A REEVALUATION OF SEED PLANT PHYLOGENY

Seed plant phylogeny is evaluated using a data set of 46 terminals (taxa) and 103 morphological and anatomical characters. Cladistic analyses using the criterion of parsimony were performed on the complete data set as well as on subsets of the data, e.g., excluding fossils and/or combining various complex taxa into single terminals. The results support the placement of the cycads as the sister group of a monophyletic group that includes several fossil ''seed ferns'' as well as extant Ginkgo, conifers, gnetopsids, and angiosperms. When fossils were included, Bennettitales (cycadeoids) were part of an ''anthophyte'' clade that included gnetopsids and angiosperms. Pentoxylon was a sister taxon to the core anthophyte clade, in some, but not all, of the most parsimonious trees. Caytonia was not found to be closely associated with the anthophyte clade, but instead was often associated as a sister taxon of the glossopterids, and these two taxa were consistently outside of the Ginkgo-conifer-anthophyte clade. In all most parsimonious trees for all analyses, Ephedra was to the outside of a clade that included all angiosperm taxa, Gnetum, and Welwitschia, thus rendering the traditional gnetopsid clade paraphyletic. New information is provided on the morphology of Caytonia and some previous interpretations of homology of the caytonian ''cupule'' are rejected. The effects of sampling, compartmentalization, and polymorphism are explored in these data, showing how different results may be obtained when polymorphic or ''summary'' terminals are used. The need for more work on gnetopsids and fossil taxa is suggested.