Medullosan seed ferns of seasonally-dry habitats: old and new perspectives on enigmatic elements of Late Pennsylvanian–early Permian intramontane basinal vegetation

Abstract

The medullosans represent a diverse group of pteridosperms that was widely distributed in forested landscapes of the late Paleozoic. These plants became widely known from the extensive tropical lowland basins of Euramerica, where they grew as slender plants with large fronds and fern-like foliage. Besides, there also exist medullosans of Late Pennsylvanian–early Permian intramontane basins of Central Europe, which have been out of research focus for more than a century. They had bigger stems with larger amounts of secondary xylem and a modified organisation of the vascular system. We provide an overview on taxonomy, anatomy and palaeoecology of these medullosans from the most important fossil localities, encompassing Chemnitz (Germany), the type locality for most taxa, Autun (France), Nová Paka (Czech Republic) and others. Late Pennsylvanian–early Permian medullosans of intramontane basins were thriving under seasonally-dry palaeoclimate on wet clastic soils showing proximity to the groundwater level. In forested landscapes, they occurred mostly sub-ordinated as part of the forest understorey. The plants' architecture and taphonomical inferences point to a (semi-)self-supporting growth habit of most of the taxa presented here. Plant architectural and anatomical peculiarities suggest a high water-conducting potential of these plants, raising questions on their ecological role in early Permian habitats. Anatomical differences with tropical relatives of Carboniferous age might reflect an evolutionary process that was driven by environmental changes during the late Paleozoic. The unusual arrangement of their stem tissues addresses the question of their role in seed plant evolution, e.g., their potential relationship with cycads.