A COMPARATIVE MORPHOLOGICAL STUDY OF PINNULES IN THE CENOZOIC OSMUNDA SUBGENUS OSMUNDA (OSMUNDACEAE): IMPLICATIONS FOR ITS HISTORICAL BIOGEOGRAPHY AND PHYLOGENY

Osmunda regalis has a worldwide distribution, except for in eastern Asia, where Osmunda japonica and Osmunda lancea can be found instead. However, the present geographic separation does not necessarily mean geographical speciation. Molecular data indicated that American O. regalis is sister to O. japonica and O. lancea rather than to European O. regalis, but the relationship between the distribution and phylogeny remained unclear. An analysis of vegetative pinnule fossils, which were abundant in the Cenozoic time, would help elucidate the historical biogeography and evolution of these species if the taxonomy of the fossils would be clarified. We measured interveinlet intervals in mature and juvenile plants of the extant species and fossils from Japan and the rest of the world, as well as the size of their pinnules, and compared them through geological time and during ontogeny. The interveinlet interval differs between O. regalis and O. japonica, and part of the Cenozoic fossils, mostly from Japan, could be classified as belonging to these two species. The venation of the O. japonica mature plants was more similar to that of juvenile plants of the same species than was the case with O. regalis, possibly due to a paedomorphic interval in O. japonica. Osmunda regalis or its ancestor had been present in eastern Asia since the Middle Eocene, while O. japonica was derived in the Miocene, possibly from O. regalis. The past sympatry of the two species may have been facilitated by ecological isolation before geographical separation.