The term ‘reef’ can mean different things for different people. To most geologists and palaeontologists reefs are rock formations which have been built by organisms in the distant past but which may no longer be alive. To the biologist a reef is a veneer of living organisms forming an ecosystem which is as complex and fragile as any on Earth. These two concepts of reefs can seem as remote from each other as forests are from coal deposits, yet they share a common past. Reefs, the geological structures, are the direct products of living ecosystems and as such their formation has always been controlled by the sorts of events that control other ecosystems, both marine and terrestrial.
Even within scientific disciplines, the term ‘reef’ can have a wide range of meanings, a range that needs to be narrowed down to be meaningful . When considering ancient reefs, it is important to distinguish ‘reefs’ from the ‘coral reefs’ of today. Corals were not the main builders of all reefs: many ancient reefs, especially those of the Palaeozoic, were not built just by corals, but by a wide array of other organisms including algae, sponges and molluscs.