Paleo Profiles: Meganeura

Meganeura monyi

A common feature of media depicting prehistoric environments is the presence of giant insects or other arthropods. In reality, the truly giant invertebrates were not as common as they are often depicted, but that does not mean that they did not exist. They exist even today - the goliath beetles can reach over 11 centimetres in length (about 4.3 inches). However, during the Carboniferous period (358-298 million years ago) arthropods got to truly staggering sizes - a millipede called Arthropleura could grow to as long as a human is high. One of these giants was the Meganeura - a giant relative of today's dragonflies. 

Discovery and Fossils

The Meganeura was discovered in 1880 in among coal in Commentry, France, and five years later a palaeontologist called Charles Brongniart. He would become one of the pioneering palaeontologists in the study of insect evolution; he would often return to the coal sites in Commentry which regularly offer new and interesting insects from the Late Carboniferous. In 1885, Brongniart would look at the fossil and name it Meganeura, (Large-Nerved), for the perfectly preserved network of veins in the wings of the dragonfly-like insect. Luckily, palaeontologists have managed to unearth many specimens from Commentry, and in 1979 another well-preserved specimen was discovered in Derbyshire, northern England. We have discovered so many specimens that it is possible to identify three different species: M. brongniarti, M. monyi, and M. vischerae.

Biology

Meganeura size, Prehistoric-Wildlife.com

Despite their appearance Meganeura were not actually dragonflies. Instead, they belonged to a now extinct order called Meganisoptera, but better known as griffinflies. Griffinflies are currently classed in the same order as contemporary dragonflies and damselflies, so they are closely related to the insects which we recognise today. Meganeura and other griffinflies, like the even larger Meganeuropsis, were far larger than any current living dragonfly. The largest member of the odonta family, a damselfly from Central and South America called Megaloprepus caerulatus, has a wingspan of 19 cm (7.5 inches) whereas the smallest Meganeura specimens had a wingspan of 65 cm (25 inches)! Meganeura was a true giant of the Carboniferous skies. The largest species, M. monyi, could have a wingspan of up to 75 cm. This is about the same size as a pigeon, so the next time you see someone throwing seed at them just imagine a dragonfly that size. Insects share several body parts which are present in Meganeura. One of these is how the Meganeura got so massive. Across the body of insects and other arthropods are holes called spiracles, which can be seen on the moth larva below:

The spiracles of an Indian moon moth larva, wikipedia.org

Spiracles lead to a series of tubes called trachae, and smaller ones called tracheoles, which allows the arthropods to breathe through a process called diffusion. Substances move from an area of high concentration to low concentration, so oxygen moves from the air to the tissues through the trachae and tracheoles while carbon dioxide moves the other way. When we look at the atmosphere of the Carboniferous this will explain why the spiracles and trachae allowed the Meganeura to grow so large.

To imagine what a Meganeura looked and lived in life just look at today's damselflies and dragonflies. Meganeura would start life in the water as a nymph preying on other aquatic life, including other nymphs, until they grew large enough to take to the air. Brightly coloured and very fast they would dart through the air of the Carboniferous swamps catching smaller insects on the fly. Dragonflies and damselflies are predators rapidly striking and catching prey so Meganeura would do the same. As modern dragonflies are territorial, to have the best perches to lunge from after food, so would the Meganeura. It takes a lot of energy to keep up the rapid wing beats needed to sustain fast and agile flight, so dragonflies need a monopoly on possible prey in their vicinity. With the smallest Meganeura being three times larger than the largest of modern dragonflies or damselflies it required a lot more energy - luckily its prey was also fairly large. Dragonflies and damselflies have a unique mating system forming a circle or heart shape with the ends of their abdomens, as well as seeing males fighting over females. Imagine seeing that with dragonflies the size of pigeons!

When and Where

Meganeura in the BBC documentary Walking with Monsters

The Meganeura lived at the end of the Carboniferous period between 305 and 299 million years ago. This was during the Paleozoic, and many forms of life we recognise today did not exist - at least on the land. The first reptiles had arrived, but they small, being able to easily fit in the palm of your hand. If you wanted to look for large vertebrates you would have to look in the water - amphibians like Proterogyrinus resembling crocodiles or monitor lizards would roam the shorelines. Meganeura has been found in the coal rich regions of Western Europe - particularly northern England, Commentry in France, and some remains in Scotland. The swamps which existed during the Carboniferous over millions of years fossilised and formed coal - the Industrial Revolution relied on the burning of plants from millions of years before the first dinosaur evolved. The Carboniferous was wetter, hotter, and richer in oxygen compared to today's atmosphere. Today, the air we breathe is roughly 21% oxygen (unfortunately that is rapidly changing due to carbon emissions), but in the Carboniferous it was around 35%. Plants and trees were the reason for this. The bacteria which decomposes foliage and release carbon dioxide into the atmosphere had yet to evolve to fully breakdown plant matter. As a result, more carbon dioxide was taken in by plants but not released through decomposition creating an atmosphere with greater oxygen content. Hotter and wetter environments not only created swamps across the world, but it also allowed more forests to flourish. This was the perfect environment for griffinflies. Lots of water to lay eggs, and flourishing plant life allowed more animal life to eat. It also allowed arthropods to grow to giant sizes.

