Palaeoecology of Macroneuropteris scheuchzeri, and its implications for resolving the paradox of ‘xeromorphic’ plants in Pennsylvanian wetlands

The medullosan pteridosperm Macroneuropteris scheuchzeri (Hoffmann) Cleal, Shute & Zodrow was widespread and abundant in the Middle to Late Pennsylvanian forests of tropical Pangaea. On the basis of its thick cuticles, few adaxial stomata, sunken stomata, and dense trichomes, it has been inferred to be a xeromorphic plant. Here we test that hypothesis by analyzing its facies distribution, especially in relation to coals, at 273 site collections from ~ 45 stratigraphic horizons accessioned in the Smithsonian National Museum of Natural History, Washington, D.C., U.S.A. The collections represent mostly (par)autochthonous assemblages and cover the taxonʹs complete known temporal range west of the Appalachians in west-central Pangaea. Chi-square analysis of facies-occurrence data indicates that M. scheuchzeri had a strong preference for mineral-enriched wetlands that immediately preceded and followed the development of peat swamps and mires (coals), particularly during the late Middle Pennsylvanian (late Moscovian) times, coincident with a period of reduced polar ice volume. In the Late Pennsylvanian, following an interval of global warming and tropical wetland reorganization, palaeoequatorial climate shifted to an overall drier mode. This was accompanied by the disappearance of M. scheuchzeri from Europe and its limitation, west of the Appalachians, to the wettest available habitats on the landscape. Thus, our data falsify the hypothesis that M. scheuchzeri was a conventional xerophyte, but highlight the paradox of a plant with apparent xeromorphic features confined to a wetland habitat. We consider several possible explanations for this association: (1) response to nutrient deficiency, (2) response to substrate salinity (due to growth in coastal mangrove habitats), (3) response to the composition of the Palaeozoic atmosphere, (4) return on investment (long leaf lifespan), and (5) phylogenetic constraint reflecting ancestral conditions. Our findings have broader implications for interpreting a wide variety of other Pennsylvanian wetland plants, which similarly show ‘xermorphic’ features.