Meganeura (Ancient Greek: μέγα (large) + νευρόν (vein or nerve)) is a genus of extinct insects from the Late Carboniferous (about 300 million years ago). It is a member of the extinct order Meganisoptera (also known as griffenflies), which 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 around , 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 midline, 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 such as 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, as insects’ largely passive respiratory system diffuses oxygen to their tissues with difficulty at large body sizes.
Much debate exists regarding 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. 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". By 1894, he had accumulated enough insect fossils to publish a detailed monograph.Atlas of Plates (XVII–LIII, 1–37) for Brongniart, Charles (1894). "Recherches pour servir à l'histoire des insectes fossiles des temps primaries". Thèses présentées a la Faculté des Sciences de Paris. 821: 1–494. 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 only three unique individuals were 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
In the decades between Brongniart's monograph and Carpenter's 1943 revision, a great deal of confusion arose regarding how many species belonged in the genus Meganeura. Beyond M. monyi, Brongniart proposed a second species, Meganeura selysii. Most subsequent studies classified M. selysii as a separate genus, Meganeurula, 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), M. brongniartiana, M. draco, and M. aeroplana (named in 1919). His 1919 report also created a new genus and species for Meunier's specimen: Meganeurella rapax.Carpenter's revision noted that the illustrations in Brongniart's monograph are rife with artistic license. 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. Meunier in 1909, and entomologist Auguste Lameere in 1917, doubted the validity of M. fafnir while upholding M. brongniarti, though they disagreed on how to diagnose it.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. 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.
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, but Handlirsch in 1919, and all subsequent authors, considered the fossil to belong to its own genus, Boltonites. According to Nel and colleagues in 2009, Meganeura vischerae, from Early Permian Russia, is an indeterminate insect. This also seems to be the case for purported Meganeura fossils from the Pictou Group of Nova Scotia.
Description
Size
With a single-wing length of , a wingspan about , 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's wing muscles could have lifted, though he did note that the procedure he used may underestimate body mass. In 2002, Gregory S. Paul estimated Meganeura's body mass at around . 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 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 suggest 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 abdomen was overall long and wide. 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. Among griffenflies, its wings were distinguished by a subnodal crossvein which sits distal to the subcosta posterior vein, as in Gallotupus and Tupus; in others, this vein was either basal to the subcosta posterior vein's apex, or opposite to it. Near each wing base was a protuberance or gibbosity, also seen in Sinomeganeura. Unlike modern dragonflies, each of M. monyi's tarsi (the last limb segment) had four joints. The tarsi bore large spines, like those of damselflies and dragonflies, which would have been used for prey capture.
Classification Meganeura belongs to the 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" (sometimes spelled griffinflies) was proposed for the group by David A. Grimaldi and Michael S. Engel in 2005.
Paleobiology
Evolution of large body size Some controversy remains 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.
High oxygen levels hypothesis