concept

Pyritization

The most generous of all preservation routes: brassy iron pyrite crystals nucleating directly onto decaying tissue, casting muscle fibres and gill filaments at nearly cellular resolution, in days. It needs three things arriving together in the right ratio - reduced sulfide, reactive iron, and, counter-intuitively, surrounding sediment that is *poor* in organic carbon. In carbon-rich mud the iron is consumed everywhere instead of at the body. On Earth the sulfide must be manufactured on site by bacteria, a race that decay usually wins - which is why pyrite fossils are rare enough for individual beds to have names.