Deep shelf biostrome of Late Permian in South China and its implications for the adaptability of calcisponges to water depth

The Yangtze platform of South China was an area of extensive carbonate accumulation during the Late Permian, with abundant benthic fossils and reef buildups. Its northern margin has long been considered to be bordered by deep shelf sediments rich in radiolarians and ammonoids. However, recent investigations found a sponge biostrome in deeper shelf settings in the uppermost Permian. The biostrome is about 0.45m thick with an organic framework formed by autochthonous calcisponges. In contrast to more diverse shallow marine reef communities at this time, the biostrome was almost entirely constructed by the sponge genus Peronidella as the only frame-building organism. This low biodiversity reflects a relatively deep-water environment. Peronidella individuals in the biostrome are apparently larger than the same genus in shallow platform settings, including the diameters of both the sponge bodies and the central tubular spongocoel, as well as the thicknesses of the body walls. This increased size may be related to the low biodiversity and therefore to reduced competition, with sponge individuals having increased space in which to develop. Biostrome development was terminated by volcanic clay deposition.