Fortunian archaeocyath sponges acquired biomineralization in the beginning of the Cambrian explosion

Abstract Archaeocyaths are biocalcified sponges largely restricted to the early Cambrian Period. Their perforated cup-shaped body facilitated filter feeding. Many of them were clonal modular animals that formed the earliest metazoan skeletal reefs. In Siberia, archaeocyaths extend from late Age 2 to Age 4 of the early Cambrian, representing an ~15 m.y. range (ca. 525–510 Ma). Elsewhere, archaeocyaths emerged later than in Siberia and, in places, survived to the middle-late Cambrian. The existing fossil record thus indicates an out-of-Siberia scenario and delayed biomineralization in archaeocyaths relative to many other animals, which acquired biomineralization in the Fortunian Age of the early Cambrian. Here we report two microscopic archaeocyath species—Primocyathus uniseriatus Wang and Xiao, gen. et sp. nov. and Sinocyathus biseriatus Wang and Xiao, gen. et sp. nov.—from the Fortunian Kuanchuanpu Formation (ca. 533 Ma) in South China. Preserved as phosphatized internal molds, they are interpreted to have had a biomineralized, two-walled, perforated, cup-shaped skeleton. They were likely filter feeders, but their solitary habit and millimetric body size indicate that they were unlikely reef framework builders. They substantially extend the stratigraphic range of archaeocyaths, challenge the out-of-Siberia hypothesis, support archaeocyath biomineralization in the beginning of the Cambrian explosion, and imply a Precambrian divergence of sponge classes.