WHAT CAN WE LEARN ABOUT SILICEOUS SPONGES FROM PALAEONTOLOGY

Abstract

In the age of molecular systematics and phylogeny palaeontological data are sometimes considered as less important. Palaeontology has, however, one important advantage, it gives time dimension. Thus, the antiquity of particular sponge groups or lineages is recognized.  The existence of two large sponge groups, i.e. Hexactinellida and Demospongiae is attested already in the Precambrian. Palaeontology helps also to understand other aspects of sponge evolution and ecology. Past occurrences of large sponge faunas, for example, show that general pattern of their depth distribution, i.e. demosponges dominating in shallower settings and hexactinellids in deeper environments, is the same today as it was in the past. It seems however, that some groups of sponges with solid silica skeleton, such as lithistids and hexactinellids with fused skeleton, inhabited in the geological past shallower environments than today. This fact could be associated with higher silica contents in the Paleozoic and some of Mesozoic seas. An example of non-actualistic ecological occurrence of siliceous sponges is the Eocene lithistid fauna of SW Australia. This very rich and diversified lithistid sponge assemblage clearly inhabited extremely shallow and near-shore water, while today’ lithistids occupy, with some exceptions only, deep-water habitats. Some Upper Cretaceous lithistid faunas are known from the chalk inhabited soft muddy substrate rather than hard rocky bottom like most Recent lithistids. The fossil record of bodily preserved sponges is very discontinuous, but in case of studies of loose, disassociated spicules, even the more common ones can give important information. The Cambrian bodily preserved sponges, for example, display very simple spiculation, while some assemblages of disassociated spicules of the same age contain much more advanced spicule types and display higher spicule diversity, with some strange morphologies unknown in bodily preserved sponges. Thus we must be cautious in our ideas of very simply organized sponges in the Cambrian. Palaeontological data give us also insight into morphological potential of particular sponge groups, as revealed by a wild variety of fossil sponge morphologies. Lack of data about sponge evolution during some intervals of geological time does not result, as it is sometimes assumed, from the poor fossil record, or the poor quality of the palaeontological material, but rather because few studies of fossil sponges have been carried out.