Heterosporous ferns – characterized by morphologically distinct spores producing unisexual gametophytes – form a monophyletic clade nested within the otherwise homosporous leptosporangiate ferns, Polypodiidae. The clade consists of two extant families: the semi-aquatic Marsileaceae, with three genera (Marsilea, Regnellidium, Pilularia) and 70 species, and the aquatic Salviniaceae, with two genera (Azolla, Salvinia) and 20 species. A recent phylogenetic analysis based on multiple genes and morphology placed heterosporous ferns as sister group to tree ferns plus "polypodiaceous" ferns, and together these comprise the three major modern fern clades. Evidence from the fossil record indicates that heterosporous ferns began to diversify during the Cretaceous (ca. 144 million years ago) at the same time as flowering plants. Marsileaceae radiated first and was the dominant heterosporous fern in the Early Cretaceous. Salviniaceae began to overtake Marsileaceae in species richness in the Late Cretaceous and was the most species-rich heterosporous fern group throughout most of the Cenozoic. In all, approximately 200 extinct species of heterosporous ferns are preserved in the fossil record, mostly in the form of microfossils with a remarkable diversity of ornamented spores. Recent phylogenetic analyses resolved the relationships among the five extant genera of heterosporous ferns, but these ferns have been largely ignored in systematic studies and infrageneric relationships are virtually unknown. We have preliminary results based on combined molecular (rbcL, trnL-F intergenic spacer, ITS) and morphological data for species-level relationships within the extant genera that we have compared using maximum parsimony, maximum likelihood, and Bayesian approaches. Traditional infrageneric classifications are largely inconsistent with our initial results, emphasizing the need for a modern systematics study of these ferns. Our phylogenetic hypothesis of extant taxa will be expanded to include morphological data from extinct heterosporous ferns, which will then serve as an explicit framework to investigate character evolution in this monophyletic group.

Key words: Bayesian, heterosporous, likelihood, Marsileaceae, parsimony, Salviniaceae