 Analysis of the Burgess Shale fossils has been important to the interpretation of the Precambrian and Cambrian fossil records , and thus to scientific understanding of the nature of early evolution . English geologist and palaeontologist William Buckland ( 1784 – 1856 ) realised that a dramatic change in the fossil record occurred around the start of the Cambrian period , 542 million years ago . The earliest Cambrian trilobite fossils are about 530 million years old , but were already both diverse and widespread , suggesting that the group had a long , hidden history . The earliest fossils widely accepted as echinoderms appeared at about the same time Because Darwin 's contemporaries had insufficient information to establish relative dates of Cambrian rocks , they had the impression that animals appeared instantaneously . Charles Darwin regarded the solitary existence of Cambrian trilobites and total absence of other intermediate fossils as the " gravest " problem to his theory of natural selection , and he devoted an entire chapter of The Origin of Species on the matter . He speculated that the phenomenon , now known as the Cambrian explosion , was a product of gaps in the sequence of fossil @-@ bearing rocks and in contemporary knowledge of those rocks . 
