Period· The columns of time

Carboniferous

359 Ma → 299 Ma

In the Carboniferous, from 359 to 299 million years ago, buried carbon pushed oxygen to its highest level of the last half billion years, and the amniotic egg freed vertebrates from the water.

Period in which carbon burial raises atmospheric oxygen to the highest level of the last half billion years, and in which the amniotic egg frees vertebrates from the aquatic environment.

What the evidence shows

The coal that names the period indicates its mechanism: equatorial swamp forests buried organic carbon on such a scale that unconsumed oxygen accumulated, to nearly a third of the atmosphere instead of the present fifth. Insects, which breathe by diffusion through tracheae whose reach depends on the external oxygen concentration, grew accordingly: relatives of dragonflies reach some seventy centimetres in wingspan. The amniotic egg is the other innovation: the embryo floats in fluid enclosed by the amnion, a second membrane handles gas exchange, a third stores waste, and a shell encloses the whole, so that reproduction no longer depends on water. Around 305 million years, the drying of the climate fragments the coal forests into islands; amphibians retreat, amniotes hold their ground. The emergent land assembles meanwhile into Pangaea.

Bounds and conventions

Two golden spikes bound the period. The oxygen value is modelled from the carbon and sulfur cycles and not measured, and the models differ on the maximum; the relation between that oxygen and insect size is a supported hypothesis and not a demonstration, since size ceased to track oxygen after the rise of birds, which indicates that predation mattered as much as the atmosphere. The oldest amniote is dated to 318 million years by its bones, and trackways published in 2025 would carry it to about 356 million.