ma (megaannua; million years ago), display some of the most spectacular cellular-level preservation known from the geologic record. The fossils include what may be metazoan blastulas, possibly animal embryos at an early stage in cell division.
The Doushantuo formation presents a classic example of phosphatic fossilization:
Careful acid baths etch away the limestone matrices, by slowly dissolving the carbonates, and reveal the phosphates that have replaced organic structures, in the manner that Dr. Chen describes. There are other means of fossilization represented in the Doushantuo formation as well.
A refinement to viewing the internal structure of fossilized embryos uses specialized microscopic three-dimensional x-ray computed tomography, a kind of micro CATscan.