Reef-building red algae from an uppermost Permian reef complex as a fossil analogue of modern coralline algal ridges

Calcareous red algae have been important components in reefal facies since the Mesozoic but their volumetric contribution to Palaeozoic reefs was usually low. Here, we report a reef-building community dominated by Parachaetetes, a genus of solenoporacean red algae, overgrowing uppermost Permian sponge reefs in Cili (Hunan Province, South China). The fossil assemblage in the community consists of about 88% Parachaetetes, 7% rugose corals, and 5% reef-dwelling organisms, including gastropods, brachiopods, foraminifera and Tubiphytes. Although the biodiversity of this community is far lower than that in modern reefs, the association of red algae and rugose corals shares several attributes with the coral–coralline algal communities of modern tropical reefs. Analysis of sedimentary facies indicates that the reef-building Parachaetetes grew in a turbulent setting near the margin of a carbonate platform, perhaps in a reef-crest environment known from modern coralgal reefs. Although calcifying red algae have been recorded from the Cambrian onwards, this is the oldest site where crustose red algae played a dominant role in reef construction.