Abundant calcareous tubeworms have been found in both shallow platform and deep basin deposits after the end-Permian mass extinction in the Cili area, South China. Tubeworms from the microbialites deposited on the shallow platform appear to be cone-shaped tubes with diameters ranging from 0.5 to 1.8 mm (mean 1.1 mm), while those attached to Claraia, the most abundant bivalve fossil preserved in the deep basin deposits after the mass extinction, are planispiral tubes with smaller diameters (0.5–1.5 mm, mean 0.9 mm). The calcareous tubeworms are identified as Microconchida (Tentaculita) according to the typical laminated sheet texture of the tubeworms found on the shallow platform. The difference in morphology between the cone-shaped tubeworms found in the microbialites and the planispiral tubeworms attached to Claraia in deeper water deposits may be related to differences in how fast the surrounding sediments were accumulating. Bacterially mediated precipitation of calcium carbonate led to rapid accumulation of the microbialites that forced the tubeworms to grow upward so as to keep up with the rate of microbialite growth and led to the cone-shaped tubes found there, whereas the slowly accumulating sediments surrounding the tubeworm-encrusted Claraia led to the development of the planispiral forms in basin deposits. Calcareous tubeworms found in the shallow platform and colonizing the shells of bivalve Claraia in basin deposits indicates calcareous tubeworms, as a significant disaster form, should have benefited from the opening of ecological space by the extinction of most marine invertebrates. Widespread oceanic anoxia has long been considered to be one of the extraordinary conditions after the end-Permian mass extinction. Tubeworm fossils flourishing in basin deposits within the short interval near the Permian-Triassic boundary implies that the deepwater environment immediately after the end-Permian mass extinction may not have been as anoxic as previously thought.

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