Hypercalcified demosponges and the end-Permian extinction

The hypothesis is presented here that the hypercalcified demosponges, characterized by a solid, largely external skeleton of spherulitic aragonite, or of high-Mg calcite, had symbiotic cyanobacteria, from the outset of their existence, that enabled them to precipitate their peculiar skeleton, and that both the skeleton and the cyanobacteria enabled them to survive the late and end-Permian extinctions, almost alone among sponge groups. The cyanobacteria would have provided them with photosynthetic oxygen to survive hypoxia (one of the proposed causes of the extinctions), and perhaps also were cropped for food, if the external bacteria, normally fed upon by sponges, were reduced by the hypoxia. The sponges' external skeleton would also have protected them in their shallow-water reefy environment from excess ultraviolet light that may have resulted from hypoxic loss of the ozone layer of the atmosphere. This is supported by the fact that the genus Guadalupia which has a lightly-calcified flat upper surface, died out by the end of the Permian, while 31 other genera of hypercalcified Demospongea as well as 3 genera of hypercalcified Calcarea were the only genera of sponges of any kind to continue from the Permian into the Triassic.