Evolution of the Fern Family Osmundaceae Based on Anatomical Studies

The fern family Osmundaceae has only 16 living species, but nearly 100 fossil species have been named on the basis of various detached organs. Most important of these are 29 species based on petrified axes which provide a partial but substantial record of the family from the Late Permian to the present. These axes and some of the Recent species have been important in the formulation of concepts of stelar evolution and in our understanding of the evolutionary relationships of living ferns, but an intensive investigation of the evolution of the species of the Osmundaceae has never been made. Thus, the purpose of the present work is to investigate the phylogenetic interrelationships of the species of the family as interpreted from anatomical and morphological characters of their stems, leaf-bases, and roots. Types or representative materials of 14 Recent and 21 fossil species were examined for variable characters. Information about nine additional species of petrified axes came from descriptions in the literature. These species were then analyzed by the ground plan method of multiple character correlation in three samples: (1) analysis of 43 species based on six characters, (2) analysis of 31 species based on 13 characters, and (3) analysis of 14 Recent species based on 18 characters. By this means the species were arranged according to lines of specialization, and when these lines are considered with respect to geologic time, nine groups of phylogenetically related species are resolved. Five of the groups are represented by the present-day genera and subgenera and their fossil members. Three groups are subdivisions of Osmundacaulis, and the remaining group includes the extinct protostelic forms. The results show large gaps between these groups. This indicates that the fossil record of the family contains only a small number of those species that existed in the past. I t is evident, however, that the modern species evolved from protostelic ancestors by intrastelar origin of a pith and subsequent (or concurrent) dissection of the xylem cylinder by leaf-gaps. The living species and most of the fossil forms are products of slight modifications of the dictyoxylic-siphonostele plan of organization. However, two of the groups of species became highly specialized in the Mesozoic but died out near the close of that era. The Recent genera Todea and Leptopteris are as distinct from one another as each is from the subgenera Osmunda, Osmundastrum, or Plenasium, but their fossil records are not sufficient to indicate their origin. Osmunda, Osmundastrum, and Plenasium were distinct lines of evolution in the Paleocene, and Osmunda and Plenasium are represented by foliage remains in the Cretaceous. No definite source for Osmunda and Osmundastrum can be determined, but Plenasium shares a number of characters with Osmundacaulis atherstonei and 0 . natalensis, both of which are from the Lower Cretaceous, and they may have evolved from forms similar to these species. The Osmundaceae probably arose during Early Permian or Pennsylvanian time from forms similar to Grammatopteris baldaufi or G . rigolloti, but neither of these species can be considered a precursor of the family. Further, no ancestors of other fern taxa can be found amonq the known Osmundaceae. Taxonomically, the protostelic forms are treated in the subfamily Thamnopteroideae with the remaining species forming the subfamily Osmundoideae.