 Burgess shale type preservation is defined as the fossilization of non-biomineralized organisms as flattened carbonaceous films in marine shales. When the animals started to decompose, their tissues collapsed under the weight of the sediment that buried them. The typical flattened fossils are outlines of tougher parts such as cuticles and jaws, which resisted decomposition for long enough to be fossilized. Soft elements, such as muscles and gut contents, were sometimes squeezed out of the decomposing organism to produce dark stains on the fossils. Organisms that lack tougher structures, such as flatworms, nemerteans and shell-less molluscs, were not preserved by this process. Very soft but chemically active tissues may be preserved by different processes. For instance during decomposition, bacteria modify the chemically unusual mid-gut glands of some organisms into the durable mineral phosphate. This change happens extremely quickly, before the corpse is compressed, and leaves a three-dimensional mold of the tissues. Gills can also be preserved in something close to their original three-dimensional shape by this process. Both preservation mechanisms can appear in the same fossil. In Burgess-like shales, organisms and parts that are only quite soft, such as muscles, are generally lost, while those that are extremely soft and those that are fairly tough are preserved. The preservation of different body parts in different ways may sometimes help palaeontologists, by suggesting whether a body part was fairly tough like an arthropod limb (preserved as flat film) or very soft and chemically active, like a part of the gut (preserved as a solid piece of mineral). These differences may also help to identify fossils, by excluding from consideration organisms whose body parts do not match the combination of types of preservation found in a particular fossil bed.
