Non-Molecular Phylogenetic Hypotheses for Ferns

-There is a consensus among fern systematists that the eusporangiate ferns, Ophioglossaceae and Marattiaceae, constitute ancient and isolated lineages, the former perhaps allied to progymnosperms. Osmundaceae, Schizaeaceae sensu lato, Gleicheniaceae, Matoniaceae, Cyatheaceae, Dicksoniaceae, and several other small or monotypic families represent early offshoots from the main line leading to higher leptosporangiate ferns, conclusions supported by fossil evidence. Among the higher ferns, there is also general agreement as to the major evolutionary groups (variously assigned subfamilial, familial, or ordinal rank), e.g., the dryopteroid, thelypteroid, blechnoid, asplenioid, polypodioid, dennstaedtioid, and pteridoid/cheilanthoid ferns. There is much less agreement on the origins and interrelationships of these more modern lineages. Highly modified ferns that occupy specialized ecological niches are especially difficult to place in a phylogenetic scheme; these include filmy ferns and heterosporous ferns. Disagreements in various phyletic schemes include the origin of the polypods (and grammitids) from either gleichenioid (exindusiate) stock or from a line leading to the largely indusiate ferns (dennstaedtioid line sensu Holttum, 1949), with sporangia having a vertical annulus. The often-suggested derivation of pteridoid/cheilanthoid ferns from schizaeoid ferns needs reexamination. In recent years, fern systematists have refrained from producing phyletic schemes based on morphological traits, and rigorous cladistic analyses of the Polypodiophyta have never been attempted, perhaps because of a supposed high incidence of reticulate evolution and homoplasy, as well as inadequate and incomplete data sets. It is the purpose of this paper to summarize ideas on the phylogenetic relationships of ferns, as based on evidence derived from such traditional sources as fossils, comparative morphology, anatomy, and chromosome number. I will approach this topic by first giving a cursory overview of characters that have been employed in the classifications of the last 250 years. These characters form the basis for most of the published evolutionary schemes, including ones by Bower (1926), Holttum (1947, 1949, 1973), Wagner (1969), Mickel (1973, 1974a), and Pichi Sermolli (1977), and also underlie commonly used classifications that lack a schematic depiction of relationships (e.g., Tryon and Tryon, 1982; Kramer in Kubitzki, 1990). The generic sequence proposed by Crabbe et al. (1975) was intended as a filing arrangement for herbaria, but the authors stated that they "incorporate the most recent evidence bearing on phylogeny" (p. 142), so that in fact it qualifies as a classification, more than many that have been advanced. Other phylogenies or classifications incorporating evolutionary principles for all or a portion of the ferns include ones by Ching (1940, 1978), Dickason (1946), Copeland (1947), Alston (1956), Mehra (1961), Nayar (1970, 1976), Bierhorst (1971), and Lovis (1977). These and others were reviewed by Pichi Sermolli (1973). The various evolutionary schemes cited above are in the form of phyletic trees-dendrograms or sausage-diagrams. The tree by Wagner (1969; Fig. 1) This content downloaded from 157.55. 39.208 on Wed, 28 Sep 2016 05:21:47 UTC All use subject to http://about.jstor. org/terms SMITH: NON-MOLECULAR PHYLOGENETIC HYPOTHESES