Period· The columns of time

Devonian

420 Ma → 359 Ma

In the Devonian, from 419 to 359 million years ago, plants acquired wood and grew into trees, vertebrates reached dry land and reefs reached their greatest extent.

Period in which plants acquire wood and tree stature, vertebrates reach dry land, and reefs attain their greatest extent.

What the evidence shows

Lignin, a rigid polymer laid down around cellulose, allows a stem to hold a column of water several metres above the ground while carrying its own weight. A fossil soil in a New York State quarry, mapped over twelve hundred square metres, shows the result: a stand of mixed ages, trees at least eight metres tall held by small anchoring roots, and woody stems running between them and probably climbing into their crowns, left by the oldest known plants producing true wood. Vertebrates leave the water in the same period, and footprints precede bones: Polish trackways eighteen million years older than the oldest tetrapod bone. At sea, stromatoporoid sponges and tabulate corals build reef tracts more extensive than any before or since, and their decline begins before the Frasnian-Famennian extinction, at 372 million years, removes them.

Bounds and conventions

The limits are golden spikes. The discussion concerns the climatic effect of forests: the drawdown of carbon dioxide and the consequent cooling were long attributed to them, but a recent reconstruction finds that gas already low before their expansion and attributes the drawdown to the shallow-rooted vegetation that preceded them. The Polish trackways are moreover read by some authors as the marks of a fish rather than a walker.