Most floral diversity in the core eudicots is based on a groundplan that includes among its most distinctive elements a bipartite perianth and pentamerous merosity combined with a bicarpellate gynoecium. To obtain insights into the origins of this groundplan, the evolution of its structural components in Hamamelidoideae (Hamamelidaceae, Saxifragales, core eudicots) is explored. Phylogenetic hypotheses were obtained from independent analyses of morphological and molecular data, and from two alternative approaches to integrating morphological and molecular data from living and fossil taxa. In spite of the large numbers of missing data cells, fossils were unambiguously placed within particular clades in the well resolved but poorly supported trees obtained from morphological data. Phylogenetic analyses of individual and combined molecular data sets using different optimization criteria resulted in congruent trees that share most strongly supported nodes. No well-supported incongruence existed among the morphological and molecular trees. A total evidence analysis including only complete taxa was congruent with molecular trees, but resolution decreased substantially as incomplete taxa and incomplete characters were included. A parsimony analysis of morphological data constrained with the strongly supported nodes from the total evidence tree resulted in the same backbone relationships as the molecular trees and in the unambiguous placement of fossil taxa; however, relationships within some clades were unique. The floral characters of ancient fossils and their nested placement in the phylogeny indicate stasis in floral structure since the origin of major lineages of Hamamelidoideae. Individual elements of floral structure display independent evolutionary histories, suggesting that the core eudicot floral groundplan evolved through the accretion of components which originated at different points during angiosperm history: some outside the eudicots, and others, namely the bipartite perianth and the pentamerous merosity, most likely within core eudicots.
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