We will examine how the probabilistic interaction model between the environmental evolution and the biological evolution can account for the mass extinctions by the influence of the calcium biochemistry.

Let us point out the cycle carbonate-silicate schematically. The silicated rocks disintegration (eruptive rocks majority), containing between 45 to 76 % of silica, releases in solution the calcium ions and the bicarbonate ions (CaSiO3 + 2 CO2 + H2O > Ca++ + 2HCO3 _ +SiO2). These ions are transported by the waterways and the rivers towards the oceans where the organisms use them to secrete calcium carbonate shells (Ca++ + 2HCO3 _ > CaCO3 + CO2 + H2O). The carbonated sediments, deposited at the marine resources, are again reconverted out of calcium silicates and CO2 (CaCO3 + SiO2 > CaSiO3 + CO2) when the oceanic plates are pushed under the other plates by the plate tectonics subduction. CO2 re-enters, again, in the atmosphere, by the volcanic eruptions (Martin Soo Hoo, according to Kasting 1995).

The higher levels of the oceans are supersaturated in calcium and carbonate ions. These conditions are carried out especially in not very deep and hot water, with low CO2 levels, rather than in deep and cold water with higher CO2 levels In the first case, the CO3Ca precipitates easily, inorganically or by means of organisms which can build their shell. It is one of the biomineralization processes. When these organisms die, their shells or mineralized parts fall at the ocean floor where they accumulate or dissolve, according to the depth, temperature and pressure levels. The limit between the secretion and dissolution levels of CO3Ca is entitled " lysocline ". The CO3Ca precipitation is an availability of CO2 function. The CO2 increase tends to decrease the pH and increases the carbonate solubility (Yale-New Haven Teachers Institute - The Calcium Cycles 1998).

The biomineralization processes are at inorganic chemistry and biology limit. The mineralized structures go from the bacterial magnetic structures to the vertebrates bones and teeth while passing by the diatoms silica exoskeleton, the aragonite or calcite molluscs shells, the corals aragonite, etc... (Levine 1998). Many protists, algae and animals, autotrophic and heterotrophic, secrete chalky skeletons, extracting from the calcium and carbonate ions of the oceans water. The molecular processes which control mineralization remain still largely unknown (Fincham 1998).The same mechanisms which cause the CO3Ca accumulation in water are varied and not yet completely elucidated.

At the marine current food chain base is the planktonic unicellular algae trinity: coccolithophoridae (nannoplankton with calcareous exoskeleton appeared with higher Trias towards - 230 M.A.), diatoms with silica walls and dinoflagellates with organic walls. Coccolithophoridae are the second more significant primary producer (30 % of the primary production) in the current oceans, second after the diatoms. They are more abundant than the diatoms in tropical hot water (Maddocks 1998). They constitute the zooplancton food, including the Copepoda and tunicates (Winter and Siesser 1994). The planktonic Foraminifera abundance (Globigerines) is controlled by that of the phytoplankton itself controlled by the nutriments level.

If the four chemical elements O, H, N and C (95 % of the living material) are the proteins essential components, basic structures of the living organisms, and their components, the amino-acids, like those of the D.N.A. (more phosphorus), we note that the calcium, located at the third rank (with carbon and hydrogen), by abundance order, in the earth's crust, is also in the chemical components of the living material. Its abundance grows, in the organisms, from the bacterium to the plants and the animals. If the six elements C, H, N, O, P, S, are taken, they constitute 99,72 % of the bacterium, 99,60 % of the alfalfa and 97,90 % of the human body on average. One sees the place left for the calcium, the potassium and the other elements found in the living organisms. The human body contains approximately 1,50 % of calcium, that is to say about half of the proportion found in the earth's crust (Raven, Peter, Evert and Curtis 1981).The plants contains from 0,20 to 3,5 % of their dry calcium weight (Yale-New-Haven Teachers Institute - The Calcium Cycles 1998).

The constitution, in many animals, of a support significant apparatus, involves an accumulation of calcium in the organism. The bone tissue differs deeply from other shapes of conjunctive tissue by an abundant mineral overload: approximately 40 % of the dry matter. In the human being, 99 % of calcium are concentrated in the skeleton and the teeth. It is present, in ionized form, in blood, extracellular fluids and the muscular cells. It is necessary in the nervous transmissions, the cardiac beats regulation, etc... The " higher " animals internal skeleton, consisted the bones, contains from 50 to 60 % of hydroxyapatite Ca5(PO4)3(OH) and 6 to 10 % of calcium carbonate (incidentally, it can contain calcium fluoride). On the other hand, the external skeleton which forms the molluscs, eggs of birds shell, the shellfishes carapace, contains from 60 to 95 % of calcium carbonate CO3Ca (in the calcite or aragonite form) against 1 to 2 % of lime phosphate.

Among Protists and Metazoa, in many sub-kingdoms, the organisms secrete an external or internal support skeleton. Generally, this skeleton consists of organic material (Insects chitin), of silica (Diatoms) or calcium (aragonite, calcite). Calcium plays an essential role in the exoskeleton or the endoskeleton of many Invertebrates or Vertebrates , Protists and Metazoa phyla. We saw that 99 % of calcium are concentrated in the bones and the teeth in the human beings. Its importance is not less in many Invertebrates (Crinoids calcite skeleton representing between 95 and 99 % of the animal volume, Roux 1988;   Rudists thick shells...). Calcium can be regarded as a probabilistic environment stimulus to which the organisms react by the exoskeletons or endoskeletons development, as well at the Vertebrates ones as in the Invertebrates. The variations, during geological periods, of this stimulus influence in the environment, involve, according to our model, in a probabilistic way, an evolution of the organisms exoskeletons and endoskeletons. This evolution can be a morphological evolution (increase in the skeleton importance at the Dinosaurs or Rudists), a radiation (Ammonoids) or an extinction (Dinosaurs disappearance at the K/T limit or Crinoids at the P/T limit).