Meganeura: Giant Insects of the Oxygen-Rich Carboniferous

Meganeura was a giant dragonfly-like insect that lived about 300 million years ago, at the end of the Carboniferous Period, in what is now central France. With wings spanning around 70 centimetres, Meganeura monyi is one of the largest flying insects ever discovered. Since its description in the 1880s it has been the best-known example of prehistoric insect gigantism and a central case in the debate over whether high oxygen levels allowed insects to grow so large.

Background: insects in the coal forests

Insects were the first animals to fly, taking to the air by about 325 million years ago. During the late Carboniferous they diversified rapidly in the swampy Carboniferous coal forests that covered lowlands near the equator, including present-day western and central Europe.

Among the many groups that flourished were the griffinflies, formally known as the order Meganisoptera (once called Protodonata). They were related to modern dragonflies and damselflies, which together make up the order Odonata, and had a similar body plan, with two pairs of long wings and large eyes. Several griffinflies reached sizes never seen in insects today.

The coal forests also held other giant arthropods, notably the millipede relative Arthropleura, which reached an estimated 2.6 metres in length. There were no birds, bats or flying reptiles, so insects had the air to themselves.

Discovery at Commentry

Meganeura was found in the coal mines of Commentry, a town in the Allier department of central France. During the late 19th century the Commentry basin yielded thousands of fossil insects, preserved in fine-grained shales laid down in a lake surrounded by forest.

The French entomologist and palaeontologist Charles Brongniart studied these fossils. He described the species in 1884 and created the genus name Meganeura, meaning “large nerves”, in 1885, a reference to the network of veins supporting its wings. The species name monyi honours Stéphane Mony, who directed the Commentry mining company.

The original specimens are held by the National Museum of Natural History (Muséum national d’Histoire naturelle) in Paris. The rocks of Commentry are dated to the latest Carboniferous, about 300 million years ago, in the stage known in Europe as the Stephanian.

What Meganeura was like

Fossils of Meganeura consist mainly of wings and parts of the body. The wingspan is usually given as about 65–70 centimetres, and the body was probably around 30 centimetres long. By comparison, the largest living dragonflies have wingspans of up to about 16–19 centimetres.

Like modern dragonflies, Meganeura was almost certainly a predator. Its spiny legs would have helped it seize prey in flight, and strong jaws would have let it eat other insects and perhaps small amphibians. It probably hunted over open water and clearings in the forest.

Griffinfly wings differed from those of true dragonflies in details of their veins. They lacked the nodus, a small joint on the front edge of modern dragonfly wings, and the pterostigma, a dark weighted spot near the tip that helps to steady the wing in flight. Scientists debate how agile they were, but they were clearly capable fliers.

Nothing is known directly about Meganeura’s young. Modern dragonfly larvae live in water, and many researchers assume griffinfly larvae did too, although no larvae have been firmly linked to Meganeura.

The oxygen debate

Meganeura sits at the centre of a long-running scientific question: why could insects grow so large in the late Paleozoic but not today?

The oxygen hypothesis

Insects breathe through a network of air-filled tubes called tracheae, which carry oxygen directly to their tissues. Because this system relies largely on diffusion, it becomes less efficient as the insect gets larger. Higher oxygen levels in the air would make it easier for a large insect to supply its muscles, especially those used for flight.

Geochemical models, developed in particular by the American geologist Robert Berner, suggest that atmospheric oxygen reached a peak in the late Carboniferous and early Permian. Estimates commonly fall around 30–35 per cent, compared with about 21 per cent today. In 1995 Jeffrey Graham and colleagues argued in Nature that this “oxygen pulse” explains the giant insects and other features of late Paleozoic life.

Evidence and objections

In 2012 Matthew Clapham and Jered Karr compiled measurements of more than 10,500 fossil insect wings. They found that the maximum size of insects tracked estimated oxygen levels for about 200 million years. However, the link weakened from the Early Cretaceous, when birds evolved, suggesting that competition and predation from flying vertebrates later kept insects small.

Other researchers point out complications. Oxygen estimates differ between models, and one of the largest griffinflies, Meganeuropsis permiana, lived in the early Permian of North America when oxygen may already have been declining. Some argue that the larvae, living in water where oxygen is scarcer, would have been most sensitive to oxygen levels. Most scientists today see oxygen as one important factor among several, rather than the only explanation.

Relatives and extinction

Meganeura was not alone. Other large griffinflies are known from Europe and North America, and Meganeuropsis permiana, described from the early Permian of Kansas and Oklahoma, had a similar or slightly larger estimated wingspan. Together they are usually counted as the largest insects ever known.

Griffinflies survived into the Permian but disappeared by its end, around 252 million years ago, in the period of the Permian–Triassic mass extinction. Their dragonfly relatives survived and remain widespread today.

Myths and facts

Meganeura is often called a “giant dragonfly”. This is a useful shorthand, but strictly it was a griffinfly, a member of an extinct order that branched off before true dragonflies evolved. Its wings show differences in their vein patterns that separate the two groups.

It is also sometimes described as the size of a hawk or an eagle. Its wingspan of about 70 centimetres is comparable to that of a pigeon, but its body was slender and very light, so it would have weighed far less than any bird of that span.

Finally, Meganeura is frequently said to be the largest insect that ever lived. It shares that title with Meganeuropsis permiana; because both are known from incomplete fossils, it is not possible to say with certainty which was larger. No living insect comes close to their wingspan.

By the numbers

Timeline

• c. 325 million years ago: Winged insects are present in the fossil record.
• c. 300 million years ago: Meganeura lives around a lake in the Commentry basin, central France.
• c. 280 million years ago: Meganeuropsis permiana lives in what is now Kansas and Oklahoma.
• c. 252 million years ago: Griffinflies disappear by the end of the Permian.
• 1884: Charles Brongniart describes the Commentry species.
• 1885: Brongniart names the genus Meganeura.
• 1995: Graham and colleagues link late Paleozoic gigantism to an oxygen pulse.
• 2012: Clapham and Karr publish a large study linking insect size to oxygen and bird evolution.

Why it matters

Meganeura shows how different life on land was 300 million years ago, when insects ruled the air and grew to remarkable sizes. As a test case for the link between atmosphere and body size, it connects fossil insects to the history of Earth’s air. It remains one of the most popular images of prehistoric life in museums across Europe.