Pterozonium is a small genus of fourteen species of homosporous, leptosporangiate ferns that are largely endemic to the tabletop mountains of the Guayana Shield. These ferns have simple or pinnate coriaceous fronds with elongate, exindusiate sori along their free veins. In his 1967 revision of Pterozonium, D. B. Lellinger studied the evolutionary history of the genus using the groundplan divergence method. This method for constructing phylogenetic trees was developed by W. H. Wagner in the 1950s as an educational tool to illustrate systematic principles. Using "primitive" or groundplan characteristics as a starting point, this character-based method estimates the amount, direction, and sequence of phylogenetic divergence among taxa. It essentially employs a parsimony algorithm and is a precursor to modern-day Wagner parsimony. From the 1950s to the 1980s, researchers doing monographic studies frequently implemented groundplan divergence to estimate phylogenetic relationships among taxa. This method has since been replaced by a more stringent application of parsimony for the analysis of morphological data. In an attempt to better understand the interspecific relationships of Pterozonium and the relationship between groundplan divergence and maximum parsimony methods, we compare the results of Lellinger to our results of parsimony analyses of Lellinger's data and of an expanded morphological data matrix. We include several closely related genera not included by Lellinger, such as Austrogramme, Eriosorus, and Taenitis, allowing us to determine the phylogenetic root for Pterozonium and the direction of morphological character state transformation in the genus. This sets the stage for a concurrent study of the genus that has been initiated using multiple molecular markers. We describe similarities between the analyses using groundplan divergence and maximum parsimony and discuss the potential of Pterozonium as a model organism in which to study the relative roles of dispersal, extinction, and vicariance in the biogeography of homosporous ferns.

Key words: biogeography, groundplan divergence, Guayana Shield, Pteridaceae, Pterozonium