Odonatoptera del Permico Superior de la Cuenca de Lodeve (Insecta).
The Late Permian Lodeve Basin has given a very rich and diverse fauna of the superorder Odonatoptera, with about 60 specimens in five families, i.e. the large dragonfly-like Meganeuridae Handlirsch, 1908 (Paleozoic Meganisoptera) and Lapeyridae Nel et al., 1999 (sister group of Nodialata, only known from Lodeve), and the small damselfly-like Saxonagrionidae Nel et al., 1999 (oldest representative of the modern clade Panodonata), Permepallagidae Martynov, 1938, and Permolestidae Martynov, 1932 (both belonging to the Permian-Early Cretaceous clade Protozygoptera) (Nel et al, 1999a,b,c) (Fig. 1). The diverse Permian clade Protanisoptera Carpenter, 1931 is lacking in the Lodeve Basin (Fig. 2). This group, known from the Early Permian of Elmo (USA), Early Permian and Late Permian (Kazanian) of Russia, Late Permian of Brazil, and Late Permian of Australia, is clearly widespread (Huguet et al, 2002). Its real (or apparent?) absence in the Lodeve Basin is very surprising. It could correspond to particular ecological preferences, but the presence of a Protanisoptera in the Elmo Formation, supposed to have a similar palaeoenvironment, would contradict this hypothesis.
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2. The Lodeve Basin: a 'desert' with numerous aquatic odonatopteran insects?
The Odonatoptera of the Lodeve Formation are all adult specimens, mainly isolated wings, or fragments of body. Some meganeurid specimens have their four wings disposed in life position but with the body almost missing. These insects have been disarticulated after their death by necrophagous animals, by the Triopsidae (Crustacea: Branchiopoda), which are extremely frequent and associated to the fragments of body of Meganeuridae (Fig. 3).
The wings of the Meganeuridae from Lodeve can be easily separated into five categories, corresponding to at least five different species, i.e. a small species with wings 6-7 cm long, two medium species with wings 8-11.5 cm long (respectively with hind wings 16 mm wide and 26 mm wide), one large species with wings 16-17 cm long, and one very large species with wings 21-23 cm long (Figs. 49). Therefore this formation has the highest known diversity for a meganeurid fauna. It is more diverse than those of the Late Carboniferous, currently alleged period of highest diversity for this family. These species with very different wing sizes certainly had different life habits (different types of flight and sizes of preys), as for the recent Odonata: Anisoptera (dragonflies). Small species probably lived in zones with vegetation (forest, bush), while the largest ones lived in open environments.
The larvae of all Odonatoptera (except few particular cases of recent taxa with terrestrial larvae) eat and breathe in water. The great majority of the recent species develop within several months but some in few weeks. Nothing is known on the early stages of the Palaeozoic Odonatoptera, except for an undescribed record of a large Late Carboniferous aquatic larva attributable to a Meganeuridae (Kukalova-Peck, pers. comm.; Grimaldi and Engel, 2005: Fig. 2.42). As the recent Odonata have primitively aquatic larvae and as the potential sister groups of the Odonatoptera (Ephemeroptera and Paleodictyopteroid orders) have aquatic larvae, the situation was probably the same for the Paleozoic Odonatoptera. The recent Odonata are attracted by humid environments suitable for their life cycle. Some females can erroneously lead their eggs in the sea.
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Therefore, the presence of a diverse odonatopteran fauna at Lodeve implies the presence of more or less close water bodies (pool, pond, lake, stream, etc.). The palaeoenvironment is supposed to be dry with episodic and more or less short rainy periods, with the presence of waters. The presence of a diverse and rich odonatopteran fauna would contradict this reconstruction. These dragonfly-and damselfly-like insects could have migrated periodically from another more humid place, and reached this area when it was filled with temporary water bodies. This hypothesis of a migratory origin for all these Odo natoptera could explain the frequent presence of adults and the total absence of larvae. But the absence of fossilised larvae does not exclude that they could not occasionally or regularly develop there, with very rapid growth. The presence of five adult specimens of the same species of Meganeuridae on the same slab of rock also suggests that numerous specimens could fly all together, falling and dying in the wet mud in the same time (Fig. 10). A similar recent situation occurs in Mauretania, where several Odonata migrate along the ocean beach and are attracted by the water bodies after the erratic rains.
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Another hypothesis could be that every year, at the end of the wet period, the adults mate and led eggs in soft mud, then died massively. After the eggs stay in dry mud still next rain season, and then developed quickly. This implies a regular alternation of rainy and dry seasons. Note that Sheldon (2005) recently questioned the current interpretation of the red Late Permian beds as desert deposits, but could be the result of a good drainage of the soils, under warm and humid conditions (red Late Permian of Sardinia).
