The Carboniferous coal forests: giant bugs in super-oxygenated air

What happened

Vast swampy forests spread across the land in the Carboniferous Period, and because wood-decomposing fungi had not yet evolved to break it down efficiently, dead plant matter piled up and was buried rather than rotting and releasing its carbon back into the air, the same buried plant matter that would later become coal. That buildup pushed atmospheric oxygen to levels estimated as high as 35 percent, far above today's 21 percent. A peer-reviewed study on insect body size found that oxygen this abundant let arthropods sidestep a physical limit: insects breathe through narrow tubes called tracheae that ordinarily choke off oxygen supply to a large body, so with oxygen this plentiful, gigantism became viable. The result included Meganeura, a dragonfly relative with a wingspan up to 75 centimetres, and Arthropleura, a millipede that grew past 2 metres, the largest land arthropod ever known.

How we know

Oxygen levels this far back are reconstructed from geochemical models of the carbon cycle, calibrated against how much organic carbon was buried as coal during this period. The oxygen-size link itself was tested directly: researchers reared living insects under artificially raised oxygen levels and found they grew measurably larger, matching the mechanism proposed for their Carboniferous ancestors.

Sources

• Harrison et al.. Atmospheric oxygen level and the evolution of insect body size (2010) · Peer-reviewed (author-declared)pmc.ncbi.nlm.nih.gov · Cited as a "journal" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)