Fortune Head GSSP, Newfoundland

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The headland on the Burin Peninsula of Newfoundland whose exposed section carries the golden spike defining the base of the Cambrian, and therefore the base of the Phanerozoic — the largest single division in the geological time scale. The boundary is placed at the first appearance of the trace fossil Treptichnus pedum, a distinctive burrow made by an unknown animal probing sediment in a repeated stepwise pattern. Choosing a burrow rather than a body fossil looks perverse until the requirement is understood: a boundary marker must be recognisable in ordinary marine sediment worldwide, and soft-bodied Cambrian animals are only preserved in rare exceptional deposits. The choice has a real cost. Treptichnus pedum is now known from slightly below the boundary in some sections, so the marker is not quite as sharp as it was believed to be when the GSSP was ratified, and the numerical age attached to the boundary has moved — from 542 to 541 to 538.8 Ma across successive chart editions — while the physical point in the outcrop has not moved at all.