Phosphatic fossilization
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Phosphatic fossilization has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures. Such microscopic fossils are only visible under the scanning electron microscope.
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:
- 'This high-resolution fossil bed is about 30% phosphate, present as the mineral fluorapatite [Ca5(PO4)3F]. Phosphatic beds within this deposit are grainstones composed of 1- to 5-mm phosphoclasts. These derive from a phosphatic surface that formed on the sea floor, in the process recrystallizing existing surface sediments. In addition to replacing carbonate sediments, soft tissues of metazoan embryos, larvae, adults, and algae also appear to have been mineralized. The phosphatized sediment crust was then broken into small fragments by heavy current activity and then redeposited and mixed in with adjacent lime muds.' (Chen et al., 'Precambrian Animal Diversity: Putative Phosphatized Embryos from the Doushantuo Formation of China,' in Proceedings of the National Academy of Sciences of the USA vol 97, pp 4457-4462.Abstract)
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.
