Sponge spicules from the lower Cambrian in the Yanjiahe Formation, South China: The earliest biomineralizing sponge record

Sponges are the earliest metazoans, and according to molecular-clocks, have a deep history back into the Cryogenian. However, the record of their biomineralized elements below the Ediacaran-Cambrian Boundary has been controversial. Here we present the first and earliest indisputable record of hexactinellid spicules in the lowest Cambrian, Terreneuvian, below the small shelly fossil Anabarites trisulcatus-Protohertzina anabarica assemblage zone (Zone I). Spicules recovered from cherts of the lowest Yanjiahe Formation in the Yichang, Hubei province, South China, include monaxons, diaxons, triaxons, and that are all siliceous in composition. This earliest record of silica biomineralization by filter feeding metazoan suggests sponges have contributed to the cycling of silica in the oceanic system as early as the beginning of the Phanerozoic. These results, along with a review of previously reported sponge occurrences, suggest a two-step evolution on sponges of the early Cambrian.