Middle Pennsylvanian pioneer plant assemblage buried in situ by volcanic ash-fall, central Bohemia, Czech Republic

Abstract

Palaeoecological analysis of a single-age plant assemblage of the middle Westphalian age (Bolsovian = middle Moscovian) preserved in the tuff bed at the base of the Whetstone Horizon in the roof of the Lower Radnice Coal of the Štilec opencast mine in central Bohemia is provided. This plant assemblage represents a peat-forming phytocoenosis buried in situ by volcanic ash-fall as indicated by frequent occurrence of upright stems rooted in the underlying coal and large plant fragments occurring at the base of the tuff. It is a low-diversity herbaceous and subarborescent assemblage dominated by small ferns and calamites with subdominant lycopsids not taller than about 1–1.5 m. This unique herbaceous assemblage comprises four fern species ( Kidstonia heracleensis, Dendraena pinnatilobata, Desmopteris alethopteroides and Sphenopteris cirrhifolia), Calamites sp. and the small lycopsid Spencerites leismanii. Each species bears mature fertile organs with spores that indicate them to represent small but fully mature plants and not juvenile arborescent taxa. This plant assemblage is interpreted as a pioneer phytocoenosis that colonised a shallowed pond or lake, and that developed in the Lower Radnice Coal mire after flooding. Comparison of the phytocoenosis preserved in tuff bed at the base of the Whetstone Horizon in the Štilec opencast mine with coeval plant assemblages from the same bed in other localities reveals its unique character. Comparison of the palynological record from the roof of the Lower Radnice Coal with the taphocoenosis preserved in the “bělka” tuff bed overlying this coal and the allochtonous taphocoenosis of the laminated tuffite above bělka indicates a close co-existence of this low-diversity herbaceous phytocoenosis with the high-diversity lepidodendrid lycopsid dominating assemblage.