Maiaspora: the hallmark of gleichenioid ferns (Gleicheniales) from the early Carboniferous

The Palaeozoic origin of gleichenioid ferns (Gleicheniales) has aroused curiosity for almost a century. The oldest definite sign of this ancient clade is the spore wall ultrastructure of Radiitheca dobranyana from the Moscovian of central Europe. However, familial assignment of this fern fossil is uncertain. A comparison of its in situ spores with those of the dispersed genus Maiaspora, from the Visean, suggests that they may represent the same group of leptosporangiates. Although these spores have a common metareticuloid sculpture, the ultra‐thin structure of the dispersed counterparts remained unknown. Study of the ultrastructure of Maiaspora concava and M. panopta from central Russia and Northumberland (UK) using transmission electron microscopy shows that the wall of Maiaspora has a middle exospore layer composed of foliated lamellae. These spread from the base of the aperture slit to the subequatorial margins of kyrtome areas. The lamellae do not overlap the aperture slit and terminate at between two‐thirds and three‐quarters of the slit height. The miospores have a three‐layered exospore in the interradial areas, while non‐apertural parts of the exospore consist of two layers. Such an ultrastructure is shared by extant Gleicheniales and fossil R. dobranyana. The combined evidence from the morphology, ultrastructure and spatiotemporal distribution of Maiaspora, as well as radioisotope dating and additional plant macrofossils, suggests that the origin of the Gleicheniales stem was related to closure of the Rheic Ocean. Apparently, the first Maiaspora‐producing gleichenioids emerged as neoendemic terrestrial ferns on the peri‐Gondwanan volcanic arcs.