An ultrastructural investigation of early Middle Pennsylvanian megaspores from the Illinois Basin, USA

Four megaspore genera: Cystosporites, Rotatisporites, Zonalessporites, and Valvisisporites were isolated from palaeokarst fill deposits of early Middle Pennsylvanian age, hosted within Ordovician limestones at Central Quarry in Central, Kendall County, Illinois, U.S.A. Five uncompacted megaspore species were identified and studied using scanning and transmission electron microscopy (SEM and TEM). The species Valvisisporitessculptus Bhardwaj is emended based on improved morphological and new ultrastructural data. The genus Pseudovalvisisporites Lachkar is recognized as a junior synonym of Valvisisporites. The ontogenetic development of aborted and functional Cystosporitesdiabolicus megaspores is investigated and the timing of sporopollenin deposition in both the exine and massa is reported. In Carboniferous megaspores, the importance of detailed investigation of the periapertural region is emphasized by the discovery of fusiform lamellate thickenings in Valvisisporites and Rotatisporites. The taxonomic and systematic importance of these features are considered in light of the otherwise highly conserved ultrastructure that is recognized in many lycopsid megaspores of this age.