Giant dragonfly-like insects and the oxygen peak

Meganeura monyi was a predatory insect with a wingspan of about 65 to 70 centimetres, the largest flying insect known. It is not a true dragonfly but a griffinfly, a member of the extinct group Meganisoptera. French paleontologist Charles Brongniart described it in the 1880s from fossils found at Commentry in central France, in rocks about 300 million years old. Related giants lived across Europe and North America.¹

Their size is tied to the Carboniferous atmosphere. Burial of vast amounts of plant material in the coal forests removed carbon from circulation and left extra oxygen in the air. Geochemical models by Robert Berner put oxygen at about 30 to 35 percent, compared with 21 percent today. Insects breathe through tubes called tracheae, which move oxygen by diffusion, so higher oxygen could allow larger bodies and the long wings that gliders and hunters need. Experiments by Jon Harrison and others showed insects grow bigger in oxygen-rich air. The link is not proven for every giant, and other factors, such as the absence of flying vertebrate predators, probably helped.¹²⁴

The giants faded as oxygen fell in the Permian. They died out in the Great Dying about 252 million years ago, after earlier decline. Meganeura is a vivid example of how the atmosphere shapes animal size. Paleontologists also point to Meganeuropsis, a Permian relative from Kansas and Oklahoma with wings of similar span, as the other record holder among giant insects.¹³