A Middle Pennsylvanian (early Asturian) tropical dry forest, Atokan-Desmoinesian boundary, Illinois Basin, USA

Resumen

Six late Atokan (early Asturian) floras from seasonally dry environments are described and quantitatively analysed from adpressions and palynomorphs. Collections are from the eastern margin of the Illinois Basin, USA, in an 80 km N-S transect. Plant fossils occur in sedimentary rocks below the underclay (paleosol) of the Minshall-Buffaloville Coal Member (thus, not “roof-shale” assemblages), uppermost Brazil Formation. Growth of floras under season dryness is indicated by outcrop and lithological features that suggest deposition in flashy discharge streams, including intraformational conglomerates, plant fossils that cross bedding planes indicating rapid, episodic burial, and local rhythmically laminated sediments. Common charcoal clasts are consistent with seasonal climate. Cordaitalean foliage dominates the macroflora, accompanied by the dryland elements Lesleya sp., Taeniopteris sp. cf. T. multinervia, and Sphenopteridium sp. Two unusual forms of foliage are presumed to be rare or novel dryland species. Small numbers of wetland/periwetland taxa include calamitaleans, Linopteris neuropteroides, Karinopteris/Eusphenopteris sp., marattialean fern foliage, Senftenbergia plumosa, cf. Zeilleria avoldensis and Sphenopteris sp. The palynoflora is dominated by marattialean tree ferns, wetland plants with broad dispersal capacities and environmental tolerances. Subdominant elements also have strong wetland affinities. Palynoflora-macroflora mismatches may reflect primary ecology, palynomorph reworking, or both. The occurrence of these floras near the Atokan-Desmoinesian (~ Bolsovian-Asturian) boundary coincides with an array of physical and geochemical data that indicate change from weak rainfall seasonality to marked seasonality at all phases of glacial-interglacial cycles, reflected prominently in the stratigraphic record from the localities studied.