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 spores from Eurasia and extant R. diphyllum. The microspores attached to or associated with M. lobata have a reticulate surface formed from muri of an outer perine layer and a thick, two-layered exine, whereas those of fossil and Recent Regnellidium have a closely packed rugulate-papillate sculpture and a thin exine that consists of a single, slightly spongiose layer. These characteristics, therefore, distinguish the Cretaceous fossils from Eocene-Recent members of this group. On the basis of available morphological characters of megaspores and microspores, M. lobata is interpreted as sister taxon to Regnellidium or to the Regnellidium + Pilularia clade. The geographically widespread records of Molaspora, some species of possibly related Arcellites, and fossil megaspores attributable to Regnellidium show that the parent plants of all of these reproductive organs were widely dispersed in the geological past and that the current distribution of Regnellidium-which is restricted to Brazil, Uruguay, and Argentina-is relict.