 It has often been suggested that this type of preservation was possible only when sediments were not disturbed by burrowing animals or the anchors of plants . However a similar type of preservation has been found in fossils from the Late Riphean period , about 850 to 750 million years ago , but in no known fossils between the end of that epoch and the start of the Cambrian . This suggests that such bioturbation has little to do with the appearance and disappearance of Burgess Shale type preservation . Such preservation may depend on the presence of clay @-@ like minerals that inhibit decomposition , and ocean chemistry may only have favoured the production of such minerals for limited periods of time . If so , it is impossible to be sure when the animals known as " Burgess Shale fauna " first appeared or when they became extinct . A few fossils of animals similar to those found in the Burgess Shale have been found in rocks from the Silurian , Ordovician and Early Devonian periods , in other words up to 100 million years after the Burgess Shale . 
