Angiosperm wood evolution and the potential contribution of paleontological data

Wood anatomy is often viewed as a source of independent data that may be used to assess evolutionary relationships among angiosperms. Comparative anatomical studies document suites of correlated characters that have been interpreted as general evolutionary trends, of which several have been asserted to be irreversible. Paleobotanical data summarized by Wheeler and Baas provide broad chronological corroboration of some wood anatomical trends, such as evolution from scalariform to simple perforation plates and long to short vessel elements. However, the focus on general evolutionary trends rather than on analyzing character distribution patterns in a cladistic phylogenetic context obscures a more detailed understanding of the evolution of wood anatomical features. Patterns of character evolution, including the assertions of irreversibility, need to be tested through cladistic analyses. In this paper selected wood anatomical features from families of Magnoliidae and "lower" Hamamelididae are summarized and mapped onto previously published cladograms as a preliminary means of testing previous hypotheses of wood evolution. The results show that many of the characters are homoplasious and have evolved both in accord with, and counter to, the hypothesized general 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 with other features as part of a cladistic analysis. Fossil woods have not yet contributed significantly to phylogenetic studies, but in the very few cases where they have been linked to fossil reproductive structures, the woods have provided a better understanding of wood anatomy in early members of some families. Data from fossil wood expand the diversity of anatomical structure known in some angiosperm taxa and thus provide additional evidence that might be used in phylogenetic analyses. Fossil woods have the greatest potential to affect phylogenetic analyses where they can be linked to other fossil organs. The best chance for establishing such a linkage is through the study of fossil charcoalified woods that co-occur with other dispersed mesofossils.