Meganeura

Meganeura (Ancient Greek: μέγα (large) + νευρόν (vein or nerve))^[1] is a genus of extinct insects from the Late Carboniferous (approximately 300 million years ago). It is a member of the extinct order Meganisoptera (also known as griffenflies^[2]), which are closely related to and resemble dragonflies and damselflies (with dragonflies, damselflies and meganisopterans being part of the broader group Odonatoptera). Like other odonatopterans, they were predatory, with their diet mainly consisting of other insects.

The genus belongs to the Meganeuridae, a family including other similarly giant dragonfly-like insects ranging from the Late Carboniferous to Middle Permian. With single wing length reaching 32 centimetres (13 in) and a wingspan about 65–75 cm (2.13–2.46 ft), M. monyi is one of the largest-known flying insect species. Fossils of Meganeura were first discovered in Late Carboniferous (Stephanian) Coal Measures of Commentry, France, in 1880. In 1885, French paleontologist Charles Brongniart described and named the fossil "Meganeura", which refers to the network of veins on the insect's wings. All valid specimens of Meganeura are housed in the National Museum of Natural History in Paris. Despite being the iconic "giant dragonfly", fossils of Meganeura are poorly preserved in comparison to other meganeurids.

In the late 1870s, the Commentry shales attracted Charles Brongniart, a pioneering paleoentomologist from the Muséum national d'histoire naturelle (MNHN, the National Natural History Museum in Paris). In 1884, Brongniart published a brief article summarizing a few gigantic insect fossils supplied by Commentry's mining engineer, Henri Fayol. One fossil was a four-winged insect, with each wing at least 30 cm (12 in) long. Brongniart found it similar to Dictyoneura, an insect now recognized as a member of the extinct order Palaeodictyoptera. He named the four-winged fossil as a new species, Dictyoneura monyi, in honor of Stéphane Mony, the recently-deceased manager of the Commentry mines.^[3]

The following year, Brongniart decided to separate Dictyoneura monyi into its own genus: Meganeura, meaning "large vein".^[1] By 1894, he had accumulated enough insect fossils to publish a detailed monograph.^[4][5] Brongniart's monograph recognized six specimens of Meganeura monyi: the original four-winged specimen and five isolated wing fragments. Half a century later, entomologist Frank M. Carpenter in 1943 realized that half of Brongniart's specimens were actually counterparts of the other half, meaning that there were only three unique individuals in Brongniart's collection.^[6] Entomologist Fernand Meunier in 1909 listed an additional Meganeura monyi specimen at the MNHN: a slab preserving portions of the thorax, wings, and spiny legs.^[7]

In the decades between Brongniart's monograph and Carpenter's 1943 revision, there was a great deal of confusion regarding how many species belonged in the genus Meganeura. Beyond M. monyi, Brongniart proposed a second species of Meganeura, Meganeura selysii.^[4] Most subsequent studies classified M. selysii as a separate genus, Meganeurula,^[8][7][9] though some authors only reluctantly maintained separation between the two genera.^[6][10]

Entomologist Anton Handlirsch named five new Meganeura species based on the illustrated M. monyi wing fragments in Brongniart's monograph, but he did not inspect the fossils in person. Handlirsch's Meganeura species include M. brongniarti, M. fafnir (named in 1906),^[8] M. brongniartiana, M. draco, and M. aeroplana (named in 1919).^[9] His 1919 report also created a new genus and species for Meunier's specimen: Meganeurella rapax.^[9]

Carpenter's revision noted that the illustrations in Brongniart's monograph are rife with artistic license.^[6] Broken slabs are illustrated as whole, and the reconstructed complete wing diagram is much broader than the fossils indicate. Handlirsch's species were inspired by subtle differences in the illustrations, and these differences did not hold up to scrutiny once the fossils are inspected in person.^[6] Meunier in 1909,^[7] and entomologist Auguste Lameere in 1917,^[11] doubted the validity of M. fafnir while upholding M. brongniarti, though they disagreed on how to diagnose it.^{[note 1]} Carpenter went a step further by recognizing that Handlirsch named multiple species for the same individual, broken across part and counterpart slabs. M. aeroplana is the partial counterpart to the original four-winged specimen, M. brongniarti and M. brongniartiana are counterparts to each other, and M. fafnir and M. draco are counterparts to each other. According to Carpenter, all of these fossils, as well as Meunier's specimen, represent a single species: Meganeura monyi.^[6]