3. The Late Permian 'giant' Meganeuridae ignored that they could not be!
Atmospheric composition greatly changed during the Phanerozoic (Beerling, 1999). In particular, the Late Carboniferous is supposed to be the period of highest oxygen and lowest carbon dioxide atmospheric concentrations but on the contrary, the Late Permian shows a minimum of oxygen atmospheric concentration (Dudley, 1998; Berner et al, 2000; Berner, 2005). These changes are currently supposed to have induced the development of 'giant' arthropods during the former period, and their extinction during the latter (Graham et al., 1995; Dudley, 1998; 2001; Spicer and Gaston, 1999; Chapelle and Leck, 1999; Gans et al., 1999; Berner et al., 2000). Increased oxygen concentration would have permitted insects to become larger by increasing diffusive permeation (Graham et al., 1995).
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'Gigantism' affected myriapods, scorpions, mayflies, palaeodictyopterids, and Odonatoptera (Grimaldi and Engel, 2005). Some of these alleged 'giants' were erroneous considered as terrestrial, viz. Selden et al. (2005) recently demonstrated that Megarachne servinei is an aquatic Eurypterida. The giant Odonatoptera: Meganeuridae are the 'symbol' and best known of these 'giant' Car boniferous and Early Permian insects, with 34 described species. The largest representatives are Meganeura monyi (Brongniart, 1884) (wing length about 320 mm), Meganeuropsis permiana Carpenter, 1939 (wing length about 330 mm), Meganeuropsis americana Carpenter, 1947 (wing length 305 mm), and Megatypus shucherti Tillyard, 1925 (wing 195 mm long) (Carpenter, 1947, 1992; Rasnitsyn and Pritykina, 2002; Grimaldi and Engel, 2005). The Meganeuridae are supposed to have decreased in size and diversity during the Late Permian. The family is absent in the Triassic and probably became extinct somewhere during a period of 20 M.a. between the Late Permian and the Early Triassic.
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The Late Permian is supposed to be the period with lowest oxygen atmospheric concentrations. Thus after the preceding theory, large to very large Meganeuridae should be already extinct during this period. But the present discoveries alienate this theory:
The inference of wing length for the incomplete specimen Ld LAP 497 gives a wing length of 210-230 mm and a wing span of about 430-470 mm. The two specimens Ld LAP 569 and Ld LAP 567 have wings 160-170 mm long and a body 10 mm wide (wing span 330-350 mm). Therefore 'giant' Meganeuridae were still present in the Late Permian of Lodeve (Figs. 7-9).
Furthermore, if the Meganeuridae had the largest known wing span, their bodies were not very large, less than 30 mm wide and 70 mm long forMeganeura monyi. They were smaller than some recent large Coleoptera: Scarabaeidae or Cerambycidae. Therefore, they cannot be qualified as 'giant' insects in term of body size and weight, except by comparison with the recent Odonata. One very well preserved abdomen of Meganeuridae from Lodeve has large, clearly functional respiratory spiracles (Fig. 11). Nothing is known on the abdominal spiracles of the Late Carboniferous Meganeuridae. It is not possible to definitively establish these structures were secondarily developed by the Late Permian Meganeuridae, as a response to the decrease of the oxygen or if they belong to the ground plan of the Odonatoptera. Similar abdominal spiracles are also present in recent Anisoptera: Gomphidae or Austropetaliidae, but not in the more derived family Libellulidae. Therefore the latter solution is the most probable.
The large to very large Carboniferous and Permian Meganeuridae were not dependent of a high oxygen atmospheric concentration for their flight capacities or their body metabolism. The appearance and diversification of very large insects and arthropods during the Late Carboniferous could have very different origins, i.e. the absence of flying vertebrate predators, and the pressure of selection related to the increasing of the prey and predator sizes.
Received: 15/07/06/ Accepted: 27/12/06
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A. N. Nel (1) *, G. Fleck (1), R. Garrouste (1), G. Gand (2)
(1) CNRS UMR 5202, Museum National d'Histoire Naturelle, CP 50, Entomologie, 45 rue Buffon, F-75005, Paris, France e-mail: firstname.lastname@example.org
(2) Laboratoire de Biogeosciences, CNRS, Universite de Bourgogne, Centre des Sciences de la Terre; 6, boulevard Gabriel, F-21000 Dijon, France
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|Author:||Nel, A.N.; Fleck, G.; Garrouste, R.; Gand, G.|
|Publication:||Journal of Iberian Geology|
|Date:||Jan 1, 2008|
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