This change happens extremely quickly, before the corpse is compressed, and leaves a three-dimensional mold of the tissues.[25] Gills can also be preserved in something close to their original three-dimensional shape by this process.[3] Both preservation mechanisms can appear in the same fossil.[4] 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.[25]