Period· The columns of time

Devonian

419 Ma → 359 Ma

Plants invent wood and become trees, vertebrates walk out of water, and the reefs reach a size never equalled since.

What the evidence shows

Lignin, a stiff polymer laid down around cellulose, lets a stem hold a column of water metres above the ground and carry its own weight while doing it. A quarry floor in New York State, mapped over twelve hundred square metres, shows what came of it: a mixed-age stand of trees at least eight metres tall, held in place by small anchoring roots, with woody stems running between them and probably climbing into their crowns, left by the earliest plants known to grow true wood. Vertebrates leave the water in the same period, and the tracks come before the bones, trackways in Poland eighteen million years older than the oldest skeleton of a four-legged animal. In the sea, sponges called stromatoporoids and tabulate corals build reef tracts larger than anything before or since, the Great Barrier Reef included, and they are already shrinking before the extinction at the Frasnian-Famennian boundary, 372 million years ago, finishes them.

Bounds and conventions

The bounds are golden spikes. What the forests did to the climate is where the argument sits: they were long credited with drawing carbon dioxide down and so cooling the world, and a recent reconstruction finds carbon dioxide already low before they spread and points instead to the shallow-rooted vegetation that came before them. The Polish trackways are also read by some as the marks of a fish rather than of a walking animal.

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. Stein et al. (2007), Nature · Giant cladoxylopsid trees resolve the enigma of the Earth's earliest forest stumps at Gilboa
3. Stein et al. (2012), Nature · Surprisingly complex community discovered in the mid-Devonian fossil forest at Gilboa
4. Stein et al. (2020), Current Biology · Mid-Devonian Archaeopteris roots signal revolutionary change in earliest fossil forests
5. Niedźwiedzki et al. (2010), Nature · Tetrapod trackways from the early Middle Devonian period of Poland
6. Copper (2002), Palaeogeography, Palaeoclimatology, Palaeoecology · Reef development at the Frasnian/Famennian mass extinction boundary
7. Dahl et al. (2022), Nature Communications · Low atmospheric CO2 levels before the rise of forested ecosystems