FOSSIL PLANTS/Calcareous Algae 433 rise to a cluster of radiating lateral branches The tips of the laterals expand into hexagonal facets that unite to form an unbroken outer globular surface The resulting swollen thallus is usually several centimetres in diameter Common genera, including Apidium, Coelosphaeridium, and Cyclocrinites, are locally abundant The affinities of Cyclocriniteae have been complicated because they have also been compared with, and sometimes placed in, receptaculitids – a problematic Palaeozoic group that shows some similarities with sponges However, although cyclocrinitids were originally regarded as animals, their morphology is similar to that of the modern weakly calcified dasycladalean Bornetella Charophyceae Figure Dasycladalean sections The aragonitic skeletons (now calcite) form a cast of the cell axis and lateral branches, which remain as matrix filled voids The oblique section (upper) partly shows the size and shape of axis and laterals whilst the tangential section (lower) indicates the arrangement of the lat erals Paleocene, western Pyrenees, Spain Scale bar 500 microns Present-day Polyphysaceae (five genera) also occupy shallow tropical to warm temperate seas Some species tolerate high salinity fluctuations in shallow-water lagoons Members of this family are characterised by the presence of a cap of gamete-producing laterals at the top of the plant as in Acetabularia, or alternating with sterile laterals in Halycorne Calcification mostly takes place in the cap Possible dasycladaleans (e.g., Yakutina) are known from the Cambrian, but the earliest firm records are Ordovician Dasycladaleans are common in platform carbonate deposits since the Carboniferous (see Sedimentary Environments: Carbonate Shorelines and Shelves) They reached high diversity in the Late Jurassic to Early Cretaceous and Eocene The species richness of the group decreased after the Eocene, and extant species can almost be considered ‘living fossils’ (Figure 1) Dasycladaleans are used in biostratigraphy of shallow-water carbonates from the Late Palaeozoic to the Palaeogene Zonations based upon dasycladalean genera and species have relatively poor time-resolution and their relation to standard chronostratigraphic scales is imprecise However, together with benthic foraminifers, dasycladaleans are often the only fossils of biostratigraphic value found in extensive shallow platform carbonates Cyclocriniteae Cyclocrinitids, or Cyclocriniteae, are a group of probable dasycladaleans (see Dasycladales above) mainly of Mid-Ordovician to Early Silurian age They are characterized by an apically-inflated main axis giving These chlorophytes are the closest algal relatives of embryophytes (mosses and vascular plants) Living charophyceans are grouped in the order Charales Two additional extinct orders occur in the Palaeozoic The group is characterized by complex bushy thalli, up to many decimetres in height, consisting of alternating nodes and internodes The internodes are mostly made up of elongate cells The nodes exhibit whorls of short branches (‘leaves’, and spine cells in some species) and branches of unlimited growth (Figure 8) Most charophyceans are anchored in soft sediment by rhizoids and have the appearance of subaqueous shrubs The stem and branches can be encrusted by low-magnesium calcite and, more rarely, aragonite Reproductive structures occur at nodes Male gametes (spermatozoids) are produced in spherical structures called antheridia Female gametes form in an ovoid oogonium that consists of an egg cell surrounded by sterile cells (Figure 8) In some species, following fertilization of the egg the surrounding cells become encrusted by calcite, while the remainder of the oogonium decays The resulting calcified structures, gyrogonites, are the most common fossil remains of charophyceans (Figure 9) Gyrogonites are made up of vertically elongate cells in primitive members of the Sycidiales, ranging from Late Silurian to Early Carboniferous; of dextral spiral cells in Trochiliscales, ranging latest SilurianPermian, and of sinistral spiral cells in the extant order Charales, first recorded in the Devonian The number of cells and the type of opening in gyrogonites are family-level taxonomic characters Members of the Mesozoic family, Clavatoraceae possess gyrogonites with an external calcified cover (utricle) of various shapes The Charales achieved greatest diversity in the Cretaceous to Palaeogene It is a small, almost relict, group at the present day (Figure 1) Charaleans grow in freshwater streams, ponds, and lakes and are especially abundant in calcareous