Explosion cambrienne

The explosion Cambrien does not describe the geological period during which a great number of macroscopic multicellular complexes appeared suddenly there are approximately 542 and 530 million years. This period indicates a marked transition, on the level of the traces Fossile S, with the appearance of the first members of the Phyla of métazoaires (animal multicellular). The appearance " explosive" from these organizations results from a rapid change evolution naire and limits from technologies available previously, which did not make it possible to analyze with precision the microphone-fossils which constituted the base of the fossils which succeeded to them.

With time, the improvement of the microscopes revealed a range of older microphone-fossils little by little. Before the discovery in 1909 of Burges Shale, published partially at the time and largely adapted to the already existing categories like " précurseurs" : a whole still unknown side of life was then consigned in a temporal category désuette, the " pre-cambrien".

More recent discoveries of microphone-fossils showed than the " pre-cambrien" concealed more than mono-cellular organizations (protista) or than of diplobastic fauna (two cellular layers allowing the cells to be in contact with an aqueous environment rich in minerals). In 1994, animals triploblastic (with more than two cellular layers, which can thus have internal bodies enabling them to store food and waste), discovered were preserved in the form of embryos phosphatized in stone of the South of the China. These fossils are estimated to have 570 million years and were thus older than the fauna of Ediacaran found in younger layers of 10 million years.

Fossils

This period of the evolution gave place to the fossils among most astonishing never discovered. Only one place, Burgess shale, provided one from the best points of view over this period of dramatic change and expérimentaion évolutionnaire which constituted the base of the majority of the large modern animals. A quantity of enigmatic and exotic cellular configurations also appeared at this period, which do not have a bond with the modern animals.

Before the explosion, the fossil traces were dominated by mono-cellular organizations, except for the fauna of Ediacaran, with the soft body, and certain microphone-fossils showing that multicellular forms of life had already appeared 30 million years earlier.

With the explosion cambrienne, the evolution of the shells and other body parts hard started to evolve/move. As the shells are more easily préservables in the sediments than the soft bodies, that makes forms of life of this period definitely easier to study according to the fossil traces than their Precambrian antecedents. That also contributes to the idea of a dramatic change in the fossil traces.

Causes of the explosion cambrienne

The explosion cambrienne could have been precipitated by a series of environmental changes during and before this period. In first, the glaciation of Varangia formed a snow-covered ground where all the oceans or almost were entirely covered with ice. That was followed of a deglaciation and a rapid total warming right before the beginning of the explosion itself.

In the modern Arctic environments, one finds organizations mono-cellular wide under the layers of ice in order to maximize their exposure to the sun. It is possible that adaptations useful for the survival of colonies of this type also contributed to the formation of the first animals triploblastic (of more than two cellular layers), 570 million years ago. Moreover, the icy environment of the ground would have allowed the development only few ecological niches, which explains the quick change (the explosion) having followed this deglaciation and this warming rapids.

References

Hakes, Allen G. " Metazoa: Fossil record". Retrieved Dec. 14,2005.
Wang, D.Y. - C., S. KUMAR and S.B. Hedges (1999). " Divergence time estimates for the early history off animal phyla and the origin off seedlings, animals and fungi." Proceedings off the Royal Society off London, Series B, Biological Sciences 266 , 163-71.
Xiao, S., Y. Zhang, and A. Knoll (1998). " Animal Three-dimensional safeguarding off algae and embryos in has Neoproterozoic phosphorite." Natural 391 , 553-58.

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