Monday, 15 October 2012

Insect nymphs from the Carboniferous Montceau-les-Mines Lagerstätte of France.

The majority of modern Insects have a distinct larval stage, that does not resemble the adult and which then metamorphoses into the adult form upon reaching maturity. This is one of the reasons for the great success of the group; the young are able to exploit completely different ecological niches to the adults. However the more primitive groups of Insects do not undergo a metamorphosis, but rather produce a number of nymphal stages that become progressively more like the adult, as the case in, for example, Dragonflies. Most fossil Insects know from the Carboniferous belong to nymph-producing groups (unsurprisingly), though the nymphs of these groups are not well known, due to their low preservational potential. 

In a paper published in the journal PLoS One on 25 September 2012, a team of scientists led by Russell Garwood of the School of Materials and School of Earth, Atmospheric, and Environmental Sciences at The University of Manchester, describe two Insect nymphs identified within siderite nodules from the Carboniferous Montceau-les-Mines Lagerstätte of France, using X-ray Micro-tomography, a method that allows fossils to be studied without freeing them from the rock matrix. This method has greatly improved our understanding of a number of invertebrate groups in recent years, as it allows for the study of specimens that simply could-not be accessed by conventional methods, without causing them severe damage or even destroying them.

The term lagerstätte is used by Palaeontologists to describe a particularly rich fossil source. The Late Carboniferous Montceau-les-Mines of the Massif Central in France comprises siderite nodules from within coal seems containing a large number of fossils, mostly plants. Siderite is a form of Iron Carbonate (FeCO₃), associated with hydrothermal systems.

The first nymph described is named Anebos phrixos, meaning young and bristling (technically conventions on zoological nomenclature require that Insect species only be formally named from adult specimens, but how this applies to palaeontological specimens is unclear). It is a 21.8 mm aquatic nymph, similar to that of a Mayfly or Dragonfly, though Garwood et al. make no conclusions about the taxonomic affinities of the Insect, since this is thought to be the ancestral state in Insects, and the nymph could easily come from a group that did not survive the Carboniferous or even a group which has subsequently evolved a different larval stage.

X-ray tomographic image of Anebos phrixos. This is entirely a computer model; the specimen has never been removed from the module in which it was found. Abbreviations: an = antenna; e = eye; L1–3 = legs 1–3; mp= maxillary palps (?). Scale bar is 50 mm. Garwood et al. (2012).

Three dimensional reconstruction of Anebos phrixos. Garwood et al. (2012).

The second nymph described is not assigned a specific name but described as a 'roachoid nymph', implying it belongs to an earlier member of the Dicoptera, the Insect group which includes Cockroaches, Termites and Mantises, and which produce non-aquatic nymphs. The early members of this group were essentially Cockroach-like, but are not referred to as Cockroaches since the three groups had not diverged at this time. The specimen is a 21.1 mm terrestrial nymph.

X-ray tomographic image of the Roachoid Nymph. Abbreviations: an = antenna; ce = cerci;  fb = femur break, reconstruction artefact resulting from the switch between pyrite infill and void; L1–3 = legs 1–3; mr = median ridge; ms = mesonotum; mt = metanotum; pn = pronotum. Scale bars are 5 mm. Garwood et al. (2012).

Three dimensional reconstruction of the Roachoid Nymph. Garwood et al. (2012).

See also A fossil Insect from the Late Devonian of BelgiumAn early Cricket from southeast India, New Ommatid Beetles from the Mesozoic of China, Fishflies from the Middle Jurassic of Inner Mongolia and Snakeflies in amber from the Early Cretaceous of northern Spain.

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