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, 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.

Meganeura Temporal range: Gzhelian, 304–299 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Original (and most complete) specimen of Meganeura monyi, MNHN R51142
Close view of the wings of the original Meganeura monyi specimen
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Infraclass:	Palaeoptera
Superorder:	Odonatoptera
Order:	†Meganisoptera
Family:	†Meganeuridae
Genus:	†Meganeura Brongniart, 1885
Type species
†Dictyoneura monyi Brongniart, 1884
Synonyms
Genus synonymy Meganeurella Handlirsch, 1919 Species synonymy M. aeroplana Handlirsch, 1919 M. brongniarti Handlirsch, 1906 M. brongniartiana Handlirsch, 1919 M. draco Handlirsch, 1919 M. fafnir Handlirsch, 1906 M. rapax Handlirsch, 1919

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)^[2] and a wingspan about 65–75 cm (2.13–2.46 ft),^[3][4][5] 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.^[6]

History

Meganeura monyi

Meganeura sculpture in Commentry, France

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.^[6]

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.^[7]

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.^[8][9] Brongniart's monograph recognized six specimens of Meganeura monyi: the original four-winged specimen and five isolated wing fragments. Half a century later, Frank M. Carpenter (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.^[10] Fernand Meunier [fr] (1909) listed an additional Meganeura monyi specimen at the MNHN: a slab preserving portions of the thorax, wings, and spiny legs.^[11]

Other supposed species

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.^[8] Most subsequent studies classified M. selysii as a separate genus, Meganeurula,^[12][11][13] though some authors only reluctantly maintained separation between the two genera.^[10][14]

Meunier (1909)'s specimen, MNHN R52938 (named as ''Meganeurella rapax'' by Handlirsch, 1919)

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),^[12] M. brongniartiana, M. draco, and M. aeroplana (named in 1919).^[13] His 1919 report also created a new genus and species for Meunier's specimen: Meganeurella rapax.^[13]

Illustrations of Meganeura monyi fossils in Brongniart's 1894 monograph

Carpenter's revision noted that the illustrations in Brongniart's monograph are rife with artistic license.^[10] 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.^[10] Meunier (1909)^[11] and Auguste Lameere (1917)^[15] 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.^[10]

A few studies have attempted to identify Meganeura fossils outside Commentry, though none are considered valid. In 1914, Herbert Bolton described a large meganeurid wing from the discard heap of Radstock colliery in Somerset, England. He named it Meganeura radstockensis,^[16] but Handlirsch (1919) and all subsequent authors considered the fossil to belong to its own genus, Boltonites.^[13] According to Nel et al. (2009),^[14] Meganeura vischerae, from Early Permian Russia,^[17] is an indeterminate insect. This also seems to be the case for purported Meganeura fossils from the Pictou Group of Nova Scotia.^[18][14]

Paleoecology

Life restoration of Meganeura monyi

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.^[6] 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.^[19]

Paleobiology

Size

There has been some controversy as to how insects of the Carboniferous period were able to grow so large.

• Oxygen levels and atmospheric density. 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 Harlé (1911) that Meganeura was able to fly only because the atmosphere of Earth at that time contained more oxygen than the present 20 percent. This hypothesis was initially dismissed by fellow scientists, but has found approval more recently through further study into the relationship between gigantism and oxygen availability.^[20] If this hypothesis is correct, these insects would have been susceptible to falling oxygen levels and certainly could not survive in our modern atmosphere. Other research indicates that insects really do breathe, with "rapid cycles of tracheal compression and expansion".^[21] Recent analysis of the flight energetics of modern insects and birds suggests that both the oxygen levels and air density provide an upper bound on size.^[22] The presence of very large Meganeuridae with wing spans rivaling those of Meganeura during the Permian, when the oxygen content of the atmosphere was already much lower than in the Carboniferous, presented a problem to the oxygen-related explanations in the case of the giant dragonflies. However, despite the fact that meganeurids had the largest-known wingspans, their bodies were not very heavy, being less massive than those of several living Coleoptera; therefore, they were not true giant insects, only being giant in comparison with their living relatives.
• Lack of predators. Other explanations for the large size of meganeurids compared to living relatives are warranted.^[2] Bechly (2004) 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.
• Aquatic larvae stadium. Another theory suggests that insects that developed in water before becoming terrestrial as adults grew bigger as a way to protect themselves against the high levels of oxygen.^[23]

See also

• Paleontology portal

• List of largest insects

Notes

1. Meunier (1909)'s perception of "Meganeura brongniarti" was not based on any of Brongniart's fossils. Instead it was inspired by a specimen which Handlirsch (1919) would later name Meganeurina confusa. Carpenter (1943) considered it a species of Meganeurula, while Nel et al. (2009) tentatively treated Meganeurina as a valid genus of tupine meganeurid.