HETEROSPOROUS FERNS: PHYLOGENY, ORIGIN AND RADIATION

The leptosporangiate fern families Marsileaceae (Marsilea, Pilularia, Regnellidium) and Salviniaceae (Azolla and Salvinia) represent the most recent acquisition of heterospory—production of separate male gametophytes in microspores and female gametophytes in megaspores. Read directly from the fossil record, the first appearance of heterosporous ferns reliably dates to the earliest Cretaceous and their radiation is broadly coincident with the radiation of flowering plants and derived ferns. Updating the work of Kovach and Batten (1993) and placing taxa in a phylogenetic framework, we investigate the likely timing of heterosporous fern origins and trace their diversification through time and space. Utilizing the record of dispersed megaspores, stratigraphic confidence intervals may be calculated for each of the clades delimited in the phylogeny. Confidence intervals place the divergence of the lineages leading to the crown groups in the Late Jurassic, consistent with the occurrence of the macrofossil Regnellites in the Berriasian. Under the assumption that the dispersed megaspore genus Molaspora belongs to crown-group Marsileaceae, the family most likely appeared in the latest Jurassic/earliest Cretaceous. This date is consistent with macrofossil records of Marsileaceae, the first of which (Regnellites) occurs in the Berriasian. Crown-group Salviniaceae probably arose in the earliest Cretaceous. Azolla most likely originated by the Cenomanian although macrofossils appear only in the Paleocene. The origin of Salvinia is enigmatic. No megaspores/macrofossils are known before the Paleocene and no fossils join the branch leading to Salvinia in our phylogeny suggesting a very long ghost lineage. Analyses of diversity changes illustrate that marsileaceous diversity rose first, peaking in the early Late Cretaceous and they were replaced by Salviniaceae in the latest Late Cretaceous, as also found by Kovach and Batten. Salviniaceae remained the most diverse clade through the Paleogene and Neogene. However, extant Marsilea (with ~70 species) is by far the most diverse genus in either family suggesting a missing record or recent radiation.