PLANT–FUNGUS–ARTHROPOD INTERACTIONS IN WETLAND ECOSYSTEMS FROM THE LATE CARBONIFEROUS CALHOUN AND EARLY PERMIAN SHANXI COALS

Interactions among plants, fungi and arthropods long have been recognized as a critical trophic step in the evolution of wetland ecosystems, despite a sparse and uninformative fossil history. Our study examines fungal fossils and their associated plants and arthropods from anatomically preserved, permineralized coal balls representing the Late Carboniferous Kasimovian–Gzhelian age (Mattoon Formation) Calhoun Coal deposit from Berryville, Illinois, USA, and Early Permian Asselian–early Sakmarian age (Taiyuan Formation) Coal deposit from Xiedao Village, Shanxi Province, China. The relationship of the Chinese Taiyuan floras have been determined as a time transgressive expression of similar plant assemblages established earlier in wetland communities of Euramerica. The Cathaysian floras diverged during the Mississippian and continued into the Early Permian. For earlier Calhoun vegetation, plant–fungus–arthropod interactions occurred principally in the extinct marattialean tree-fern Psaronius, the dominant plant constituent, and less so for the subdominant seed fern Medullosa and the sphenophyte Sphenophyllum. For Psaronius, symbiotic and parasitic fungal structures have been found endophytically, respectively, in root and rachis tissues. Spores and vesicular and arbuscular mycorrhizae with intra- and intercellular hyphal structures frequently occur in root tissues. Fungi in rachis cells feature complete reproductive and vegetative life stages. In one endophytic association between an insect galler and its Psaronius plant host, we note fungal colonization in the vacuities among insect coprolites (fossilized fecal pellets) and galled tissue, probably indicating a diffuse, tritrophic association. By comparison, the plant–fungus–arthropod interactions in the Shanxi plant assemblage exhibit more pervasive relationships with subdominant cordaites than the dominant lycopsids. Additionally, the use of SEM with energy dispersive spectroscopy has elucidated the nature of fossil preservation that contributed to the exceptional mycorrhizal structure in the coal-ball specimens.