Para ilmuwan berpikir bahwa invertebrata menjadi fosil, dan jaringan lunak mereka tetap utuh pada daerah semacam Burgess Shale, karena secara tiba-tiba tertutupi oleh lumpur dengan kandungan oksigen sangat rendah.
Scientists think that invertebrates were fossilized, and their soft tissues kept intact in regions such as Burgess Shale, by being suddenly covered in mud with a very low oxygen content.