Endemic families of Madagascar. XII. Resurrection and taxonomic revision of the genera Mediusella (Cavaco) Hutchinson and Xerochlamys Baker (Sarcolaenaceae)

The position of Leptolaena subgenera Mediusella Cavaco and Xerochlamys (Baker) Cavaco with respect to Leptolaena Thouars and Sarcolaena Thouars has long been debated along with the circumscription of species within these two groups. Recent advances in the molecular phylogeny of Sarcolaenaceae indicate that both Mediusella and Xerochlamys are not part of Leptolaena, requiring a generic recircumscription of the entities in the Leptolaena-Sarcolaena-Mediusella-Xerochlamys clade. The results of a previous multivariate analysis of morphological characters also showed that Mediusella and Xerochlamys include two and eight entities respectively, which are recognized here as distinct species. Three species of Xerochlamys (X. coriacea, X. itremoensis and X. undulata) are described as new, and Mediusella arenaria requires a new combination. The three new species are distinguished by their habit, the size, pubescence and margin of their leaves, and the structure of their inflorescences. A key to the species and information on their distribution, ecology and phenology are given. The conservation status of each species is evaluated according to IUCN Red List criteria; six species are considered endangered (EN), two are vulnerable (VU), one is near threatened (NT) and the last is of least concern (LC).