ARTICULATED SPONGES FROM THE EARLY CAMBRIAN HETANG FORMATION IN SOUTH CHINA

The Early Cambrian Hetang Formation at Lantian, southern Anhui Province of South China, contains a well-preserved benthic, epifaunal assemblage characterized by diverse sponges. These sponges are from the "stone coal" beds of the lower Hetang Formation. Biostratigraphic correlation suggests that the "stone coal" beds and the sponges therein are Meishucunian¨CQiongzhusian (=Diandongian¨Cearly Qiandongian) in age. In Siberian terminology, they are probably Tommotian¨CAtdabanian, approximately 530¨C520 Ma. The Hetang sponges are taxonomically diverse and morphologically complex. Thirteen articulated siliceous sponges, including both demosponges and hexactinellids, occur in the "stone coal" beds; but no calcareous sponges are known in the Hetang Formation. The Hetang and other Neoproterozoic¨CCambrian sponge fossils, at face value, indicate that hexactinellids evolved no later than the Nemakit-Daldynian¨CTommotian and probably in the late Neoproterozoic, and the demosponges and calcareans evolved no later than the Atdabanian. In comparison with the eumetazoans, which probably diverged about 600 Ma, the sponges (particularly 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¨Ca topology supported by increasing molecular evidence.