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Premise of research. The taxonomic affinities of nonmarine Proterozoic and Paleozoic microfossils are often difficult to determine. Given that the preservability (degradation resistance) of cell walls displaying distinctive features is widely regarded as a key feature allowing the recognition and classification of fossil protists, we examined the retention of diagnostic cell wall features after high-temperature chemical hydrolysis of several modern filamentous algal genera previously hypothesized to be related to particular Proterozoic or Paleozoic microfossils.

Methodology. We collected and in some cases cultured filamentous algae from modern terrestrial sites or freshwaters of arid locales hypothesized to model ancient nonmarine habitats. We subjected these and other samples of Vaucheria (Stramenopila, Xanthophyceae), Cladophora (Chlorophyta, Ulvophyceae), Stigeoclonium (Chlorophyta, Chlorophyceae), and Oedogonium (Chlorophyta, Chlorophyceae) to acetolysis, an extremely degradative hydrolytic process widely used in palynology to select for resistant organic materials. We imaged the remains using bright-field, polarizing, and fluorescence LM and also SEM.

Pivotal results. Filaments of all xanthophycean and chlorophytan green algal genera tested resisted acetolysis and retained distinctive structural traits previously used to classify Proterozoic and Paleozoic microfossils as algae. Features of cell wall remains revealed by polarizing microscopy and SEM suggested that degradation resistance results largely from the presence in cell walls of cellulose types that are more resistant to degradation than are celluloses of land plants and streptophyte algae. In the case of Cladophora, specific autofluorescence properties also suggest the presence of a previously undetected phenolic layer in the primarily cellulosic cell wall.

Conclusions. Our results are more or less consistent with previous classifications of certain ancient microfossils with genera of modern filamentous algae and explain degradation resistance of their cell walls. The results justify the use of cell wall features to classify filamentous microfossils and suggest steps that might yield even more convincing identifications.