Scleractinia

Scleractinia is a group of corals existing since the Mesozoic era, for more than 240 million years. The beginnings if Scleractinia dates back to 5 milion years after the mass extinction at the end of the Permian (about 250 million years ago). As it was already said before Scleractinia has a polyphyletic origin, which means that different groups evolved separately. We don’t know how they originated, there are two hypothesis about that but none of them have been prooved. First says that they derived from Rugosa, the second one that from not producing skeletons anemones.

During this 240 milion years of existing of Scleactinia we observe changes of coral diversity, expressed by increase and reduction of fauna. At the beginning of Scleractinia’s development there were distinguished four groups different in microstructure.

1.		Corals having very thick wall and rudimentary septa. It is called pachythecal group. This is the group, which probably originated from Rugosa corals.
2.		Corals with septa built from thick structures, resembling little beams, called trabecules. This group is called thicktrabecular.
3.		Corals with septa built from thin trabecules. This group is called minitrabecular.
4.		Corals with septa not build from trabecules, but from columns being bunches of aragonit fibres. This group is called non-trabecular, fascilcular.Corals with septa not build from trabecules, but from columns being bunches of aragonit fibres. This group is called non-trabecular, fascilcular.

During the Mesozoic and Cenozoic eras amount of families and orders of all these groups have been changing. Every period ended with extinction of more or less coral families. End of every period is related with geological and environmental changes, which caused so called mass extinctions, among others, extinction of many coral families. Probably the most important changes causing those extinctions were: changes of sea level and tectonical movements, especially movements caused by drift of continental plates of the Earth , called plate-tectonics and related changes of shore lines which caused changes of oceanic currents and of oxygenation of ocean water, as well as climatic changes.

Two biggest crises took place in scleractinian history at the reach of the Triassic period and in the middle of Cretaceous. Crises related to environmental changes caused evolution of corals.

Integration of colony means grade of union of organisms living in colony. In the Mesozoic many corals built , so called, pseudocolonies. Pseudocolonies (some scientist agree minds, that they were colonies) were colony-like branching coral constructions not joined with body, only with skeleton. Today such corals building pseudocolonies are relicts. Today most corals build colonies with a different grade of integration. Some of them are joined with coenosark (see asexual reproduction), some colonies has common coelenteron.

1.		Change of way how colony integrate.
2.		Changes of functions of many parts of skeleton.
3.		Changes of life style.