Archaeocyathids – sponge-like reef builders

Archaeocyathids were important reef-building animals, but they were not the first reef-builders.The earliest reefs consisted of stromatolites – structures that build up over thousands of years when marine bacteria trap sediment. Stromatolites became much less common when grazing animals such as snails evolved in the early Cambrian.

In the Cambrian, another type of reef-building organism appeared – sponge-like organisms called archaeocyathids, meaning ‘ancient cups’. Their hard bodies formed the distinctive patterns seen in limestone from this time. Fossils of archaeocyathids show that they built up reef structures by growing on top of the skeletons of dead archaeocyathids, and in doing so created reef environments that were important habitats for other marine life.