Phosphatic Preservation in Burgess Shale-type Fossils
Soft-tissue fossils, such as those found in the Burgess Shale, are rare. In some cases their internal organs are replicated in phosphate. The phosphate mainly comes from the tissue itself, and may later be replaced by calcium carbonate. A low pH makes CaCO3 less likely to precipitate, clearing the way for phosphate to be laid down. This is facilitated by the absence of oxygen in the decaying tissue. Accordingly, (secondary) phosphate is generally only preserved in enclosed spaces, such as a tightly-closed bivalve shell.
Higher concentrations of phosphate in the sea water do not enhance phosphatization, as may seem natural; rather, it increases the rate at which the organism breaks up, perhaps because the mineral "fertilizes" the decay micro-organisms.
Phosphatization can happen quickly: The chitinous structures that support bivalve gills can be replaced by calcium phosphate, with a little help from co-occurring bacteria, in just two to six days. The gill axes and musculature of bivalves can also be preserved in phosphate. The structures that most famously preserved in phosphate in the Burgess Shale are the midgut glands of Leanchoilia, perhaps on account of their central position and plausibly a low pH?
Phosphatization can be microbially mediated, especially in decay-resistant groups such as arthropods; or substrate-dominated, where phosphate-rich tissue leads the mineralization process (as in fish). Cephalopods fall somewhere between these two extremes.
Read more about this topic: Phosphatic Fossilization
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