As we discussed earlier, insects breathe using spiracles leading to tracheoles. They are also used for thermoregulation - water can exit and enter the spiracles, so arthropods can close the spiracles to stop water loss. An atmosphere rich in oxygen gave arthropods the opportunity to grow larger - more oxygen allows larger bodies without compromising diffusion. This is why the Carboniferous forests became coal after millions of years; humans are now releasing their carbon. The humidity affected this as well. Larger bodies mean larger spiracles, and a larger surface area to lose water from; a humid environment heavily reduced the amount of water lost. Carboniferous swamps allowed Meganeura to grow large without suffocating or dehydrating. We see this today - the largest damselfly is found in the rainforests of Central and South America, and the goliath beetles are found in the rainforests of central Africa. Scientists at Arizona State University raised insects in controlled oxygen rich environments, and found that, over just a few generations, their size rapidly increased. Another theory has emerged which also explains the size of Meganeura and other arthropods. Unlike adults, larva cannot control their oxygen intake, and oxygen can be dangerous in high quantities - that is why you get light headed when you breathe directly from oxygen canisters. In an oxygen rich environment regular sized larva could potentially be killed from too much oxygen. However, a larger size means that larva can safely take in oxygen without posing a health risk, this leads to larger adults. Finally, palaeontologists have also stated that Meganeura could grow to such large sizes as they had few large predators. The only fully terrestrial animals were either invertebrates and small reptiles, so Meganeura could become large thanks to lack of competition.

Extinction

Why then did the Meganeura go extinct? The same thing which is currently driving thousands to millions of species to extinction now: climate change. While we know what is causing contemporary climate change (our own actions), we still are unsure of what caused the Carboniferous climate shift. From around 305 to 300 million years ago the planet became both warmer and drier. This devastated the swamps which covered particular Europe and North America. As swamps started disappearing this put increased pressure on the habitats of Meganeura and other large invertebrates, like the 2 m long millipede Arthropleura. Swamps also trap carbon dioxide, so as swamps vanished the levels of carbon dioxide in the atmosphere rose. The bacteria which decomposed foliage and trees also began to evolve, and decompose dead trees in higher numbers releasing carbon dioxide which otherwise would not have been released. Between shrinking forests and bacteria releasing more carbon dioxide the Meganeura could not adapt. A small mass extinction event happened - the Arthropleura went extinct around 300 million years ago and the last Meganeura followed it 299 million years ago. Griffinflies continued to exist until the early Permain with the even larger Meganeuropsis existing until 283 million years ago. The drop in oxygen levels and disappearance of giant swamps meant that griffinflies could not compete, especially as reptiles bounced back quickly from the Carboniferous extinction event. The Age of Invertebrates soon gave way to the Age of Reptiles.

The sources I have used are as follows:

-Robert Dudley, 'Atmosphere Oxygen, Giant Paleozoic Insects, and the Evolution of Aerial Locomotor Performance', The Journal of Experimental Biology, 201, (1998), 1043-1050

-'Meganeura', Prehistoric-Wildlife.com, [Accessed 22/06/2019]

-Alan Cannell, 'The Engineering of the Giant Dragonflies of the Permian: Revised body mass, power, air supply, thermoregulation, and the role of air density', Journal of Experimental Biology, 221, (2018), 1-7

-Michael May, 'Heat Exchange and Endothermy in Protodonata', Evolution, 36:5, (1982), 1051-1058

-PBS Eons, 'The Age of Giant Insects', YouTube, 18/09/2017, [Accessed 22/06/2019]

-Gauthier Chapelle and Lloyd Peck, 'Polar Gigantism dictated by Oxygen Availability', Nature, 399, (1999), 114-115

-'Reptile's Beginnings', Walking with Monsters, (2005), BBC, 15 December

Thank you for reading and I hope you found it interesting. For other Paleo Profiles we have a list here. For future blog updates please see our Facebook or catch me on Twitter @LewisTwiby.