Cambrian Explosion

Rapid evolutionary radiation beginning ~538.8 Ma when most major animal phyla and mineralized skeletons appeared.

Specifications & Metrics

type: Standard Stratigraphic Chronology
timespan: ~538.8 – 518 Ma (Early Cambrian Series, Paleozoic Era)
defining boundary: First appearance of Treptichnus pedum burrows at Fortune Head GSSP, Newfoundland (538.8 Ma)
proponents or Body: Charles Walcott (Burgess Shale, 1909); ICS Phanerozoic boundary
key evidence: Burgess Shale (508 Ma), Chengjiang biota (518 Ma), Small Shelly Fossils, trilobites

Marking the base of the Phanerozoic Eon at 538.8 Ma, the Cambrian explosion saw the fossil appearance of nearly all modern bilaterian animal phyla — including arthropods, molluscs, brachiopods, echinoderms, and chordates — within a 20- to 25-million-year span.
Soft-bodied Konservat-Lagerstätten such as China's Chengjiang biota (518 Ma) and Canada's Burgess Shale (508 Ma, discovered by Charles Doolittle Walcott in 1909) preserve complete marine ecosystems with compound eyes, gills, and guts.
Organisms simultaneously evolved calcium carbonate and calcium phosphate biomineralization (the 'Small Shelly Fauna' and trilobite exoskeletons with calcite crystal lenses) alongside vertical substrate burrowing that transformed seafloor sediment chemistry.