The three hypnobryalean orders of pleurocarpous mosses, Hookeriales, Hypnales, and Leucodontales, are difficult to define using morphological criteria, with "defining" characters of a group often appearing elsewhere and species and genera repeatedly shifting allegiances. Similarly, the circumscription and relationships of the eubryalean pleurocarps, and their relationship to the hypnobryalean pleurocarps and acrocarpous outgroup taxa, is also problematic. Results of phylogenetic analyses of pleurocarpous mosses using molecular sequence data, from the rbcL, trnL-trnF, rps4 and 18S genes, have underscored this problem. In the current analysis, using molecular characters, the hypnobryalean orders form a monophyletic group with exemplars from the Hookeriales in a clade sister to exemplars from the Hypnales and Leucodontales, but with these two orders paraphyletic and poorly resolved. This pattern, seen also in earlier studies, indicates a rapid evolutionary radiation, with very short internodes that are difficult to resolve satisfactorily with the molecular sequence data at hand. However, whereas molecular sequence characters are generally thought to evolve in a regular, "clock-like" fashion, morphological characters are thought to evolve following a different pattern, with long periods of stasis interspersed with periods of rapid change. Morphological characters may therefore be able to resolve the relationships of taxa involved in rapid radiation events. Many moss morphological features are difficult to define as characters and states for cladistic analysis, especially those "gestalt" characters that have played a large part in taxonomic concepts. Other features show extensive and complex variation that may represent several different characters. Branching architecture and rhizoid morphology are two such complex features that have been deconstructed to yield characters for inclusion in cladistic analyses, and to provide a basis for study of the evolution of these features.

Key words: evolutionary radiation, morphology, pleurocarpous mosses, sequence data