Article: Periodicity in Devonian coral growth

Fine growth-ridges on Middle Devonian rugose corals are seen to be grouped into regular bands of about thirty ridges up the length of the epitheca. Several hypotheses on the origin of these bands are discussed. It is concluded that the banding is most probably related to a lunar breeding periodicity and that the Middle Devonian year contained 13 lunar months each of 30 1/2 days.