Meganeura

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 32 centimetres (13 in), a wingspan around 65–75 cm (2.13–2.46 ft), and mass estimates ranging from 17.8 g (0.039 lb) to 100–150 g (0.22–0.33 lb) 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 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. 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. 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 Fernand Meunier 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.

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