434 FOSSIL PLANTS/Calcareous Algae is imprecise Nonetheless, charalean fossils permit stage, or even sub-stage, level dating of deposits that often lack other biostratigraphic fossils Groups of Uncertain Affinity Gymnocodiaceae In vegetative construction, gymnocodiaceans are similar to halimedaleans (see Halimedales above), but rounded cavities within the cortex of some specimens resemble reproductive structures and suggest a red algal affinity Gymnocodiaceans have been compared with the modern weakly calcified chaetangiacean Galaxaura This enigmatic group, including Gymnocodium, Permocalculus, Nanjinophycus, and possibly Succodium, is most widespread and diverse in the Late Permian Permocalculus also occurs in the Cretaceous and has been reported from the Paleocene and Miocene Gymnocodiaceans appear to be shallow tropical marine in distribution, and locally are important bioclastic sediment producers Phylloid Algae Figure Charalean plant (A), showing the male reproductive structure (antheridium) at a node (B), and a female reproduct ive structure (oogonium) (C) The spiral cells in the oogonium are encrusted by calcite after fertilization The resulting calcified structures, gyrogonites, are the most common fossil remains of Charales (Adapted from Engler and Prantl (1898 1900).) Figure Sections of charalean gyrogonites Lower Cret aceous, Transylvania, Romania Scale bar 500 microns oligotrophic waters Some species live in brackish lagoons Fossil gyrogonites are used in biostratigraphy of non-marine deposits, especially from the Early Cretaceous to the Palaeogene The biostratigraphic resolution of charalean zones is relatively low and correlation with marine chronostratigraphy The term ‘phylloid algae’ describes fossils with flattened leaf-like shape that are especially common in Upper Carboniferous and Lower Permian reefs Their recognition as a group is practical because the internal structure of these fossils is commonly poorly preserved, but it overlooks important morphological and systematic variations, and phylloids are a highly heterogeneous group that probably includes both green and red algae Their internal structure is gradually being elucidated by discovery of well-preserved specimens and by use of cathodo-luminescence Two morphological subgroups can be distinguished: (i) erect blades with medullary and cortical structure resembling that of halimedalean green algae, e.g., Anchicodium, Eugonophyllum, Ivanovia and, doubtfully, Calcifolium (see Halimedales); (ii) prostrate crusts with internal cells and possible conceptacles, e.g., Archaeolithophyllum that has differentiated multicellular filaments together with surficial conceptacle-like structures (Figure 10) This latter subgroup is compared with red algae, particularly peyssonneliaceans (see Peyssonneliaceae above) Phylloid algae formed skeletal frame reefs in the Late Carboniferous to Early Permian Calcified Cyanobacteria Originally named blue-green algae, the prokaryotic cell organization of this group clearly allies it with bacteria In natural environments, cyanobacteria extrude polymeric substances to form protective