Megaspores and Microspores of the Extant and Paleogene Marsileaceous Fern Regnellidium and Cretaceous Molaspora: Evolutionary and Phytogeographic Implications

In common with the single extant species of the water fern Regnellidium, Regnellidium diphyllum, megaspores of Eocene and Oligocene representatives of this genus are characterized by having twisted, leaflike folds forming an acrolamella around the triradiate suture. The fossil and modern megaspores also have very similar wall ultrastructure and sculpture, and the morphology of the attached microspores of both is indistinguishable. Specimens of one of the species of the Cretaceous megaspore genus Molaspora—namely, Molaspora lobata, with attached microspores from Spain—and spores in the sporocarp Regnellidium upatoiensis from Georgia, United States, share some characters with those of extant Regnellidium, but the surface ornament of the microspores and the ultrastructure of the megaspore and microspore walls are different. The inner layer of the inner perine of M. lobata megaspores is thinner than the outer layer and of more compact, spongiose construction than that of Eocene and Oligocene Regnellidium spor...