 The Burgess Shale animals were probably killed by changes in their environment either immediately preceding or during the mud @-@ slides that buried them . 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 . 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 . 
