THE ROLE OF EIFFELIA IN EARLY SPONGE EVOLUTION

The Cambrian sponge Eiffelia has been known to possess calcareous hexaradiate spicules forming a single layer in a globose body in a manner reminiscent of the coeval hexactinellid Protospongia. Recently, Botting & Butterfield (2005; PNAS 102:5, 1554–1559) discovered that in addition to the hexaradiates, the spicular skeleton of Burgess Shale Eiffelia also contains hexactinellid-type spicules. They further observed a bilayered structure of the hexaradiate spicules and proposed that this reflects a dual mineralogical composition: a core of magnesium calcite being overlaid by an opaline siliceous envelope. They concluded that siliceous sponges (hexactinellids and demosponges) were likely derived from early calcareous sponges by silicification of the skeleton. We have tested this hypothesis by investigating the mineralogical structure in Eiffelia spicules from other types of deposits, namely the Lower Cambrian carbonate sequences of South Australia. These spicules are mostly preserved in calcium phosphate, which is a common type of preservation of calcarous skeletons in those deposits. In addition, however, spicules preserve a siliceous core which does not appear to be of diagenetic origin. Botting & Butterfield’s proposal of a dual mineralogy of Eiffelia spicules thus appears correct, but with the mineral phases inversed from what they assumed. We investigate the implications of spicule homology between calcareous and siliceous sponges reversing the polarity from the proposed one, i.e. involving a transition from siliceous to calcareous mineralogies.