 Briggs and Whittington started experimenting with cladistics in 1980 to 1981 and the results , while full of uncertainties , convinced them that cladistics offered reasonable prospects of making sense of the Burgess Shale animals . Other fossil beds discovered since 1980 – some rather small and others rivalling the Burgess Shale – have also produced similar collections of fossils , and show that the types of animals they represent lived in seas all over the world . It appears that most of the major animal lineages had arisen before the time of the Burgess Shale , and before that of the Chengjiang and Sirius Passet lagerstätten about 15 million years earlier , in which very similar fossils are found , and that the Cambrian explosion was complete by then . In the 1990s it was suggested that some Ediacaran fossils from 555 to 542 million years ago , just before the start of the Cambrian , may have been primitive bilaterians , and one , Kimberella , may have been a primitive mollusc . By 1996 , with new fossil discoveries filling in some of the gaps in the " family tree " , some Burgess Shale " weird wonders " such as <unk> and Opabinia were seen as stem members of a total group that included arthropods and some other living phyla . 
