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 preeruption 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 peatforming 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.

Lycopsida from the lower Westphalian (Middle Pennsylvanian) of the Maritime Provinces, Canada

“…It is possible that the absence of the remains of smaller plants (undergrowth, lianas) at this locality may have been the result of catastrophic flooding, removing part of the floral association; but this is conjectural. In contrast, Opluštil et al (2009) recorded a considerable diversity of floral elements in volcanic-ash deposits associated with the Radnice coals of Bolsovian age in the Czech Republic. They distinguished canopy, understorey, lianas and ground cover/ climbers.…”

T0 peat-forming plant assemblage preserved in growth position by volcanic ash-fall: A case study from the Middle Pennsylvanian of the Czech Republic

“…They usually occur on discrete horizons often associated with certain types of facies, thus indicating that their origin is related to particular environments and processes. Fundamental controls on the origin of these phenomena are related to rapid burial in a geological instant, preserving the plant relationships in space and time and removing or reducing many taphonomical biases (Opluštil et al 2009a, DiMichele & Falcon-Lang 2011, Rößler et al 2012. Such conditions are achieved especially by crevasse splay deposition, earthquake-induced subsidence which catastrophically drops the vegetated area beneath the water table, rapid eustatic sea-level rise, permineralization of vegetation by hot springs, or its burial by ash falls proximal to volcanic centres (DiMichele & Falcon-Lang 2011).…”

“…Such taphocoenoses often preserve even subtle ecological details, e.g. lianas scrambling along trees, epiphytes still attached to supportive plant (= phorophyte) or such exceptional details as an arachnid preserved on a Cordaites leaf (Opluštil et al 2009a, Selden & Penney 2010. Limited taphonomic bias makes these assemblages ideal for the analysis of tree density, spatial heterogeneity, assemblage diversity, whole-plant reconstructions, assessment of plant interactions and other ecological "parameters" that can be rarely obtained from any other type of fossil preservation (e.g.…”

“…Limited taphonomic bias makes these assemblages ideal for the analysis of tree density, spatial heterogeneity, assemblage diversity, whole-plant reconstructions, assessment of plant interactions and other ecological "parameters" that can be rarely obtained from any other type of fossil preservation (e.g. Opluštil et al 2009a, b, Wang et al 2009, DiMichele & Falcon-Lang 2011, Pše-nička et al 2005. Careful taphonomical analysis of T 0 assemblages thus significantly contributes to much better understanding of the fossil ecosystems they represent (Johnson 2007).…”

“…Whereas lateral changes in the composition of plant fossil assemblages reflect spatial heterogeneity of vegetation cover, the vertical distribution of plant remains within a tuff includes separation of litter and ground cover from tree crown foliage, lianas and epiphytes (Burnham & Spicer 1986, Wing et al 1993, Burnham 1994, Opluštil et al 2009a. Evidence of interactions between animals and plants also may be preserved, not uncommonly (Prokop & Nel 2010, Scott & Taylor 1983, Rößler et al 2012.…”