The Cambrian explosion

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The interval in which almost all animal body plans that exist today first appear in the fossil record, together with mineralised skeletons, complex burrowing, eyes and predation. The word "explosion" is doing two jobs and only one of them is safe. As a description of the fossil record it is accurate: the change from the Ediacaran to the Cambrian assemblages is abrupt and it is worldwide. As a description of evolutionary history it is contested, because molecular clocks place the divergences of the major animal lineages substantially earlier, in the Ediacaran or the Cryogenian, implying a long cryptic history of small, unskeletonised, rarely preserved animals. Several developments could each make an existing lineage suddenly visible without any new lineage arising: biomineralisation, an increase in body size, the onset of deep burrowing, and the appearance of predators driving the evolution of hard parts. The base of the Cambrian is defined at Fortune Head in Newfoundland and dated on recent ICS charts at 538.8 ± 0.2 Ma, revised down from the 541 Ma figure cited widely through the 2010s.

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