摘要
The receptaculitid genus Calathium is an extinct genus of sessile organisms of uncertain systematic position but which has most often been attributed to sponge or calcareous Green algae, having a somewhat similar appearance to dasyclads-which still survive at the present day. Calathium ranges in stratigraphic age from Ordovician to Silurian and is particularly notable because it played a major role in reef building during the Early and Middle Ordovician. The base of the Early Ordovician saw the initiation of a major new phase in composition of reefs within shallow marine carbonate settings-in which the mud mounds previously dominantly constructed by microbes became joined in varying proportions by Calathium and by a limited diversity of metazoans, mainly sponges-particularly siliceous lithistid sponges. Finally, reefs with Calathium-dominated eventually vanished in the Late Ordovician, at the same time as the innovation occurred of significantly more diverse metazoan reef communities including corals and stromatoporoid sponges-leading to still more developed and complex reef systems.