Articulated sponges from the Lower Cambrian Hetang Formation in southern Anhui, South China: their age and implications for the early evolution of sponges

A well-preserved benthic, epifaunal assemblage of articulated sponges is described from the stone coal beds of the Lower Cambrian Hetang Formation at Lantian, southern Anhui Province, South China. These sponges are Meishucunian-Qiongzhusian (=Diandongian-early Qiandongian) in age. In Siberian terminology, they are probably Tommotian-Atdabanian, approximately 535-520 Ma. The Hetang sponge fauna is taxonomically diverse and morphologically complex. Eleven species of both demosponges and hexactinellids, including three new taxa (Choia? striata sp. nov., Protospongia gracilis sp. nov., and Lantianospongia palifera gen. et sp. nov.), are described. Two undetermined forms are also illustrated. The Hetang and other Neoproterozoic-Cambrian sponge fossils, at their face value, indicate that that hexactinellids evolved no later than the Nemakit-Daldynian-Tommotian and perhaps in the late Neoproterozoic, and the demosponges and calcareans evolved no later than the Atdabanian. The divergence of sponge classes therefore appears to be part of the Cambrian Radiation event. In comparison with the eumetazoans which probably diverged at ca. 600 Ma, however, the sponges (particularly the demosponges and calcareans) appear to have a missing fossil record in the late Neoproterozoic and earliest Cambrian. The minimum implied gaps (MIGs) of the calcareans and demosponges are substantial (tens of Myrs to perhaps more than 100 Myrs), particularly if the calcareans constitute a sister group of the eumetazoans-a topology supported by currently available molecular evidence.