Evolution of the Fern Genus Osmunda

The phylogenetic background of the living species of Osmunda is interpreted from seven species of fossil rhizomes, six of which are assigned to the genus in this account. Osmunda pluma, 0. precinnamomea, and 0. arnoldii are new species from the Paleocene of North Dakota that are described in the subgenera Osmunda, Osmundastrum, and Plenasium respectively. A fourth new species, Osmunda nathorstii, is based on a fossil from the Upper Tertiary of Spitsbergen that was previously included under the binomial Osmundites spetsbergensis (Nathorst) Kidston and GwynneVaughan. This species is assigned to the subgenus Osmunda. Osmundites schemnicensis (Pettko) Unger, from the Miocene-Pliocene of Hungary, is transferred to the subgenus Osmunda under the new name Osmunda iliaensis; and Osmundites oregonensis Arnold, from the Eocene of Oregon, is placed in this subgenus as a new combination. The living Osmunda claytoniana L. , which is usually treated in the subgenus Osmundastrum, is regarded as a member of the subgenus Osmunda because of its anatomical similarity to other members of this subgenus. Osmundites chandleri Arnold is treated as a synonym of Osmunda dowkeri (Carruthers) Chandler, a species in the subgenus Plenasium from the Eocene; and fossil rhizomes from the Upper Tertiary of Washington are identified as Osmunda cinnumomea L. Furthermore, Osmundacaulis nom. nov. is used in place of Osmundites Unger, which is invalid, and the species of this form genus are listed in groups according to their phylogenetic relationships. The three subgenera of Osmunda have been well-defined lines of specialization since the Paleocene, and the subgenera Osmunda and Plenasium are in evidence in the Early and Late Cretaceous respectively. The Recent species evolved almost unchanged from their Tertiary ancestors with most modifications reflected in the various arrangements of petiolar sclerenchyma. An important exception to this is the development of the internal endodermis in Osmunda cinnamomea which occurred sometime between the Paleocene and the present. Three of the living species of the subgenus Osmunda are the products of a line of development represented by 0. pluma, 0. oregonensis, and 0. iliaensis, whereas 0. claytoniana arose on a divergent branch of this line through 0. nathorstii from a form similar to 0. iliaensis. In the subgenus Osmundastrum, Osmunda cinnamomea is viewed as a more or less direct descendant of 0. precinnamomea. The Recent species of Plenasium arose from Osmunda arnoldii or a similar form, and 0. dowkeri represents an extinct line of specialization within Plenasium that converges with the main line in the Cretaceous. EVOLUTION OF FERN GENUS OSMUNDA 141 Although it has not been possible to trace the origin of any of the subgenera to specific forms of Osmundacaulis, the subgenera Osmunda and Osmundastrum show greatest affinity with the Osmundacaulis herbstii group while Plenasium probably arose from an unknown form of the Osmundacaulis skidegatensis group. To recognize the probability that the genus Osmunda is polyphyletic and that its subgenera have been independent lines of specialization for at least 70 million years, i t is recommended that Osmunda, Osmundastrum, and Plenasium be raised to genera.