Meganeura (Ancient Greek: μέγα (large) + νευρόν (vein or nerve)) 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), which are closely related to and resemble dragonflies and damselflies (with dragonflies, damselflies and meganisopterans being part of the broader group Odonatoptera). While various species of Meganeura have been named, only one is now considered valid: the type species, M. monyi.
Fossils of Meganeura were first discovered in Late Carboniferous (Stephanian) Coal Measures of Commentry, France, in 1880. In 1884, French paleontologist Charles Brongniart described the type specimen and assigned it to the genus Dictyoneura. The following year, he assigned it to a genus of its own, Meganeura, a name which refers to the network of veins on the insect's wings. Several specimens have been discovered since, though all are poorly preserved and some have been reassigned to other genera. All valid specimens of Meganeura are housed in the National Museum of Natural History in Paris. The genus belongs to the Meganeuridae, a family including other similarly giant dragonfly-like insects ranging from the Late Carboniferous to Middle Permian. Despite being the iconic "giant dragonfly", fossils of Meganeura are poorly preserved in comparison to most of its relatives.
With single wing length reaching , a wingspan of about , and mass estimates ranging from to M. monyi is one of the largest known flying insect species and is considerably larger than the biggest modern dragonfly, Petalura ingentissima. Based on other meganeurids, it would have had large compound eyes which met along the midine, similar to some modern dragonflies. It was equipped with large spines on its tarsi (the end segments of its limbs), like those of modern odonatopterans, which would have been used for prey capture. Like other odonatopterans, meganeurids like Meganeura were predatory, with their diet mainly consisting of other insects. Their size and wing anatomy means that they were not capable of the same kinds of abrupt direction changes as modern dragonflies, and they may have spent much of their time perching to properly oxygenate their tissues.
There is much debate over how Meganeura attained such a large body size, with hypotheses ranging from a correlation with the high oxygen content of the Late Carboniferous (though this is contradicted by the presence of giant meganisopterans in the Late Permian, after oxygen levels had dropped) to the absence of other big aerial predators. It has been suggested that the extinction of meganisopterans as a whole and the absence of similarly large flying insects may correlate with the evolution of flying tetrapods such as pterosaurs.
Discovery
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 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.
The following year, Brongniart decided to separate Dictyoneura monyi into its own genus: Meganeura, meaning "large vein". 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. Entomologist in 1909 listed an additional Meganeura monyi specimen at the MNHN: a slab preserving portions of the thorax, wings, and spiny legs.
Other species
left|thumb|Meunier (1909)<nowiki>'s specimen, MNHN R52938 (named as </nowiki>Meganeurella rapax<nowiki></nowiki> by Handlirsch, 1919)
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. though some authors only reluctantly maintained separation between the two genera.
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), doubted the validity of M. fafnir while upholding M. brongniarti, though they disagreed on how to diagnose it. 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. but Handlirsch in 1919, and all subsequent authors, considered the fossil to belong to its own genus, Boltonites. is an indeterminate insect. This also seems to be the case for purported Meganeura fossils from the Pictou Group of Nova Scotia. and an estimated body length of up to , Meganeura monyi is one of the largest-known flying insect species, five times the length and twice the thoracic width of the largest modern dragonflies. Mass estimates for M. monyi have varied. In 1982, Michael L. May provided a very low mass estimate of based on an evaluation of the maximum amount that M. monyi<nowiki/>'s wing muscles could have lifted, though he did note that the procedure he used may underestimate body mass. Graham E. Dorrington calculated that the similarly sized Meganeuropsis permiana weighed . In 2018, Alan E. R. Cannell contended that May's work yielded a major underestimate. He gave a larger estimate and determined that specimens whose wingspans exceeded may have had a body mass of . According to Cannell, the necessary muscle power would indeed have been present.
Anatomy
thumb|Life restoration of Meganeura monyiThough 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<nowiki/>' eyes suggests that griffenflies like Meganeura had extremely good vision and a similar "hawking" ecology to those modern taxa. Unlike modern dragonflies, the thorax and abdomen of M. monyi were roughly equal in thickness. 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 dragonflies, it and other meganeurids lacked nodal flexion structures on their wings, reducing their mobility. Unlike modern dragonflies, each of M. monyi<nowiki/>'s tarsi (the last limb segment) had four joints. This hypothesis was initially dismissed, though did find some support after further study into the relationship between gigantism and oxygen availability. 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". 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. with a wingspan of 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. A 2026 study by Edward P. Snelling and colleagues suggested that there was little correlation between body size and atmospheric oxygen content in invertebrates, based on the fact that the space occupied by tracheoles does not significantly differ between small and large species, despite the fact that an increase with body size would likely be advantageous in that it would reduce the diffusion distance for oxygen; if such a correlation existed, then an increase would be expected. Snelling and colleagues therefore concluded that there is probably no connection between diffusive oxygen transport mechanisms and a large body size.
Air dominance hypothesis
As such, other explanations for the large size of meganeurids compared to living relatives are have been put forward. 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.
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. The wingspans of certain taxa (such as Meganeurites) may have further hindered their mobility.