The fouling of skeletons and shells of benthic organisms and the formation of multi-tiered skeletal formations are characteristic of carbonate-producing Porifera, which participated in the structure of reef communities during the Phanerozoic. The accretion of flint sponges, Hexactinellidas, to the solid elements of the substrate is a little-known phenomenon, since many representatives of this group were fixed on the sediment surface by rhizoid outgrowths.
