Angiosperm wood evolution and the potential contribution of paleontological data

Wood anatomy is often viewed as a source of independent data that may be us ed to assess evolutionary relationships among angiosperms. Comparative anat omical studies document suites of correlated characters that have been inte rpreted as general evolutionary trends, of which several have been asserted to be irreversible. Paleobotanical data summarized by Wheeler and Baas pro vide broad chronological corroboration of some wood anatomical trends, such as evolution from scalariform to simple perforation plates and long to sho rt vessel elements. However, the focus on general evolutionary trends rathe r than on analyzing character distribution patterns in a cladistic phylogen etic context obscures a more detailed understanding of the evolution of woo d anatomical features. Patterns of character evolution, including the asser tions of irreversibility, need to be tested through cladistic analyses. In this paper selected wood anatomical features from families of Magnoliidae a nd "lower" Hamamelididae are summarized and mapped onto previously publishe d cladograms as a preliminary means of testing previous hypotheses of wood evolution. The results show that many of the characters are homoplasious an d have evolved both in accord with, and counter to, the hypothesized genera l trends in different groups of flowering plants. In general, changes that confirm generalized trends are more common than changes that are counter to those trends. Future studies should combine wood anatomical characters wit h other features as part of a cladistic analysis. Fossil woods have not yet contributed significantly to phylogenetic studies, but in the very few cas es where they have been linked to fossil reproductive structures, the woods have provided a better understanding of wood anatomy in early members of s ome families. Data from fossil wood expand the diversity of anatomical stru cture known in some angiosperm taxa and thus provide additional evidence th at might be used in phylogenetic analyses. Fossil woods have the greatest p otential to affect phylogenetic analyses where they can be linked to other fossil organs. The best chance for establishing such a linkage is through t he study of fossil charcoalified woods that co-occur with other dispersed m esofossils.