Ediacara biota
The base of the Cambrian is officially defined as the first appearance of a certain trace fossil, "Treptichnus pedum". However, this fossil also appears in older rocks than the locality which officially marks the start of the period. However, there is a period of biological change which makes this time period a good one to demarcate the Cambrian from the earlier Phanerozoic eon's periods and the Precambrian supereon. Consensus holds that fossils of the Ediacara biota disappear here, as do some shelly fossils and acritarchs; and a new small shelly fossil biota emerges.