The five more significant well-known mass extinctions are the extinctions at the Ordovician/Silurian, late Devonian, Permian/Trias, Trias/Jurassic and Cretaceous/Tertiary limits, the most significant being at the P/T limit, the most well-known at the K/T limit (Gore Rick 1989, Sepkoski J.J.Jr, 1986).

If the mass extinctions reality is an object of consensus in the scientific community, the causes of these extinctions induce many and debatable assumptions. The extinction at the K/T limit caused a flourish of hypothesis, some of them are serious, like the marine regressions and climatic deteriorations which resulted from this (Ginsburg 1964), others rather whimsical or speculative like the supernova thesis, a viral epidemic, groups senescence, etc... The theory in vogue of a meteorite impact with the earth (Alvarez 1980) is the most popular.

The causes most often proposed in these five mass extinctions, which rest on paleontological, paleogeographic, paleoenvironmental observations, etc... can be summarized as follows: climatic variations (glaciations, reheatings), oceanic levels changes (transgressions and regressions), volcanicity, orogeny, impact (especially with limit K/T), anoxia, hypersalinity. These causes are called upon, either in a univocal way, or gathered in a plural way (Gerta Keller 1996).

One can however note a certain number of common characteristics to various extinctions: fauna and tropical reefs preferential disappearance during the 5 extinctions, extinctions repetition for certain groups (Ammonoids in Devonian, P/T, K/T and Trias; Trilobites in Ordovician, Devonian and P/T), gradual and sudden extinctions (Dinosaurs and Ammonoids with the Cretaceous; Coccolithophoridae with the finish-Cretaceous), coolings and glaciations (Ordovician, Devonian, P/T, K/T), anoxic oceans (Devonian, P/T), regional or total extinctions.

In these various observations light, it appears clearly that any mass extinctions general explanation must be able to account for the various parameters which characterize them. It can be only plurifactorial or synthetic. Another mass extinctions fundamental parameter relates to their selectivity. It is where the unifactorial causes seem most insufficient.

The Vertebrates and Invertebrates groups examination implied in the mass extinctions reveals that they are primarily animal groups with a calcium compound endoskeleton or exoskeleton:

Protists: chalky algae, coccolithophoridae, foraminifera.

Metazoa: spongiae, cnidaria, bryozoa, conodonts, brachiopods, molluscs, arthropoda, echinodermata and vertebrates.

Often, these groups, entirely decimated, present a very significant metabolism: Hippuritids with very thick chalky shells, big sizes Orbitoids going up to 6 cm at the Cretaceous end, many Mesozoic Reptiles gigantism, Permian Crinoids. The organisms with siliceous skeleton or of organic material, generally, are saved: Annelids, Insects with chitin skeleton (only exception at the P/T limit, Protists (Silicoflagellates, Diatoms, Dinoflagellates, Radiolaria). The plants are very variously affected by the extinctions (Angiospermae at the Cretaceous end).

Calcium necessary to the organisms skeleton constitution their is provided either by the food chain or by the biomineralization. We saw that the biomineralization processes, complexes, can favour or be disturbed by many factors. We thus propose that, according to the probabilistic model, the biomineralization disturbing factors and the calcium food chain are the probabilistic causes of the mass extinctions.

Probabilistic favorable factors to the calcium biomineralization (dependent or independent):

1 Supersaturated Ca++ ions: hot higher levels and not very deep water (Maddocks 1998).

2 Hot or tropical temperature: the corals cannot remain below 18°C.

3 The low CO2 level, less soluble in hot water, supports the CO3Ca precipitation (Maddocks 1998).

4 Orogenetic calm: clear water and terrigenous sedimentation low level (Maddocks 1998); low volcanicity (little CO2 expulsion) (Mc Lean 1997).

5 pH neutral or alkaline (Mc Lean 1997), average salinity.

6 Favorable atmospheric and dissolved oxygen level (Holland 1984).

7 Intact food chain: from primary producers (phytoplankton) to the first consumers (zooplankton) etc... (Maddocks 1998).

1 Under-saturated Ca++ ions in deep and cold water (Maddocks 1998).

2 Glaciations, fresh or cold temperature: in general, the chalky organisms are replaced by siliceous diatoms in high productivity and low temperatures areas . These conditions are generally met during the glaciations (Archer 1991).

3 The higher CO2 level, more soluble in cold water, interferes with the biomineralization and supports the CO3Ca dissolution (Maddocks 1998).

4 Orogeny and volcanic activity support terrigenous sedimentation, the CO2 expulsion and disturb the water purity (Mc Lean 1997).

5 Acid pH (deleterious effects on haemoglobin affinity for oxygen - Bohr effect - Knoll 1996), hypersalinity (Maddocks 1998).

6 Anoxia (Holland 1984).

7 Food chain breakage: the zooplankton abundance (planktonic Foraminifera) is controlled by that of the phytoplankton itself controlled by the nutriments level (Maddocks 1998). The disparition of an element of the food chain (coccolithophoridae) destroyed all the food chain.

In the various mass extinctions, a certain number of the food chain disturbing probabilistic factors or biomineralization occurs. It results from it an abnormal operation from these processes which results in a more or less organisms with calcium skeleton major attack. We will note thus that the most significant mass extinction, at the Permian end, cumulates the disturbing factors majority whereas that of finish-Triassic, well less significant, counts only two of them. We will examine, in turn, the five more significant mass extinctions. The factor 1 presence, particularly difficult to highlight for the periods spent, will not be taken into account.