Calcareous algae make their first appearance near the base of the Tommotian. The oldest confirmed occurrences are of ?Gemma, Obruchevella and Renalcis in the Nemakit-Daldyn Horizon and late Yudomian of Siberia. A little higher in the sequence, in the Aldanocyathus sunnaginicus Zone of the earliest Tommotian, Angulocellularia, Bajanophyton, Batenevia, Botomaella, Girvanella, Hedstroemia, Korilophyton, Proaulopora, Subtifloria, and Tarthinia appear. Immediately above this, in the Dokidocyathus regularis Zone of the middle Tommotian, Chabakovia appears with the epiphytacean genera Epiphyton, Gordonophyton and Tubomorphophyton. Kordephyton appears in the Dokidocyathus lenaicus Zone of the late Tommotian and Razumovskia in the early Atdabanian. These fossils constitute three calcareous algae assemblages (CAA 1, 2, 3) characteristic of the Nemakit-Daldyn Horizon/late Yudomian, early Tommotian, and middle Tommotian/Atdabanian respectively. Most of the genera are long-ranging, but their rapid appearance and diversification near the base of the Tommotian represents an important event in the history of calcareous algae and augments faunal data concerning evolution and biostratigraphy near the Precambrian/Cambrian boundary.--Modified journal abstract.