Numerous mesofossil floras, deposited approximately 80-115 million years ago (Ma), containing charcoalified and lignitized flower, fruits, seeds and leaves, have been found along the Atlantic and Gulf Coastal Plains of the United States. Although angiosperms from these localities have been intensively studied, other plant remains have not been examined in as much detail. The megaspore floras and fern mesofossils that have been recovered from three localities are described. Cumulatively, at least ten genera and thirteen species of megaspores are recognized from Northeast Plaza (Aptian, 115Ma, Maryland), Mauldin Mountain (Cenomanian, 99Ma, Maryland) and Upatoi Creek (Santonian, 84Ma, Georgia) localities. Megaspores identified so far have affinities either to heterosporous lycophytes—Bohemisporites, Erlansonisporites, Minerisporites, Paxillitriletes, and three additional "genera"—or to heterosporous ferns—Arcellites, Ariadnaesporites, and Molaspora. On the basis of recently described sporocarp remains, Molaspora is a dispersed megaspore belonging to the extant genus Regnellidium (Marsileaceae). At both sites in Maryland, lycopsid megaspores are more diverse and abundant than fern megaspores. At Upatoi Creek, lycopsid megaspores are more diverse, but approximately equivalent in abundance to fern megaspores. Fern mesofossils—pinnules with and without sporangia, and rachis fragments—are present at all sites. At Mauldin Mountain, isolated pinnules enclosing sporangial clusters are among the most common fossils. At Mauldin Mountain and Upatoi Creek, fertile pinnules assignable to Onychiopsis, a fern with affinities to Dicksoniaceae and previously known only from sediments of Cenomanian age and older, have been identified.

Key words: Cretaceous, megaspores, palynology, petridophytes