Period· The columns of time

Carboniferous

359 Ma → 299 Ma

Forests bury carbon faster than decay returns it: oxygen reaches the highest level of the past half-billion years, insects grow enormous, and an egg frees vertebrates from water.

What the evidence shows

The coal that names it is the point: equatorial swamp forests buried organic carbon on a scale that left the oxygen unspent and free to accumulate, to somewhere near a third of the air instead of today's fifth. Insects, which breathe by diffusion through tubes, grew as the oxygen rose, dragonfly relatives reaching wingspans of about seventy centimetres. The other invention is smaller and more consequential. The amniotic egg carries its own pond, an embryo floating in fluid inside a membrane, a second membrane for breathing, a third for waste, and a shell around the whole: vertebrates no longer had to return to water to breed. Around 305 million years ago the climate dried, the coal forests broke into islands, amphibians lost ground and amniotes kept theirs. Meanwhile the world's landmasses were gathering into Pangaea.

Bounds and conventions

Two golden spikes bound it. The oxygen figure is modelled from the carbon and sulfur cycles rather than measured, and the models disagree on the peak; the link between that oxygen and insect size is a well-supported hypothesis rather than a demonstration, since insect size stopped following oxygen once birds had spread, which suggests predators mattered as much as the atmosphere. The oldest amniote is dated 318 million years by bones, and tracks reported in 2025 would push it to about 356.

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. Berner et al. (2003), Annual Review of Earth and Planetary Sciences · Phanerozoic atmospheric oxygen
3. Clapham & Karr (2012), PNAS · Environmental and biotic controls on the evolutionary history of insect body size
4. Sahney, Benton & Falcon-Lang (2010), Geology · Rainforest collapse triggered Carboniferous tetrapod diversification in Euramerica
5. Long et al. (2025), Nature · Earliest amniote tracks recalibrate the timeline of tetrapod evolution
6. Stampfli et al. (2013), Tectonophysics · The formation of Pangea