Article: Principal floras of Palaeozoic marine calcareous algae
The stratigraphic distribution of eighteen groups of fossils commonly assigned to the calcareous algae reveals three major floras in shallow marine carbonate deposits of Palaeozoic age: (1) Cambrian flora, (2) Ordovician flora, (3) Carboniferous flora. The Cambrian flora appears abruptly near the Precambrian-Cambrian boundary and is dominated by cyanophytes. The Ordovician flora appears quickly during the lower and middle Ordovician and is dominated by chlorophytes, ?rhodophytes, and problematic groups. The Carboniferous flora appears gradually, mainly during the Carboniferous, and is dominated by rhodophytes, chlorophytes, and problematic groups. Important extinctions occurred near the ends of the Devonian, Carboniferous, and Permian.The succession of floras is reflected in the changing sedimentological roles of Palaeozoic calcareous algae. Cambrian reefs are dominated by Epiphyton-Renalcis assemblages which reappear briefly in the Devonian. During most of the middle Palaeozoic algae are subordinate to metazoan reef-builders, but Solenoporaceae, Rothpletzella, and Wetheredella are nevertheless important locally. Following a hiatus during the lower Carboniferous, Donezella, Ungdarella, phylloid algae, and Tubiphytes were important reef-builders. Skeletal oncoids built by Girvanella, Hedstroemia, Ortonella, and Rothpletzella, together with Solenopora rhodoliths, are common at many levels in the Palaeozoic, but skeletal stromatolites are generally rare. Nodules formed by Archaeolithophyllum and Cuneiphycus occur in the upper Palaeozoic. Sand- and gravel-size fragments, mainly of chlorophytes and rhodophytes, increase in abundance from the Ordovician onwards.