Vegetation heterogeneity on a Late Pennsylvanian braided-river plain draining the Variscan Mountains, La Magdalena Coalfield, northwestern Spain

The spatial heterogeneity and community ecology is reconstructed for Late Pennsylvanian (Stephanian B sensu lato) vegetation preserved in La Magdalena Coalfield, northwestern Spain. The ≈ 1500 m thick basin-fill accumulated rapidly along the margin of the Variscan Mountains, and the principal sedimentary facies comprise the deposits of large braided streams that dissected extensive wetlands containing large lakes. Quadrat analysis of 93 mostly (par)autochthonous megafloral assemblages indicates that pteridosperms and ferns dominated communities, with three taxa (Pecopteris spp., Callipteridium pteridium, and Neuropteris ovata) accounting for ≈ 58% of all plant remains. Sphenopsids and lycopsids were less common but widespread, and cordaitaleans were rare. At the local scale, laterally exposed bedding planes reveal that communities comprised a complex and heterogeneous mosaic of species. At the landscape scale, ecological gradients are evident from multivariate analyses of quadrats in a facies context. Pteridosperms dominated marginal wetlands adjacent to steep basin margins. A greater proportion of ferns occurred in or adjacent to braided channel belts, consistent with their opportunistic growth and prolific reproductive capacity, characteristics advantageous in frequently disturbed habitats. In interfluve wetlands distant from channel influence, communities consisted of low-diversity or monospecific patches of slow-growing pteridosperms enclosed in a fern-dominated matrix. Pteridosperms in interfluve wetlands were arranged along an environmental gradient: Alethopteris and Pseudomariopteris preferred more disturbed sites, Neuropteris, Linopteris, and Odontopteris occupied more stable settings, and Callipteridium was cosmopolitan. Ephemeral wetland swamps were dominated by tree ferns with lesser Sigillaria, and Omphalophloios was a major constituent in long-lived rheotrophic mires that resulted in thick coal seams. In addition to improving knowledge of Late Pennsylvanian tropical ecosystems in marginal settings, the findings demonstrate how the spatial structure of ancient plant communities can be reconstructed from multiple quadrats sampling a vertical succession.