Species of the water-fern megaspore genus Molaspora from a Cenomanian deposit in western France: occurrence, sporoderm ultrastructure and evolutionary relationships

The partially reticulate sculpture of Molaspora aspera sp. nov., a marsileaceous megaspore from a Cenomanian deposit in western France, distinguishes it from other species of Molaspora. An acrolamella entirely surrounds and obscures a small tetrad scar, a feature that has been demonstrated hitherto within members of the genus only in M. fibrosa. It was also encountered for the first time in M. lobata, with which the new species is associated in the same French mesofossil assemblage. The ultrastructure of the sporoderm of M. aspera is similar to that of M. lobata, but differs particularly in that the inner epispore is markedly thicker and may also contain numerous large, homogeneous spherules or, alternatively, holes of comparable dimensions and only a few small spherules. It is possible that these are a response to some hostile bacterial or other activity when the developing sporoderm was partially permeable. The cavity replacing part of the epispore in one of the specimens, and in the specimen of M. lobata examined, may be a preservational feature or have served to increase buoyancy of the spore in water. Molaspora lobata is very similar to megaspores of fossil and extant Regnellidium, but M. aspera bears some resemblance to other members of extant Marsileaceae and certain species of Cretaceous Arcellites, although there are significant differences between them. This suggests that Molaspora is a heterogeneous taxon embracing megaspores produced by water ferns of more than one natural genus, of which only Regnellidium has survived to the present day