ARE HYPERCALCIFIED DEMOSPONGES A KEY TO THE END-PERMIAN EXTINCTION?

Unlike the other members of this silica-secreting class, the hypercalcified Demosponges have in addition to, or instead of, siliceous spicules, a massive, largely external, skeleton of CaCO3, built either of spherulitic, penicillate, or microgranular aragonite, or in a few species, of microgranular Mg-calcite. They were at their acme in the Permian and Triassic, and almost alone among sponges, they were so little affected by the end-Permian extinction that individual genera (31 genera in 21 families!) continued from the Permian into the Triassic, building reefs in the Tethyan tropical belt in both periods (see Finks and Rigby, 2004, pp. 585-734). The recent discovery of a probable large impact structure in East Antarctica (von Frese and Potts, 2006) roughly antipodal to the extensive Siberian flood-basalts of end-Permian age, provides an adequate cause for the extinction event, especially if the impactor was an icy body from the Kuiper Belt that added large amounts of CO2 and CH4 to the atmosphere. Widespread black shale in the early Triassic suggests reduction of atmospheric oxygen as one of its consequences (Benton, 2003, p. 269). The hypercalcified sponges may have survived because they had symbiotic cyanobacteria, previously suggested as aids in the secretion of their calcareous skeleton (Vacelet, 1983), that may have provided them with photosynthetic oxygen, as well as food, during immediate post-impact time. The living demosponge Verongia aerophoba (not hypercalcified) contains large quantities of symbiotic cyanobacteria, cropped by phagocytosis, and many other living demosponges also contain them, the only metazoa to do so (Sara and Vacelet, 1973).