Macroflora of the Krkonoše-Piedmont Basin (Pennsylvanian–early Permian); Bohemian Massif, Czech Republic

Abstract

The Krkonoše-Piedmont Basin is one of several land-locked post-collisional Variscan basins of the Bohemian Massif. The fossil record of the basin is evaluated, based on published data and unpublished lists of floras collected in borehole cores and during mapping. The occurrence ranges of plant taxa are plotted against radioisotopically calibrated lithostratigraphic units. The compiled dataset is used to evaluate species richness and vegetation patterns throughout Late Pennsylvanian and Asselian times. The species richness of particular intervals has been affected by taphonomic and sampling biases of varying intensities. Coal-bearing strata and grey lacustrine intervals represent well-documented fossil-rich taphonomic windows with 20–45 species. In contrast, red beds are fossil-poor and under-sampled, typically providing only a few species. It is assumed that coal-bearing and grey lacustrine intervals represent periods of increased climatic humidity, during which wetlands covered extensive areas of basinal lowlands or shallows and coastal areas of meromictic lakes. Most habitats were colonised by hygrophytes and mesophytes. However, sparse conifer shoots found in association with wetland taxa indicate seasonality to a sufficient degree that xerophytic taxa were able to get close to sites of deposition and wetland habitats. During the deposition of red beds, wetlands may have almost completely disappeared, resulting in the basinal landscape being colonised by gymnosperm-dominated forests of walchian conifers with co-dominant cordaitaleans. Wetland assemblages of low diversity and small populations may have survived in the basin, although many species may have disappeared from the basin, having dispersed elsewhere to habitats where their survival was possible. In the late Middle Pennsylvanian and early Upper Pennsylvanian, such habitats existed in the Intra-Sudetic Basin farther east, and in other basins located in western and central parts of the Bohemian Massif. These climatically-driven oscillations in vegetation patterns, which occurred on scales of hundreds of thousands to a million years, were superimposed on a long-term drying trend marked by an increase in gymnosperms in the species pool, as well as an abundance of their remains in the fossil record, at the expense of free-sporing plants. Among the gymnosperms, there was a gradual drop in the diversity and abundance of medullosalean pteridosperms and a concomitant rise of peltasperms and conifers.