Marsileaceae sporocarps and spores from the Late Cretaceous of Georgia, USA

A new species provisionally assigned to the extant genus Regnellidium Lindm . (Regnellidium upatoiensis sp. nov.) is established for isolated sporocarp s assignable to the heterosporous water fern family Marsileaceae. Three spo rocarps and hundreds of dispersed megaspores were recovered from unconsolid ated clays and sifts of the Eutaw Formation (Santonian, Late Cretaceous) al ong Upatoi Creek, Georgia, U.S.A. The sporocarps are ellipsoidal and flatte ned, contain both megasporangia and microsporangia, and possess a two-layer ed wall-an outer sclerenchymatous layer and an inner parenchymatous layer. In situ megaspores are spheroidal, with two distinct wall layers-an exine, differentiated into two layers, and an outer ornamented perine also differe ntiated into two layers. The megaspores also possess an acrolamella consist ing of six (five to seven) triangular lobes that are twisted. In situ micro spores are trilete and spheroidal, with a strongly rugulate perine, and sho w modification of the perine over the laesura to form an acrolamella. Compa rison of the fossil sporocarps with those of four extant species of Marsile aceae reveal marked similarity with Regnellidium diphyllum Lindm., particul arly in megaspore and microspore morphology. If found dispersed, the in sit u megaspores would be assigned to Molaspora lobata (Dijkstra) Hall and the microspores to Crybelosporites Dettmann based on their size, shape, and orn amentation. Regnellidium upatoiensis sp. nov. extends the stratigraphic ran ge of the genus back to the Santonian, nearly contemporaneous with the firs t evidence of Marsilea, and implies that the diversification of the Marsile aceae into its extant lineages occurred in the mid-Cretaceous.