Halysitid tabulates: sponges in corals' clothing

Abundant pyritic pseudomorphs of monaxonic siliceous spicules (ophirhabds and ?heloclones) have been found entrapped in the calcareous skeleton of the halysitid tabulate Quepora ?agglomeratiformis (Whitfield) from late Ordovician limestones of Frobisher Bay, Baffin Island, Canada. The finding indicates a poriferan (choristid or sublithistid) affinity of halysitids, early Palaeozoic marine fossils related so far to corals. They probably derived from a monaxonic group of early demosponges that adapted during the Ordovician to Ca2+ stress conditions in epicontinental seas by excreting the excessive Ca2+ influx to their tissues as variously designed chains of basally secreted calcareous tubes.