Aspects of evolutionary differentiation of the Hamamelidaceae and the Lower Hamamelididae

New investigations on the flower and fruit structure of extant Hamamelidaceae and other Lower Hamamelididae together with new finds of fossil flowers and seeds from the Upper and Lower Cretaceous provide the outline of an increasingly more differentiated picture of the early evolution of the subclass. Three patterns of valvate anther dehiscence are recognized in the subfamily Hamamelidoideae (and the subclass Hamamelididae). The basic (plesiomorphic) type within the Hamamelididae has 2 valves per theca. The type with 1 valve but 2 pollen sacs per theca is both consistent and exclusive for the 5 southern genera of the Hamamelidaceae. They seem to be the remnants of a homogeneous group that originated before the Upper Cretaceous. This is supported by fossil hamamelidaceous flowers from the Upper Cretaceous that have thecae with 1 valve. Since several-seeded Hamamelidaceae predate one-seeded forms in the fossil seed record (in Europe) and the systematic structure of the one-seeded group is relatively more homogeneous, several-seeded groups are considered to be more ancient. Several parallel evolutionary trends are recognized within the Hamamelidaceae as well as within the Lower Hamamelididae: anther dehiscence with 2 valves per theca → 1 slit or 1 valve; pollen sacs per theca 2 → 1; pollen tricolpate → polyforate; exine coarsely reticulate → finely reticulate; loss of perianth (tepals or petals and sepals) and concomitant loss of fixed number of floral organs; differentiation of exposed nectaries.