Towering sponges in an Early Cambrian Lagerstätte: Disparity between nonbilaterian and bilaterian epifaunal tierers at the Neoproterozoic-Cambrian transition
Epifaunal, suspension-feeding bilaterian animals in the Cambrian lived close to the sediment-water interface, and hence their ecological tiering levels were low (<10 cm). Here we report an Early Cambrian (Diandongian or probably Tommotian-Atdabanian) Lagerstätte from the Hetang Formation in Anhui Province, south China. The Hetang biota is characterized by high-tiering (to 50 cm) sponges and small (<0.5 cm) bilaterians (including orthothecid hyoliths and bivalved arthropods). Nonbilaterian suspension feeders (sponges, cnidarians, and archaeocyathids) as high-tiering animals and bilaterian suspension feeders as low-tiering animals also characterize other Neoproterozoic-Cambrian assemblages, such as the Ediacaran, Chengjiang, Burgess Shale, and Sinsk biotas. These data are consistent with medium- to high-tiering levels in Neoproterozoic-Cambrian epifaunal communities, but suggest that nonbilaterians achieved such tiering levels long before bilaterian suspension feeders did so in the Early Ordovician. The disparity between bilaterian and nonbilaterian tierers during the Neoproterozoic-Cambrian transition and the delayed appearance of high-tiering bilaterians demand phylogenetic and ecological explanations. The Cambrian substrate revolution may have triggered a cascade of ecological evolution, including the rise of bilaterian animals in high-tiering levels during the Ordovician radiation of the Paleozoic fauna.