DECLINE OF SILICEOUS SPONGES AND SPICULE MINIATURIZATION INDUCED BY MARINE PRODUCTIVITY COLLAPSE AND EXPANDING ANOXIA DURING THE PERMIAN-TRIASSIC CRISIS IN SOUTH CHINA

Variations in sponge form diversity, abundance, and spicule size provide a potential record of paleoenvironmental changes associated with the end-Permian biotic crisis. Changes in three common spicule forms (Oxea A, Oxea B, and Orthopentactines) were analyzed in uppermost Permian (Changhsingian) and lowermost Triassic (Induan) strata at two localities in South China. The deeper-water Dongpan section exhibits decreasing spicule size as well as a strong decline in spicule-form diversity prior to the end-Permian crisis. In contrast, the shallower-water Maanying section exhibits an increase in spicule size and a more limited loss of form diversity over the same interval. In both study sections, the end-Permian crisis was accompanied by a miniaturization stage marked by a rapid decline in average spicule sizes (by >50% for Oxea A and Oxea B). Paleoproductivity proxies suggest that the decline of sponges was related to a general collapse of marine productivity. The timing and intensity of the sponge biocrisis varied between the two sections, however, with an earlier onset of miniaturization and a complete disappearance of sponges prior to the Early Triassic at Dongpan, versus a later onset of miniaturization and survival of several sponge forms into the Early Triassic at Maanying. These differences are attributed to relatively less hostile conditions (e.g., less frequently or less intensely reducing) in shallower-water environments during the end-Permian crisis.