Period· The columns of time

Silurian

444 Ma → 419 Ma

A short period of recovery in which vertebrates acquire jaws, land animals leave their first fossils, and the sky becomes a filter.

What the evidence shows

Barely twenty-five million years long, one of the shortest periods on the chart, it carries three firsts. In South China, rocks of 439 to 436 million years hold the oldest teeth of a jawed vertebrate and, three million years later, complete jawed fish: the jaw begins as a modified gill arch, and every jawed mouth since is a variation on it. On land, a millipede from the Scottish island of Kerrera, about 425 million years old, is the oldest body fossil of an animal living out of water, though molecular clocks place the crossing considerably earlier. And above both, the air changes: the plants spreading over the land since the Ordovician build tissue far poorer in phosphorus, carbon for carbon, than anything the sea grows, and bury enough carbon to lift oxygen to modern levels, high enough for ozone to hold a shield against the ultraviolet that breaks DNA.

Bounds and conventions

Two golden spikes hold it. The dates inside are less settled: the oldest body fossil of a land animal gives 425 million years, molecular clocks give considerably older, and the gap between the two is narrower than it is often made out to be, without being closed. The rise of the ozone shield is inferred from models of oxygen and phosphorus rather than measured, and one recent estimate revises the Proterozoic ozone column that preceded it sharply downward.

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. Andreev et al. (2022), Nature · The oldest gnathostome teeth
3. Zhu et al. (2022), Nature · The oldest complete jawed vertebrates from the early Silurian of China
4. Tihelka et al. (2022), Biology · Was there a Cambrian explosion on land? The case of arthropod terrestrialization
5. Lenton et al. (2016), PNAS · Earliest land plants created modern levels of atmospheric oxygen