Era· The columns of time

Paleozoic

539 Ma → 252 Ma

Two hundred and eighty-seven million years, from the first shelled animals to the worst extinction in the record. Life leaves the water in this era and never fully returns.

What the evidence shows

It opens on the Cambrian explosion, in which most animal body plans appear within a few tens of millions of years. The name makes the boundary sound sharper than it was: the diversification begins in the late Ediacaran, before the era does, and carries on well into it. Plants reach land around 470 million years, and remake the air and the ground they grow in. It closes at 251.9 million years on the end-Permian extinction, dated to within a hundred thousand years and emptying the sea of the large majority of its species: the reason the era that follows looks so unlike this one.

Bounds and conventions

Both of its bounds are golden spikes, boundaries pinned to a chosen outcrop rather than to a round number, which makes this era one of the best bracketed on the chart. What its name says is another matter: ancient life is a nineteenth-century judgement, passed when everything older than it was still invisible.

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. Wood et al. (2019), Nature Ecology & Evolution · Integrated records of environmental change and evolution challenge the Cambrian Explosion
3. Rubinstein et al. (2010), New Phytologist · Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana)
4. Shen et al. (2011), Science · Calibrating the end-Permian mass extinction