DISTRIBUTION OF VARIOUS CALCIFIED DEMOSPONGEA THROUGH TIME, SINCE THE PERMIAN PERIOD - ARGUMENTS FOR THE INFLUENCE OF THE OCEANIC PARAMETERS ON THE BIOMINERALIZATION PROCESS IN CALCIFIED SPONGES

In recent years, the renewal of major concepts about taxonomy of calcified Demospongiae, and the discoveries of numerous new fossil data, brought fundamental transformation in our ideas about the evolution of these organisms. Plotting the presence of these various groups of sponges since the Upper Palaeozoic time, with special attention to the original mineralogy of their skeletons, an alternate distribution of calcitic and aragonitic forms can be clearly demonstrated. In this paper, dealing with the sponge faunas from the Permian to Present, three major inversions occur in the fossil data, with regards to the major groups of sponges: - the Carboniferous/Permian limit shows the beginning of aragonitic skeleton sponges (astrosclerids and ceratoporellids, for instance), with a very important acme during the Triassic; - the Triassic-Jurassic boundary demonstrates a very clear inversion: the aragonitic sponges disappear in the calcareous facies, and are replaced by other calcified families, with distinct microstructural features; - after the Mesozoic-Tertiary, the aragonitic calcified families reappear in the reef facies, but with subordinate position with regard to the dominant position of Cnidaria as reef builders. The synchronic behaviour of these phylogenetically distinct groupes of calcified sponges implies an external factor dependance in their biomineralization processes. It is suggested that sponges, as primitive metazoans, may have inferior regulating mechanisms, and are more sensitive to the oceanologic condition with regard to the formation of their skeletons. This conclusion is supported by the striking correspondance of these biologic alternances with the repartition of the mineralogy of carbonate cements during the same geological periods.