 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 . 
