A relatively short interval of rapid intense evolution that supposedly occurred in the early to mid-Cambrian period, some 540 to 520 million years ago. The supposition is based on the sudden appearance in the fossil record from this time of many diverse and novel forms, particularly marine animals, among which can be found representatives of all major modern groups. Notable well-preserved fossil assemblages dating to this period include the Burgess shale fossils of Canada and the Chengjiang fossils of China. There is debate about whether such an explosion actually occurred or whether the evidence supporting it merely reflects discontinuity in the fossil record – that is, Precambrian ancestors of these fossil specimens did exist but were simply not fossilized.
Subjects: Biological Sciences.