Zygospores of the Zygnemataceae (Division Chlorophyta) and Other Freshwater Algal Spores from the Uppermost Pliocene St. Erth Beds of Cornwall, Southwestern England

Abstract

Zygnemataceous zygospores are reported for the first time from upper Pliocene marine clays of a transgressive sequence known as the St. Erth Beds. The marine clays were deposited under the warm shallow waters of a coastal embayment. The freshwater zygnemataceous zygospores, and other algal spores of suspected freshwater origin, are not in situ although they are considered essentially contemporaneous. These algal spores may have been washed directly into the embayment: alternatively they are reworked from freshwater sediments that may have existed along the coast prior to marine transgression. Palynological analysis shows the algal spores to be rare, with vascular plant tissues and non-algal sporomorphs dominating the St. Erth assemblages. The algal spores are typically distinct in having unstructured hyaline walls that do not readily accept safranin-o stain. A total of 18 formal and informal taxa have been documented: these include the extant zygnematacean species Debarya glyptosperma (perhaps constituting the oldest known record of zygospores assignable this species) and representatives of the fossil genera Lecaniella (formae 1-10, some resembling zygospores of the Zygnemataceae), Gelasinicysta n. gen., and Planctonites (which includes at least one possible representative of the Desmidiaceae). The presence of these algal spores records a hinterland with freshwater habitats of probably ephemeral nature and subject to seasonal warming: detailed interpretations are impeded by incomplete understanding of the biology of related extant taxa. Fluorescence microscopy (FM) shows the algal spores to be among the brightest fluorescing objects in the stained palynological residues. FM may therefore be an effective, as well as non-destructive, way to rapidly locate scarce algal spore specimens in stained hylogen-rich samples. Gelasinicysta n. gen. (type species: Gelasinicysta vangeelii n. gen., n. sp.) is proposed for hyaline-walled fossil algal spores with a continuous equatorial fissure and dimpled ornament. Gelasinicysta shangsica (He 1984) n. comb. (al. Lacunalites shangsicus He 1984) is transferred accordingly. Planctonites nagyae (Nagy 1969) n. nom., n. comb. (al. Deflandridium stellatum Nagy 1969) is transferred on the basis of its similarity with P stellarius (R. Potonie), the type, from which it differs in its larger size.