The Burgess Shale animals were probably killed by changes in their environment either immediately preceding or during the mud-slides that buried them.[1] Proposed killing mechanisms include: changes in salinity; poisoning by chemicals such as hydrogen sulfide or methane; changes in the availability of oxygen; and changing consistency of the sea floor.[13]: 238 The death event was not necessarily related to the burial, and there may have been multiple death events between burial events; but only organisms killed immediately before a burial event would stand any chance of being fossilised, instead of rotting or being eaten.[13]