Devonian, and the even larger ones to come in the Permian and even today.
Corals were still abundant in those warm Mississippian seas, and both colonial and solitary types were common. Tabulate corals were colonial, tall columns growing together with horizontal segments, tabulae, providing the platform on which the individual zooids lived. One species, named Lithostrotion basaltiforme, has a remarkably similar appearance to the polygonal columns of basalt that form as a result of cooling – though the corals are much smaller.
There are biologic features in the Mississippian that can truly be called reefs, although purists might call them bioherms, meaning a relatively tall structure that might be a pile of material or a construction like a true reef, or a combination of both. The words “reef” and “bioherm” are sometimes used almost interchangeably, but to me a reef means a relatively large structure that is mostly composed of the actual skeletons of the animals that lived there, rather than a pile of debris. I’m not a specialist in this area, so take that as just my opinion about the terminology.
—Richard I. Gibson
Drawing from an old text (public domain)