A Taxonomic Revision of Monopteryx (Leguminosae): a Florally Divergent and Ancient Papilionoid Genus of Large Amazonian Trees

Abstract— Monopteryx is a florally divergent genus of Dipterygeae, an early-branching papilionoid legume clade largely marked by winged papilionate floral architecture, expanded upper calyx lobes often assuming a wing-shaped orientation, and petals differentiated into standard, wings, and a keel enclosing the basally connate stamens. In contrast to the remaining Dipterygeae genera, Monopteryx has differentiated petals but the marginally coherent keel with interlaced trichomes exposes the free stamens and the expanded upper calyx lobes are nearly entirely fused with a standard-like dorsal orientation. Monopteryx species are restricted to the Amazonian rainforests, where they have diversified since the last ∼15 Ma, but the divergence of the genus is estimated to be as old as ∼39 Ma. They grow as large buttressed trees usually with a uniquely “flying” architecture, which are arched from the trunk to the ground and separated from one another, unlike that found in any other species of leguminaceous trees. Its fruits are elastically dehiscent pods and in some species they bear marginally crimped wings along the sutures. Our taxonomic revision of this ecologically and evolutionarily important, ancient genus includes an analysis of about 135 specimens from across 14 herbaria, including both type and historical collections, as well as recently collected samples from our extensive fieldwork across remote areas of the Amazon. Grounded on a densely-sampled dated molecular phylogeny of nuclear and plastid data, here we recognize three phylogenetically and morphologically distinct taxa: M . angustifolia , M . inpae , and M . uaucu . After a careful revision of their nomenclatural history, we also found that M . inpae was not validly published. We subsequently have provided typification of all names associated with species of the genus. This revision also includes morphological descriptions, illustrations, and distribution maps for all species. We also discuss the phylogenetic relationships between the species and the evolution of selected taxonomically key morphological characters in the context of the entire Diptergyeae clade.