Integrating molecular phylogenetic and paleobotanical evidence on origin of the flower

Inferences on the origin of the angiosperm flower require consideration of other seed plants, especially fossils. Molecular data favor a relationship of Gnetales to conifers rather than to angiosperms, and both alternatives are equally parsimonious in terms of the morphological data set presented here. However, if molecular relationships among extant taxa are accepted, morphology still associates glossopterids, Pentoxylon, Bennettitales, and Caytonia with angiosperms. Bennettitales had flowerlike structures, but if Caytonia is sister to angiosperms, aggregation of fertile parts probably occurred independently in Bennettitales and angiosperms. These results and developmental genetic data are consistent with homology of the angiosperm bitegmic ovule with the cupule of glossopterids and Caytonia, while the carpel could represent a leaf and a cupule-bearing axillary branch. Origin of an adaxial cross zone could produce a uniovulate, ascidiate carpel, as in living basal angiosperms. Stamens may represent similar units bearing two microsynangia. However, ovulate structures of Pentoxylon and Bennettitales are more difficult to interpret, and any homologue of the carpel wall in Caytonia is unclear. Further progress may require better understanding of homologies in known fossils and/or recognition of closer stem relatives of angiosperms. A proposed Cretaceous stem relative, Archaefructus, is more likely a crown-group angiosperm related to Hydatellaceae (Nymphaeales).