Period· The columns of time

Cambrian

539 Ma → 485 Ma

Nearly every way of building an animal appears within a few tens of millions of years, and the radiations that followed explored those designs rather than adding new ones.

What the evidence shows

Between roughly 540 and 518 million years ago the rocks fill with arthropods, molluscs, segmented worms and the first chordates. The pace can be measured rather than asserted: rates of change in trilobite anatomy give the radiation a duration of some twenty million years, which is fast for evolution and slow for an explosion. It begins quietly, with a rain of millimetre shells, cones and tubes and plates freed from limestone by acid, most of them pieces of animals whose bodies are still unknown. Eyes are there early, and already excellent: a South Australian shale laid down about 515 million years ago has yielded a compound eye two to three centimetres across with at least sixteen thousand lenses, a match for the keenest compound eyes alive today. And none of it would be legible without a handful of muds, at Chengjiang and in the Canadian Rockies, that preserved gills, guts and nerve cords as films of carbon.

Bounds and conventions

A golden spike in Newfoundland at 538.8 million years, placed on the first appearance of a burrow rather than of a body, makes its base one of the best defined on the chart. What the period is famous for is far less sharp. Whether the explosion is a true burst of evolution or a burst of preservation and of hard parts has never been settled; molecular clocks put the origin of many animal groups well before it, although for trilobites the clocks and the fossils now agree on a start inside the Cambrian.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Paterson et al. (2019), PNAS · Trilobite evolutionary rates constrain the duration of the Cambrian explosion
3. Zhang & Shu (2021), PalZ · Current understanding on the Cambrian Explosion: questions and answers
4. Gaines et al. (2012), PNAS · Mechanism for Burgess Shale-type preservation
5. Paterson et al. (2011), Nature · Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes
6. Shu et al. (2003), Nature · Head and backbone of the Early Cambrian vertebrate Haikouichthys