Pollen and diatom analyses as well as radiocarbon datings were used to study the development of the ancient lake-basin Pyysuo in SW Finland with mass occurrence of Trapa natans subfossil fruits in its sediments. The basin was isolated from the Baltic Sea c. 4300 BP, after which it continued to develop as a shallow, eutrophic, alkaline and vegetation-rich small lake until the final overgrowth by a mire at c. 2500 BP. The fossil fruits of Trapa natans appeared in the sediments c. 3500 BP, and reached their maximum abundance just prior to filling-in. In view of the great time span of its finds now in Finland (8500-2500 BP), its occurrence was evidently not directly limited by the mean temperature, but the habitat availability and local conditions in the lake basins themselves.
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Glückert, G.1976: Post-glacial short-level displacement of the Baltic in SW Finland. Annales Academiae Scientiarum Fennicae A III. 118, 1-92.
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Gams, H.1958: Beiträge zur Variabilität der Früchte von Trapa natans . In Welten, M. and Zoller, H., editors, Festschrift Werner Lüdi, Veröffentlichungen der Geobotanischen Institutes Rübel in Zürich 33, 108-15.
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