428 FOSSIL PLANTS/Calcareous Algae Calcareous Algae J C Braga, University of Granada, Granada, Spain R Riding, Cardiff University, Cardiff, UK ß 2005, Elsevier Ltd All Rights Reserved to coarse and fine bioclastic fragments Postmortem disintegration of a wide variety of calcified algae and cyanobacteria has probably extensively produced mud-sand-gravel sediment, although often only coarser components are readily recognizable Introduction Algae are anatomically simple photosynthetic plants, lacking vascular tissues, roots, and leaves They constitute a heterogeneous group comprising many divisions (see Palaeontology) (botanically equivalent to phyla) In addition, it is convenient to consider some photosynthetic bacteria (see Biosediments and Biofilms), such as Cyanobacteria (blue-green algae) along with eukaryotic algae The algal body (thallus) can be unicellular, or multicellular with a certain degree of complexity due to cell differentiation Although most algal groups are aquatic, some inhabit soils and subaerial environments Calcareous algae precipitate CaCO3 during life at particular sites on or within their thalli Calcification is a cross-systematic feature, occurring in diverse divisions, in freshwater and marine environments, and in planktic and benthic forms However, fewer than 10% of extant benthic marine algae calcify, and they principally belong to red and green algae Only one extant calcified genus of brown algae (Padina) is known, and cyanobacteria rarely calcify in marine environments at the present day although they did so at times in the past In freshwater, charaleans and a few other chlorophytes calcify, and cyanobacterial calcification is common in calcareous streams and lakes Photosynthesis raises pH and increases saturation state with respect to CaCO3 minerals However, the degree of biological control over calcification depends mainly on the location of sites for precipitation within the organism In general, cyanobacterial calcification is environmentally controlled, and calcification by chlorophytes is also environmentally dependent In contrast, coralline red algae and coccolithophores calcify intracellularly and relatively closely control their calcification This is reflected in their wider latitudinal distribution in marine environments Fossils that may be calcified algae have been reported from the Neoproterozoic but the substantial record of calcified cyanobacteria and algae is mainly Phanerozoic (Figure 1) Their long geological history and wide environmental distribution result in calcified bacteria and algae being significant producers of calcium carbonate sediment in marine and freshwater deposits of many ages Sedimentary products range from in-place reefal masses (see Sedimentary Environments: Reefs (‘Build-Ups’)), through nodules Rhodophyta (Red Algae) Corallinales Coralline algae are the major extant group of calcified marine red algae Their thalli consist of branched cell filaments with a coherent unified, pseudoparenchymatous, organization Growth is achieved by addition of new cells through cell division at the tip of each filament Most present-day corallinaleans are heavily calcified by high magnesium calcite precipitated in the cell wall (Figure 2) This consists of a layer of crystals parallel to the wall from which radial crystals grow towards the cell interior In non-geniculate (non-articulated) corallines all vegetative cells, Figure Stratigraphic range of the major groups of calcareous algae Width of bars indicates the relative diversity of the group in each period Pr: Proterozoic, C: Cambrian, O: Ordovician, S: Silurian, D: Devonian, Ca: Carboniferous, P: Permian, T: Trias sic, J: Jurassic, K: Cretaceous, Ce: Cenozoic