Abstract: Recent advances in angiosperm phylogenetics driven by molecular data have led to the establishment and refinement of the Angiosperm Phylogeny Group (APG) classification system, which prioritizes monophyly in delimiting orders and families. However, this approach has resulted in some highly heterogeneous groups that lack clear morphological synapomorphies, necessitating further critical study. The Gyrostemonaceae-Resedaceae-Forchhammeria-Tirania (GRFT) clade in Brassicales is one of these groups, and its family-level classification has long been controversial. Here, we used an integration of phylogenetic and dating analyses, based on the combined dataset of five DNA regions and 35 morphological characters, to determine the familial classification within this clade. We further used plastome data to confirm the classification. Our results show that the GRFT clade contains five major lineages: Gyrostemonaceae, Resedaceae s.str., Borthwickia, Forchhammeria, and Neothorelia-Tirania-Stixis. Gyrostemonaceae is the earliest-diverging lineage in the GRFT clade, followed by Borthwickia. Neothorelia-Tirania-Stixis is sister to the clade containing Resedaceae s.str. and Forchhammeria. Within this phylogenetic framework, the evolution of selected characters is inferred, and diagnostic morphological characters at different taxonomic levels were identified and discussed. The GRFT clade started to diversify at about 55 Ma. Borthwickia originated around 48 Ma. Neothorelia-Tirania-Stixis, Forchhammeria, and Resedaceae s.str. diverged at approximately 38-36 Ma, which are older than 42 families recognized in the APG classification system. By integrating lines of evidence from phylogeny and divergence times obtained from the combined molecular and morphological dataset, we divided the GRFT clade into five families, i.e., Gyrostemonaceae, Borthwickiaceae, Stixaceae, Forchhammeriaceae, and Resedaceae, in which Forchhammeriaceae was newly described.

Key words: classification, divergence times, Forchhammeriaceae, morphology, phylogeny, Stixaceae