Dimorphic fronds and in situ spores of Drynaria (Polypodiaceae) from the upper Pliocene of Southwest China

Two types of fronds belonging to a new species, Drynaria dimorpha sp. nov., are described from the upper Pliocene Mangbang Formation in western Yunnan, southwestern China. The base frond is lobate and the lobes are incised up to 2/3 or more of the width. The foliage frond is preserved as individual fertile or sterile pinnae with serrated margin and constricted basal part. Circular sori are arranged in one row on either side of the primary vein. In situ spores are monolete and planoconvex, elliptical in polar view and bean-shaped in equatorial view. A detailed morphological comparison suggests that none of the previously described drynarioid fossils and extant species is in accord with the present ferns. The discovery of this new fern indicates that a developed humus-collecting structure of Drynaria had existed and a species-level radiation of this genus had taken place at least by the late Pliocene.

► Dimorphic fronds of Drynaria are described from the upper Pliocene of Southwest China.
► Sporangium and in situ spores are examined under SEM.
► Base frond has a function of humus collecting.
► Developed humus collector has existed at least by the late Pliocene.
► Different ecological habitats promote the diversity on the species-level.