A Middle Pennsylvanian (Bolsovian) peat-forming forest preserved in situ in volcanic ash of the Whetstone Horizon in the Radnice Basin, Czech Republic

Abstract

The precursory mire of the Middle Pennsylvanian (Bolsovian) Lower Radnice Coal was buried in situ by volcanic ash, preserving the taxonomic composition, spatial distribution, vertical stratification, and synecology of this peat-forming ecosystem in extraordinary detail. Plant fossil remains represent the pre-eruption vegetation of the swamp, which resulted from accumulation of peat in a high-ash, planar (rheotrophic) mire situated in a narrow palaeovalley containing an active fluvial system. A tuff bed (the Bělka) at the base of the volcaniclastic Whetstone Horizon was exposed in two contiguous excavations over an area of 50 m 2 in the Radnice Basin of western Bohemia, Czech Republic. Twenty-seven morphotaxa were identified, representing 20 whole-plant species with a wide variety of growth forms. The canopy of the peat-forming community was dominated by Cordaites borassifolius trees together with the arborescent lycopsid “ Lepidodendron” (= Paralycopodites), whereas Lepidophloios cf. acerosus was subdominant. Evidence suggests that the laterally extensive “crowns” of these arborescent lycopsids would have overlapped during the final phase of their life cycles, but differences in the height of tree species resulted in a complex and vertically variable canopy interrupted by randomly distributed gaps. The understorey was dominated by medullosan pteridosperms and marattialean tree ferns, whereas zygopterid ferns and sphenophylls comprised the bulk of the ground cover. In comparison with the canopy, understorey and ground cover species were less abundant and patchier in distribution, with almost complete absence beneath the deep shade of C. borassifolius trees. Lianas that entwined arborescent trees were an important component of the peat-forming forest. Three lyginopterid pteridosperm species along with a sphenophyll had a lianescent habit based on their close association with upright or prone lycopsid trunks and “canopy” branches. Species richness in the swamp superficially appears low. However, considering the small area of excavation, along with the higher diversity known from the same tuff bed in the adjacent, former opencast Ovčín Mine, it appears that species richness in the forest was comparable to some of the less diverse Westphalian peat-forming swamps in the U.S.A. The Lower Radnice mire vegetation was compositionally homogeneous, but had a heterogeneous distribution with patchiness occurring at a very fine scale. The preserved plant assemblage most resembles mires dominated by medullosan pteridosperms and Paralycopodites described from upper Westphalian coal balls in the U.S.A., which were characterised by high diversity in all storeys and involved plants centred in high-ash peat-forming swamps.