Oxygen and Gigantism
One of the most striking features of Carboniferous life is the enormous size of many insects and other arthropods. Meganeura with 70-cm wingspans, Arthropleura exceeding 2 meters in length, and many other species far larger than any living relatives. The leading explanation for this phenomenon is the exceptionally high atmospheric oxygen levels of the Late Carboniferous, which peaked at approximately 35%, compared to 21% today.
The Oxygen Hypothesis
Insects breathe through a system of tracheal tubes that deliver oxygen directly to tissues via diffusion. This system works well at small body sizes but becomes increasingly inefficient as body size increases, because the tubes must be longer and diffusion distance becomes a limiting factor. At modern oxygen levels (21%), this places an effective upper limit on insect body size.
When atmospheric oxygen is higher, diffusion is more efficient, and the tracheal system can supply oxygen to a larger body volume. This allows insects to grow larger before reaching their physiological limit. At 35% oxygen, the theoretical maximum body size is significantly larger than at 21%.
Experimental Evidence
Laboratory experiments have supported the oxygen-gigantism link. Researchers have raised modern insects (particularly beetles and dragonflies) in atmospheres with elevated oxygen concentrations and observed increased body sizes compared to controls raised at normal oxygen levels. Conversely, insects raised in low-oxygen atmospheres tend to be smaller. These experiments demonstrate that the relationship between oxygen and body size is not merely theoretical.
Other Contributing Factors
Oxygen alone does not fully explain Carboniferous gigantism. Other factors likely contributed:
The absence of flying vertebrate predators (birds and bats had not evolved) meant that large flying insects faced less predation pressure in the air. The abundant food resources in Carboniferous swamp forests supported large body sizes. The warm, humid climate may have also been favorable. And there may have been developmental or ecological feedbacks: once some insects became large, there was selective pressure for their predators and competitors to also increase in size.
The Decline
As oxygen levels declined during the Permian period (falling from ~35% to perhaps 15% by the end of the Permian), giant insects became increasingly rare. Meganeuropsis in the Early Permian was the last of the truly giant flying insects. By the Triassic, no insects approached Carboniferous sizes. When oxygen levels rose again in the Mesozoic, insects did not return to giant sizes, possibly because flying vertebrates (pterosaurs, birds) now occupied the large-bodied aerial predator niches.