A few studies have attempted to identify Meganeura fossils outside Commentry, though none are considered valid. In 1914, palaeontologist Herbert Bolton described a large meganeurid wing from the discard heap of Radstock colliery in Somerset, England. He named it Meganeura radstockensis,^[12] but Handlirsch in 1919, and all subsequent authors, considered the fossil to belong to its own genus, Boltonites.^[9] According to Nel and colleagues in,^[10] Meganeura vischerae, from Early Permian Russia,^[13] is an indeterminate insect. This also seems to be the case for purported Meganeura fossils from the Pictou Group of Nova Scotia.^[14][10]

With a single-wing length of 32 centimetres (13 in),^[15] a wingspan about 65–75 cm (2.13–2.46 ft),^[16][17][18] and an estimated body length of up to 60–75 cm (1.97–2.46 ft),^[19] Meganeura monyi is one of the largest-known flying insect species. Mass estimates for M. monyi have varied. In 1982, Michael L. May provided a very low mass estimate of 17.8 g (0.039 lb) based on an evaluation of the maximum amount that M. monyi's wing muscles could have lifted.^[20] Graham E. Dorrington calculated that the similarly sized Meganeuropsis permiana weighed 34 g (0.075 lb).^[21] In 2018, Alan E. R. Cannell gave a far larger estimate and determined that specimens whose wingspans exceeded 70–75 cm (2.30–2.46 ft) may have had a body mass of 100–150 g (0.22–0.33 lb). According to Cannell, the muscle power necessary to lift such a heavy weight would indeed have been present, though would have come at a major cost in thermoregulation.^[22]

Though Meganeura is the archetypal griffenfly, its overall anatomy is poorly known. Griffenflies as a clade had greatly enlarged compound eyes, which in the case of Meganeurites gracilipes, met along the midline for most of their length; among modern dragonflies, the same is seen in darners and several "libelluloid" clades. The position and morphology of Meganeurites' eyes suggests that griffenflies like Meganeura had extremely good vision and a similar "hawking" ecology to those modern taxa.^[23] Unlike modern dragonflies, the thorax and abdomen of M. monyi were roughly equal in thickness.^[22] The abdomen was overall long and wide.^[22][24] The wings of M. monyi could be distinguished from those of modern dragonflies by three morphological traits: firstly, the radius of the hindwing was unbranched, whereas that of the forewing bore two veins which emerged closely together; secondly, M. monyi had an unveined precostal area (a short part of the wing between the costal vein and the wing edge); and thirdly, the subcostal veins extended almost to the wingtips. Unlike modern dragonflies, Meganeura's tarsi (the last limb segment) had four joints.^[25] The tarsi bore large spines, like those of damselflies and dragonflies, which would have been used for prey capture.^[23][10]

Meganeura belongs to Meganisoptera, an order of extinct insects often improperly referred to as giant dragonflies; due to their phylogenetic remoteness and morphological dissimilarity from damselflies and dragonflies (Odonata), the term griffenflies was proposed for the group by David A. Grimaldi and Michael S. Engel in 2005.^[2][10]

There has been some controversy as to how insects of the Carboniferous period were able to grow so large. The way oxygen is diffused through the insect's body via its tracheal breathing system puts an upper limit on body size, which prehistoric insects seem to have well exceeded.

It was originally proposed by railway engineer Édouard Harlé in 1911 that Meganeura was able to fly only because the atmosphere of Earth at that time contained more oxygen than the current 20%.^[26] This hypothesis was initially dismissed, though did find some support after further study into the relationship between gigantism and oxygen availability.^[27][28] If this hypothesis is correct, these insects would have been susceptible to falling oxygen levels and certainly could not survive in Earth's modern atmosphere. However, this hypothesis was predicated largely on the idea that insects did not actively breathe. Later research indicates that insects do indeed breathe, with "rapid cycles of tracheal compression and expansion".^[29] Recent analysis of the flight energetics of modern insects and birds suggests that both the heightened oxygen levels and air density of the Carboniferous provide an upper bound on size.^[28] A general problem with all oxygen-related explanations of giant griffenflies is the fact that very large meganisopterans (Arctotypus sp.)^[30] with a wingspan of 45 cm (18 in) also occurred in the Upper Permian of Lodève in France, when the oxygen content of Upper Permian atmosphere was much lower than any other geologic stage.^[31]

