Abstract
The hemiphractid frog genus Stefania is one of the many ancient (near-) endemic lineages of vertebrates inhabiting the biodiverse Pantepui biogeographical region in the Guiana Shield Highlands of northern South America-the famous "Lost World" of Arthur Conan Doyle. Previous molecular analyses of the genus Stefania have indicated that species boundaries and phylogenetic relationships are often incongruent with morphological traits in that clade. A substantial number of "taxonomically cryptic" species, often microendemic, remain to be described. This is notably the case for an isolated population from the summit of Wei-Assipu-tepui, a small table-top mountain at the border between Guyana and Brazil. That population was previously known as Stefania sp. 6 and belongs to the S. riveroi clade. The new species is phylogenetically distinct, but phenotypically extremely similar to S. riveroi, a taxon found only on the summit of Yuruaní-tepui in Venezuela and recovered as sister to all the other known species in the S. riveroi clade. The new taxon is described based on morphology and osteology. Data about genetic divergences within the S. riveroi clade are provided. A new synapomorphy for the genus Stefania is proposed: the presence of a distal process on the third metacarpal. Amended definitions are offered for the three other species in the S. riveroi clade (S. ayangannae, S. coxi, S. riveroi). The new species should be listed as Critically Endangered according to IUCN criteria.