The members of Hamamelidaceae have unambiguously been documented to belong to the Saxifragoid clade, within the core eudicots. Although the monophyly of the family has not been firmly established, the monophyly of subfamily Hamamelidoideae is confirmed by several independent phylogenetic studies. Insights about the early history of Hamamelidoideae have been provided by the flowers of Archamamelis (Santonian-Campanian, Sweden), and Allonia (late Santonian, Georgia, U.S.A.). A third, recently discovered flower, Androdecidua, from the late Santonian of Georgia, provides further information about morphological diversity within Hamamelidoideae. Androdecidua has two pentamerous androecial whorls, in which the stamens of the outer whorl have bisporangiate anthers, resulting from the absence of the adaxial pollen sac of each theca. A phylogenetic analysis was conducted to obtain explicit information about relationships within Hamamelidoideae, particularly the affinity of fossil genera, and about character evolution. The analysis included 38 taxa, comprising members of Hamamelidaceae, representatives of the Saxifragoid clade, and outgroups form the basal eudicot grade, and was based on 57 characters of floral form. The strict consensus of 42 most parsimonious trees resolves Archamamelis and Hamamelis as sister taxa, whereas Androdecidua and Allonia are nested within a monophyletic Loropetalinae. The obtained phylogenetic hypothesis suggests that the strap-like, circinately coiled petals, which characterize some genera of Hamamelidoideae, evolved twice; a specialized stomium configuration that results in a distinctive mode of anther dehiscence evolved once, but was subsequently lost, and stamens with bithecal anthers evolved at least two times. The highly nested placement of fossil taxa suggests that the main evolutionary lineages within Hamamelidoideae had evolved by the late Santonian. Considering that the oldest records for core eudicots are from slightly older strata (Turonian), it appears that morphological and taxic diversification proceeded quickly during the initial evolution of Hamamelidoieae, and subsequently remained stable.

Key words: floral evolution, fossils, Hamamelidoideae, perianth, phylogeny, stamens