As such, other explanations for the large size of meganeurids compared to living relatives are have been put forward.^[15] In 2004, paleontologist Günter Bechly suggested that the lack of aerial vertebrate predators allowed pterygote insects to evolve to maximum sizes during the Carboniferous and Permian periods, perhaps accelerated by an evolutionary arms race for increase in body size between plant-feeding Palaeodictyoptera and Meganisoptera as their predators. In Bechly's model, the evolution of flying tetrapods such as pterosaurs, which were more agile and less restricted by drag, may have provided a predatorial threat which the giant insects of the Palaeozoic could not overcome.^[32]

It has been suggested that the diversification of true odonatans in the Mesozoic may correspond to the extinction of meganisopterans around the time of the Permian-Triassic extinction event.^[2]

Research on close relatives Meganeurula and Meganeurites suggest that Meganeura was adapted to open habitats, and similar in behaviour to extant hawkers. The eyes of Meganeura were likely enlarged relative to body size. Meganeura had spines on the tibia and tarsi sections of the legs, which would have functioned as a "flying trap" to capture prey.^[23] An engineering examination estimated that the mass of the largest specimens with wingspans over 70 cm to be 100 to 150 grams. The analysis also suggested that Meganeura would be susceptible to overheating.^[22]

Meganeura is one of many insects recovered from coal mines on the outskirts of Commentry, France. Commentry was a major component of France's 19th century coal industry, but it also gained renown among paleontologists as one of the best sources of Carboniferous insect fossils in the world. The fossils of Commentry are from the Gzhelian stage of the Carboniferous, about 304 to 299 million years ago.^[23] Also known from Commentry is Meganeurites, on which Meganeura may have predated.^[22]

• List of largest insects

• Bechly, G (2004). "Evolution and systematics" (PDF). In Hutchins, M.; Evans, A.V.; Garrison, R.W. & Schlager, N. (eds.). Grzimek's Animal Life Encyclopedia. Vol. Insects (2nd ed.). Farmington Hills, MI: Gale. pp. 7–16.
• Chapelle, Gauthier & Peck, Lloyd S. (May 1999). "Polar gigantism dictated by oxygen availability". Nature. 399 (6732): 114–115. Bibcode:1999Natur.399..114C. doi:10.1038/20099. S2CID 4308425.
• Dudley, Robert (April 1998). "Atmospheric oxygen, giant Paleozoic insects and the evolution of aerial locomotion performance". The Journal of Experimental Biology. 201 (Pt8): 1043–1050. doi:10.1242/jeb.201.8.1043. PMID 9510518.
• Harlé, Edouard (1911). "Le Vol de grands reptiles et insectes disparus semble indiquer une pression atmosphérique élevée". Extr. Du Bulletin de la Sté Géologique de France (in French). 4 (9): 118–121.
• Nel, André; Fleck, Günther; Garrouste, Romain & Gand, Georges (2008). "The Odonatoptera of the Late Permian Lodève Basin (Insecta)". Journal of Iberian Geology. 34 (1): 115–122.
• Rake, Matthew (2017). Prehistoric Ancestors of Modern Animals. Hungry Tomato. p. 20. ISBN 978-1512436099.
• Taylor, Paul D.; Lewis, David N. (2007). Fossil Invertebrates (repeated ed.). Harvard University Press. p. 160. ISBN 978-0674025745.
• Westneat, MW; Betz, O; Blob, RW; Fezzaa, K; Cooper, WJ & Lee, WK (January 2003). "Tracheal respiration in insects visualized with synchrotron x-ray imaging". Science. 299 (5606): 558–560. Bibcode:2003Sci...299..558W. doi:10.1126/science.1078008. PMID 12543973. S2CID 43634044.

Media related to Meganeura at Wikimedia Commons

• Picture of life sized model of Meganeura monyi made for Denver Museum of Natural History.