Dévonien

The Dévonien is a geological system extending from 408 to 355 million years. It is followed by the Carbonifère and is preceded by the Silurien. The dating of beginning and end of this period are approximate from 5 to 15 million years although the stratigraphic layers of reference are known with precision. The dévonien is named according to the Devonshire in England where the outcrops of layers going back to this time are common.

Divisions of the dévonien

Dévonien is subdivided in three times, the higher Dévonien , average and lower .

The layers of Dévonien produced Pétrole and gas in certain areas.

Paleogeography

Continents of the hemisphere , the North America and the Europe formed a continent, Laurussia. The remainder of the modern Eurasia is located in the northern hemisphere. " Hun Terrane s", whose Armorica and Iberia which had been detached from Gondwana in higher Silurien carry on their road and will enter in collision with Eurasia only in higher Dévonien.

The majority of the new demonstrations of life took place in Euramérica. The sea level was high. Many grounds were immersed in not very deep seas where lived tropical reefs. Immense and deep ocean covered the rest of the world.

Dévonien is known under the name of age to greenhouse effect or of age of the Fougère S . The extent of the reefs indicates that the climate was lenient and hot. It is important to announce that an important extinction of mass takes place in higher Dévonien, between Frasnien and Famménien, which affected up to 70% of the alive species. The cause of such an extinction could have an extraterrestrial origin (meteoritic impact).

Fauna

Marine biotopes

The sea level is high. The marine animal-life is dominated by the Ectoprocta, various kinds of Brachiopode S and the corals. The Trilobite S still common but are diversified than in the previous times. The large fish with plates, the Placoderme S, were joined during the average dévonien by the first Poisson S with scales, which then diversified. The first Requin S appear at the beginning of the dévonien. The layer of Miguasha, in Quebec, is characterized by abundance from the specimens, the quality of conservation of the fossils and the representativeness of the evolutionary events at the vertebrate ones of this period. The fish with edges, some of an important size, join them soon. During the higher dévonien, our ancestors, the lobed fish with edges evolved to the first Tétrapode S, the such Tiktaalik roseae, which went on the grounds at the end of the dévonien. The forms most advanced of Graptolite S disappear.

Reefs

A large barrier of reefs, now located in the Basin of Kimberley in the North-West of the Australia, extends on nearly thousand kilometers forming a continental edge. The reefs are in general built by organizations secreting of the carbonaceous deposits resisting the action of the waves. The principal contributors with these constructions are algae limestones, organizations presenting of the similarities with the modern corals, the Stromatopore S, the Tabulate S and of the rough corals in this order of importance.

Terrestrial biotopes

On ground, the bacteria and the algae are joined early for this period by primitive plants which created the first fatty grounds and lodged Arthropode S, like the Mite S and the scorpions, and of the Myriapode S. With the higher dévonien of the forests of primitive plants exist: Lycophyte S, Sphénophyte S, Fern S and of the Progymnospermes appeared. The majority of these plants have true roots and sheets. The ferns were spéciées in giant forms similar to the trees with true wood. At the end of the dévonien the first plants with seeds appeared. The fast appearance of so much of groups of different plants is known under the name of explosion of Dévonien

The cover of greenery of the continents acted like a pump with Carbon dioxide and the reduction of the rate of this gas with greenhouse effect could cause a cooling leading to a massive Extinction.

The Vertebrate S and the Arthropode S are firmly established on ground.

The controversy of Dévonien

Establishment of Dévonien as geological period was the result of an important scientific controversy which is spread out of 1830 to approximately 1837, and of which some of the main characters count among the most eminent members of the British geological community of this time: Murchison, Of the Spade, Sedgwick or Greenough.

Before 1830, one considered that the period of the Carbonifère succeeded directly that of the Silurien. But this chronological cutting will be called in question after in particular discovered a presence coal in geological layers the period of Silurien, which was not coherent with the chronology accepted hitherto.

This controversy gave place to a study considered as important in the fields of the sociology and the history of sciences: The Great Devonian Controversy of Martin Rudwick (1985). In this work, the author seeks to highlight the sociological influences in the evolution of the polemic, and in particular, in the negotiation process towards a consensus on the scientific contents.

See too

geological Time scale

External bonds

Objective-Ground, Dévonien

my search for fossils in Dévonien (perso page of dinoland)

National park of Miguasha

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