Composition and tiering of the Cambrian sponge communities

Sponges are dominant sessile suspension feeders in the Cambrian metazoan community; thus, sponge tiering and composition in the Cambrian communities will provide important information for elucidating the evolutionary history of the epifaunal community. Based on more than two thousand articulated sponge specimens from the early Cambrian Chengjiang fauna, the present study demonstrates that the Chengjiang sponge community is dominated by demosponges with a well-developed three-level tiering, consisting of 0–5 cm, 5–15 cm and 15–30 cm tiers. The majority of the sponges are restricted to the 0–15 cm tier, suggesting rich nutrients in the bottom seawater of the Cambrian ocean. Tiering in the Cambrian sponge communities is consistent and similar to that of the Ediacaran and Phanerozoic epifaunal communities, but the fourth tier (50–100 cm) in these latter communities is absent in the Cambrian communities. Demosponges dominate the shallow-water sponge communities and occur in all tiers. Hexactinellids are relatively rare and limited in the lower tiers in the shallow-water community, but dominate in the deep-water communities. The composition variation in the Cambrian sponge communities may be controlled by the differences in skeleton architecture between the Demospongiae and Hexactinellida.