REITNER, Joachim; Univ. of Goettingen, Geobiology: Sponges � A geobiological approach

Sponges are ancestral metazoans with a great geobiological importance. They are divided in two clades, the silica forming sponges (Hexactinellida & Demospongiae), and the Mg-calcite forming Calcarea. The fossil record trace back 1.8 Mrd.y based on chemofossils. Intriguing is that all main taxa of siliceous sponges harbour diverse microbial communities – demosponges Bacteria and hexactinellids Archaea. Therefore the sponge bauplan resembles to a complex biofilm anatomy. The function and anatomy of sponge canal systems and biofilm water flow systems are very similar. It is assumed that the Precambrian stem group of sponges was a multicellular organism constructed of flagellate protozoans and microbes. First body fossils of sponges are known since the late Precambrian (Vendian), and they are the first metazoans with an enzymatically controlled silica- and Ca-carbonate biomineralization. The onset of biomineralization in the Vendian was linked with a deep global change after world wide glaciation events. All major sponge groups were evolved in the Lower Cambrian. Coralline sponges, sponges with secondary calcareous skeletons, were forming the first reefs in the Lower Cambrian and main reef-building organisms till the Middle Cretaceous, the beginning of coralgal reefs. Today coralline sponges are �living fossils� from the Mesozoic and restricted to cryptic niches in tropical reefs and deep fore reef areas. They give important phylogenetic data and their skeletons are environmental proxy archives. Bacteria-rich sponges have a high potential of preservation under certain environmental circumstances. Due to microbial sulphate reduction and ammonification alkalinity increases significantly and promote rapid calcification. This calcification process built huge carbonate microbe/sponge reefs (�mud mounds�) in the past and today in deep water environments.