REVIEW OF BIOLOGICAL AFFINITIES OF PALEOZOIC ACID-RESISTANT, ORGANIC-WALLED EUKARYOTIC ALGAL MICROFOSSILS (INCLUDING ACRITARCHS)

Acid-resistant, organic-walled microfossils attributed to three classes of green algae are present in Paleozoic rocks. Included are members of the Prasinophyceae (uppermost Precambrian-Recent); Chlorococcales (Family Botryococcaceae, Carboniferous-Recent; Family Hydrodictyaceae, Silurian, Carboniferous-Recent); Zygnematales (Family Zygnemataceae, Carboniferous-Recent). The Silurian genus Arpylorus Calandra, 1964 is apparently a dinocyst. Moyeria Thusu, 1973 (also Silurian) may be a euglenoid pellicle. Other microfossils (''acritarchs'') are probably predominantly phytoplankton cysts, although exact affinities are disputed. Among selected genera, three clades are recognized here based on wall architecture and excystment structures: Baltisphaeridium clade, Cambrian(?)-Upper Permian; Peteinosphaeridium clade, Middle-Upper Ordovician; Cymbosphaeridium clade, Lower Silurian-lowermost Devonian. Members of the Cymbosphaeridium clade resemble dinocysts more closely than do cysts of at least one living species of peridinioid dinoflagellate, but lack the paracingulum and polygonal archaeopyle required by most paleontologists for inclusion with the dinocysts. Recent advances in chemical analysis have demonstrated that sporopollenin is a class of similar polymers, rather than a single compound. Acid-resistant structures within green algae are chemically distinct from those in higher plants, while dinocysts have yet to be investigated. With further technological advances, chemical analysis of wall material holds the greatest promise for establishing possible dinocyst affinities among Paleozoic acritarchs.