sponges and related fossils

sponges and related fossils Sponges are simple multicellular organisms intermediate in grade between protozoans and metazoans and hence are often called parazoans. They are sedentary aquatic filter-feeders, using flagellae on the cells to create currents that pass into the organism through multiple external openings; these currents then circulate before exiting through a common opening. About 1500 genera of modern sponges are known, of which about 80 per cent are marine. Sponges have an internal skeleton composed of needle-like elements called spicules, which can be composed of calcium carbonate, opaline silica, or fibres of the scleroprotein spongin. Although relatively rare in the fossil record, the presence of isolated spicules shows that sponges existed at least as far back as the Cambrian. The sponges form the major part of the phylum Porifera, but there are several groups of sponge-like organisms that are found as fossils and are generally included within the phylum. These include stromatoporoids, which formed massive calcareous skeletons, and archaeocyathids, which were conical and also calcareous.

Sponges are classified on the shape and composition of the spicules and also on their external morphology. These characters are used to divide them into five major classes. Demospongea are the dominant modern group with 600 genera; they have a skeleton formed of spongin or silica, or a mixture of the two. Hexactinellids are deep marine forms with siliceous skeletons that can be solidly fused. Sponges with calcareous skeletons are included within the Calcarea, an exclusively marine shallow-water tropical group. Sclerosponges are a minor group with a skeleton of massive calcite and spicules, described from modern forms only in the 1960s but with a record that extends from the Ordovician. The stromatoporoids have a laminated calcareous skeleton with no spicules; they became extinct in the Cretaceous. In addition to these groups the archaeocyathids (which are sometimes treated as a phylum in their own right) were exclusively Cambrian and had a calcareous skeleton in the form of a porous inverted cone.

Although sponges in general show great ecological variation and adaptability, individually they are restricted environmentally by temperature, salinity, currents, turbidity, and the substrate. Because of this they show marked provinciality when viewed worldwide. Together with their incomplete fossil record this makes them generally unsuitable for use in biostratigraphic zoning, although they sometimes have value on a local scale. One exception is the archaeocyathids, which occur worldwide in the Cambrian, reaching their acme in the Lower Cambrian, where they are stratigraphically useful.

The main geological significance of sponges is as a source of biogenic silica that can contribute to the formation of chert; they have also been important as reef-formers through time, although generally in a subsidiary role to other organisms. During the Cambrian, archaeocyathids formed shallow-water reefs in association with stromatolites; during the Silurian and Devonian stromatoporoids formed reefs in association with tabulate corals. Although sponges continued to be an important element of reef structures into the early Mesozoic, their importance subsequently declined. This was probably due to the rise of the reef-building modern corals, which grow more quickly and can thus colonize space more rapidly.

David K. Elliott