Why prehistoric dragonflies grew to the size of modern seagulls

Space · Published 27 October 2026 Swipe more like this →

In short: During the Carboniferous period, insects reached terrifying proportions, with predatory dragonflies ruling the skies as large as birds. While scientists long blamed a surplus of oxygen for this gigantism, new research suggests that these giants may have evolved specifically to survive a deadly chemical hazard in their youth.

The Carboniferous period, spanning from approximately 359 million to 299 million years ago, was defined by vast lowland swamp forests. These dense ecosystems of ferns, mosses, and horsetails created an atmosphere with oxygen levels reaching around 30 percent—nearly 50 percent higher than what we experience today. This oxygen-rich environment allowed adult arthropods, including huge cockroaches and dragonflies, to grow to massive sizes while meeting their metabolic energy needs.

However, a new study by Wilco Verberk and David Bilton suggests a more complex mechanism: oxygen toxicity. Because many insects, such as stonefly larvae, live in water before becoming terrestrial adults, they are highly vulnerable to high oxygen concentrations. Larvae typically absorb oxygen directly through their skin, giving them little control over gas intake. In an atmosphere with 30 percent oxygen, the concentration of dissolved oxygen in water would have been dangerously high, potentially poisoning smaller, more sensitive larvae.

This suggests that insect gigantism might have been a survival strategy against 'oxygen poisoning.' By growing larger, these insects may have developed better mechanisms to regulate their internal chemistry or simply survived at scales where the toxicity was less lethal. This shift in perspective moves the focus from how oxygen fueled growth to how it dictated which species could survive the transition from water to land.

The era eventually saw a dramatic climatic shift. A combination of reduced volcanic activity and the massive carbon uptake by forests caused atmospheric CO2 to drop by at least five times over 50 million years. This led to cooler, drier environments, causing the collapse of spore-producing rainforests and paving the way for seed-bearing plants like conifers and the advancement of reptiles.

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